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For how long can animal body cells survive outside the body?

For how long can animal body cells survive outside the body?


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I'm researching on the topic that if bacteria have souls then animal cells and plant cells also do for an article.

1) Is there a way to make any of the body cells survive outside the body for a reasonably long time?

2) What is the longest time that human cells have survived outside the body?

3) Can we consider each human body cell as a living thing, the same or more complex than a bacteria? Why? Why not?

4) If a cell is unable to survive outside the body, is it considered to be not a living thing in biology?

5) What are the properties of a living thing definition that animal body cells do not meet?

6) What is the stance of biologists on animal cells being living things?


As @LinuxBlanket stated, that's too many questions. But I thought this interesting so wanted to answer anyway.

I'm researching on the topic that if bacteria have souls then animal cells and plant cells also do for an article.

Since there's no evidence of "souls" anywhere, the topic is moot. I encourage you to read this.

Concerning your main question

Can animlal body cells survive outside the body? For how long

That has nothing to do with a soul - if souls did exist, then they'd only really matter when the organism dies. So how long something lives has nothing to do with your topic.

You should also think about what "alive" means, as there isn't a solid definition in Biology. usually the definition is around replication and/or metabolism, but then would you say a virus is alive? If yes, what about prions? If you cut a tree to the stump, when exactly does it "die"? Where do you draw the line?

OK, to answer your questions.

  1. Yes. Think of a cell as an enclosed unit that metabolises and respirates. As long as the cell gets what it needs to continue those two processes, whether within the tissue or in a synthetic setting, it'll "survive". See this.

  2. So as I explained, you can keep a cell ticking along as long as you can keep those basic processes going. At some point, a cell will want to replicate. So now you have to think - is my 1 surviving cell becomes 2, then 4, etc., then cell number 1 dies, am I still classing it as survival? If you want to keep cell number 1 alive indefinitely, you can adjust its environmental conditions to slow the pace of its life - cool the cell down, chemical reactions for metabolism and respiration slow down and the individual cell will "live longer". You can even freeze a cell for years then thaw it, and it will continue to live. See this.

  3. That's for you to figure out. I encourage you to look up the definitions of cells, tissues, organs and organisms.

  4. If a cell dies, it's considered dead. Even in biology.

  5. Again, the definitions of "living" aren't set in stone. Personally, I reduce a living thing into these two requirements:

    • Something that replicates
    • Something that metabolises

But, I would say that animal cells meet the requirements of the general consensus more than viruses and prions (they replicate), but less than plant cells. This might be a good start to exploring why. But in short, animal cells tend to work together as tissues. Plant cells are a little more viable on their own. You can take a plant cell and grow an entire plant from it. You can only do the same with an animal if you use stem cells.

  1. You should be able to answer that question yourself now =)

1) Is there a way to make any of the body cells survive out side the body for a reasonably long time?

HeLa cells have been living outside of their owner's body since 1951. Admittedly, cancer cells are trying hard not to die in the first place…


Cell lines are cancer cells or virally or chemically transformed cells that can continue to replicate indefinitely, as long as they get fresh media. Primary cells are native cells that are grown in culture, but they eventually die off. Some primary cells are short lived, both in vitro and in vivo, like neutrophils. Other primary cells can grow, differentiate, and replicate for weeks, particularly if growth factors are supplied.


The biology of fats in the body

When you have your cholesterol checked, the doctor typically gives you levels of three fats found in the blood: LDL, HDL and triglycerides. But did you know your body contains thousands of other types of fats, or lipids?

In human plasma alone, researchers have identified some 600 different types relevant to our health. Many lipids are associated with diseases--diabetes, stroke, cancer, arthritis, Alzheimer's disease, to name a few. But our bodies also need a certain amount of fat to function, and we can't make it from scratch.

Researchers funded by the National Institutes of Health are studying lipids to learn more about normal and abnormal biology. Chew on these findings the next time you ponder the fate of the fat in a French fry.

Fat Functions

Triglycerides, cholesterol and other essential fatty acids--the scientific term for fats the body can't make on its own--store energy, insulate us and protect our vital organs. They act as messengers, helping proteins do their jobs. They also start chemical reactions involved in growth, immune function, reproduction and other aspects of basic metabolism.

The cycle of making, breaking, storing and mobilizing fats is at the core of how humans and all animals regulate their energy. An imbalance in any step can result in disease, including heart disease and diabetes. For instance, having too many triglycerides in our bloodstream raises our risk of clogged arteries, which can lead to heart attack and stroke.

Fats help the body stockpile certain nutrients as well. The so-called "fat-soluble" vitamins--A, D, E and K--are stored in the liver and in fatty tissues.

Using a quantitative and systematic approach to study lipids, researchers have classified lipids into eight main categories. Cholesterol belongs to the "sterol" group, and triglycerides are "glycerolipids." Another category, "phospholipids," includes the hundreds of lipids that constitute the cell membrane and allow cells to send and receive signals.

Breaking It Down

The main type of fat we consume, triglycerides are especially suited for energy storage because they pack more than twice as much energy as carbohydrates or proteins. Once triglycerides have been broken down during digestion, they are shipped out to cells through the bloodstream. Some of the fat gets used for energy right away. The rest is stored inside cells in blobs called lipid droplets.

When we need extra energy--for instance, when we exercise--our bodies use enzymes called lipases to break down the stored triglycerides. The cell's power plants, mitochondria, can then create more of the body's main energy source: adenosine triphosphate, or ATP.

Recent research also has helped explain the workings of a lipid called an omega-3 fatty acid -- the active ingredient in cod liver oil, which has been touted for decades as a treatment for eczema, arthritis and heart disease. Two types of these lipids blocked the activity of a protein called COX, which assists in converting an omega-6 fatty acid into pain-signaling prostaglandin molecules. These molecules are involved in inflammation, which is a common element of many diseases, so omega-3 fatty acids could have tremendous therapeutic potential.

This knowledge is just the tip of the fat-filled iceberg. We've already have learned a lot about lipids, but much more remains to be discovered.


What cells in the human body live the longest?

Although the our bodies are continuously replenishing their cells, some stick around for longer than others.

On average, the cells in your body are replaced every 7 to 10 years. But those numbers hide a huge variability in lifespan across the different organs of the body.

Neutrophil cells (a type of white blood cell) might only last two days, while the cells in the middle of your eye lenses will last your entire life.

And it’s even possible that your brain cells might have longer maximum lifespans than you do. In 2013, researchers transplanted neurons from old mice into the brains of longer-lived rats and found that the cells were still healthy after living for two whole mouse lifespans!

Brain cells: 200+ years?

Eye lens cells: Lifetime

Egg cells: 50 years

Heart muscle cells: 40 years

Intestinal cells (excluding lining): 15.9 years

Skeletal muscle cells: 15.1 years

Fat cells: 8 years

Hematopoietic stem cells: 5 years

Liver cells: 10-16 months

Pancreas cells: 1 year

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.


How long can a virus live outside a body?

It's enough to make you sick! Viruses can be stubborn little blighters to get rid of.

Asked by: Chaudhary Nikul, India

Viruses can live for a surprisingly long time outside of a body, depending on conditions such as moisture and temperature. They tend to live longer on water-resistant surfaces, such as stainless steel and plastics.

A cold virus can sometimes survive on indoor surfaces for several days, although its ability to cause infection drops dramatically over time.

Flu viruses can survive in the air for several hours, especially at lower temperatures, and on hard surfaces they can survive and remain infectious for 24 hours.

Enteric viruses, such as norovirus (pictured) and hepatitis A, can survive for weeks on a surface if conditions are suitable. The norovirus is known for causing sickness outbreaks in schools, cruise ships and hospitals.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.


Related Story

At first, the Yale group was uncertain if an “ex vivo” brain to which circulation was restored would regain consciousness. To answer that question, the scientists checked for signs of complex activity in the pig brains using a version of EEG, or electrodes placed on the brain’s surface. These can pick up electrical waves reflecting broad brain activity indicating thoughts and sensations.

Initially, Sestan said, they believed they had found such signals, generating both alarm and excitement in the lab, but they later determined that those signals were artifacts created by nearby equipment.

Sestan now says the organs produce a flat brain wave equivalent to a comatose state, although the tissue itself “looks surprisingly great” and, once it’s dissected, the cells produce normal-seeming patterns.

The lack of wider electrical activity could be irreversible if it is due to damage and cell death. The pigs’ brains were attached to the BrainEx device roughly four hours after the animals were decapitated.

However, it could also be due to chemicals the Yale team added to the blood replacement to prevent swelling, which also severely dampen the activity of neurons. “You have to understand that we have so many channel blockers in our solution,” Sestan told the NIH. “This is probably the explanation why we don’t get [any] signal.”

Sestan told the NIH it is conceivable that the brains could be kept alive indefinitely and that steps could be attempted to restore awareness. He said his team had elected not to attempt either because “this is uncharted territory.”

“That animal brain is not aware of anything, I am very confident of that,” Sestan said, although he expressed concern over how the technique might be used by others in the future. “Hypothetically, somebody takes this technology, makes it better, and restores someone’s [brain] activity. That is restoring a human being. If that person has memory, I would be freaking out completely.”

Brain experiments

Consciousness isn’t necessary for the type of experiments on brain connections that scientists hope to carry out on living ex vivo brains. “The EEG brain activity is a flat line, but a lot of other things keep on ticking,” says Anna Devor, a neuroscientist at the University of California, San Diego, who is familiar with the Yale project.

Devor thinks the ability to work on intact, living brains would be “very nice” for scientists working to build a brain atlas. “The whole question of death is a gray zone,” she says. “But we need to remember the isolated brain is not the same as other organs, and we need to treat it with the same level of respect that we give to an animal.”

Today in the journal Nature, 17 neuroscientists and bioethicists, including Sestan, published an editorial arguing that experiments on human brain tissue may require special protections and rules.


Contents

"Biology" derives from the Ancient Greek words of βίος romanized bíos meaning "life" and -λογία romanized logía (-logy) meaning "branch of study" or "to speak". [11] [12] Those combined make the Greek word βιολογία romanized biología meaning biology. Despite this, the term βιολογία as a whole didn't exist in Ancient Greek. The first to borrow it was the English and French (biologie). Historically there was another term for "biology" in English, lifelore it is rarely used today.

The Latin-language form of the term first appeared in 1736 when Swedish scientist Carl Linnaeus (Carl von Linné) used biologi in his Bibliotheca Botanica. It was used again in 1766 in a work entitled Philosophiae naturalis sive physicae: tomus III, continens geologian, biologian, phytologian generalis, by Michael Christoph Hanov, a disciple of Christian Wolff. The first German use, Biologie, was in a 1771 translation of Linnaeus' work. In 1797, Theodor Georg August Roose used the term in the preface of a book, Grundzüge der Lehre van der Lebenskraft. Karl Friedrich Burdach used the term in 1800 in a more restricted sense of the study of human beings from a morphological, physiological and psychological perspective (Propädeutik zum Studien der gesammten Heilkunst). The term came into its modern usage with the six-volume treatise Biologie, oder Philosophie der lebenden Natur (1802–22) by Gottfried Reinhold Treviranus, who announced: [13]

The objects of our research will be the different forms and manifestations of life, the conditions and laws under which these phenomena occur, and the causes through which they have been affected. The science that concerns itself with these objects we will indicate by the name biology [Biologie] or the doctrine of life [Lebenslehre].

The earliest of roots of science, which included medicine, can be traced to ancient Egypt and Mesopotamia in around 3000 to 1200 BCE. [14] [15] Their contributions later entered and shaped Greek natural philosophy of classical antiquity. [14] [15] [16] [17] Ancient Greek philosophers such as Aristotle (384–322 BCE) contributed extensively to the development of biological knowledge. His works such as History of Animals were especially important because they revealed his naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany that survived as the most important contribution of antiquity to the plant sciences, even into the Middle Ages. [18]

Scholars of the medieval Islamic world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī (828–896), who wrote on botany, [19] and Rhazes (865–925) who wrote on anatomy and physiology. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.

Biology began to quickly develop and grow with Anton van Leeuwenhoek's dramatic improvement of the microscope. It was then that scholars discovered spermatozoa, bacteria, infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop the basic techniques of microscopic dissection and staining. [20]

Advances in microscopy also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the cell. Then, in 1838, Schleiden and Schwann began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory. [21] [22]

Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species. [23] Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Although he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought his work influenced the evolutionary theories of both Lamarck and Darwin. [24]

Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck, who was the first to present a coherent theory of evolution. [26] He posited that evolution was the result of environmental stress on properties of animals, meaning that the more frequently and rigorously an organ was used, the more complex and efficient it would become, thus adapting the animal to its environment. Lamarck believed that these acquired traits could then be passed on to the animal's offspring, who would further develop and perfect them. [27] However, it was the British naturalist Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell, Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, who forged a more successful evolutionary theory based on natural selection similar reasoning and evidence led Alfred Russel Wallace to independently reach the same conclusions. [28] [29] Darwin's theory of evolution by natural selection quickly spread through the scientific community and soon became a central axiom of the rapidly developing science of biology.

The basis for modern genetics began with the work of Gregor Mendel, who presented his paper, "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), in 1865, [30] which outlined the principles of biological inheritance, serving as the basis for modern genetics. [31] However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the modern synthesis reconciled Darwinian evolution with classical genetics. [32] In the 1940s and early 1950s, a series of experiments by Alfred Hershey and Martha Chase pointed to DNA as the component of chromosomes that held the trait-carrying units that had become known as genes. A focus on new kinds of model organisms such as viruses and bacteria, along with the discovery of the double-helical structure of DNA by James Watson and Francis Crick in 1953, marked the transition to the era of molecular genetics. From the 1950s to the present times, biology has been vastly extended in the molecular domain. The genetic code was cracked by Har Gobind Khorana, Robert W. Holley and Marshall Warren Nirenberg after DNA was understood to contain codons. Finally, the Human Genome Project was launched in 1990 with the goal of mapping the general human genome. This project was essentially completed in 2003, [33] with further analysis still being published. The Human Genome Project was the first step in a globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body and the bodies of other organisms.

Chemical basis

Atoms and molecules

All living organisms are made up of matter and all matter is made up of elements. [34] Oxygen, carbon, hydrogen, and nitrogen are the four elements that account for 96% of all living organisms, with calcium, phosphorus, sulfur, sodium, chlorine, and magnesium accounting for the remaining 3.7%. [34] Different elements can combine to form compounds such as water, which is fundamental to life. [34] Life on Earth began from water and remained there for about three billions years prior to migrating onto land. [35] Matter can exist in different states as a solid, liquid, or gas.

The smallest unit of an element is an atom, which is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Individual atoms can be held together by chemical bonds to form molecules and ionic compounds. [34] Common types of chemical bonds include ionic bonds, covalent bonds, and hydrogen bonds. Ionic bonding involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, [36] and is the primary interaction occurring in ionic compounds. Ions are atoms (or groups of atoms) with an electrostatic charge. Atoms that gain electrons make negatively charged ions (called anions) whereas those that lose electrons make positively charged ions (called cations).

Unlike ionic bonds, a covalent bond involves the sharing of electron pairs between atoms. These electron pairs and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. [37]

A hydrogen bond is primarily an electrostatic force of attraction between a hydrogen atom which is covalently bound to a more electronegative atom or group such as oxygen. A ubiquitous example of a hydrogen bond is found between water molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. Two molecules of water can form a hydrogen bond between them. When more molecules are present, as is the case with liquid water, more bonds are possible because the oxygen of one water molecule has two lone pairs of electrons, each of which can form a hydrogen bond with a hydrogen on another water molecule.

Organic compounds

With the exception of water, nearly all the molecules that make up each living organism contain carbon. [38] [39] Carbon can form very long chains of interconnecting carbon–carbon bonds, which are strong and stable. The simplest form of an organic molecule is the hydrocarbon, which is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other atoms. When combined with other elements such as oxygen, hydrogen, phosphorus, and sulfur, carbon can form many groups of important biological compounds such as sugars, fats, amino acids, and nucleotides.

Macromolecules

Molecules such as sugars, amino acids, and nucleotides can act as single repeating units called monomers to form chain-like molecules called polymers via a chemical process called condensation. [40] For example, amino acids can form polypeptides whereas nucleotides can form strands of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Polymers make up three of the four macromolecules (polysaccharides, lipids, proteins, and nucleic acids) that are found in all living organisms. Each macromolecule plays a specialized role within any given cell. Some polysaccharides, for instance, can function as storage material that can be hydrolyzed to provide cells with sugar. Lipids are the only class of macromolecules that are not made up of polymers and the most biologically important lipids are fats, phospholipids, and steroids. [40] Proteins are the most diverse of the macromolecules, which include enzymes, transport proteins, large signaling molecules, antibodies, and structural proteins. Finally, nucleic acids store, transmit, and express hereditary information. [40]

Cells

Cell theory states that cells are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through cell division. [41] Most cells are very small, with diameters ranging from 1 to 100 micrometers and are therefore only visible under a light or electron microscope. [42] There are generally two types of cells: eukaryotic cells, which contain a nucleus, and prokaryotic cells, which do not. Prokaryotes are single-celled organisms such as bacteria, whereas eukaryotes can be single-celled or multicellular. In multicellular organisms, every cell in the organism's body is derived ultimately from a single cell in a fertilized egg.

Cell structure

Every cell is enclosed within a cell membrane that separates its cytoplasm from the extracellular space. [43] A cell membrane consists of a lipid bilayer, including cholesterols that sit between phospholipids to maintain their fluidity at various temperatures. Cell membranes are semipermeable, allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as ions. [44] Cell membranes also contains membrane proteins, including integral membrane proteins that go across the membrane serving as membrane transporters, and peripheral proteins that loosely attach to the outer side of the cell membrane, acting as enzymes shaping the cell. [45] Cell membranes are involved in various cellular processes such as cell adhesion, storing electrical energy, and cell signalling and serve as the attachment surface for several extracellular structures such as a cell wall, glycocalyx, and cytoskeleton.

Within the cytoplasm of a cell, there are many biomolecules such as proteins and nucleic acids. [46] In addition to biomolecules, eukaryotic cells have specialized structures called organelles that have their own lipid bilayers or are spatially units. These organelles include the cell nucleus, which contains a cell's genetic information, or mitochondria, which generates adenosine triphosphate (ATP) to power cellular processes. Other organelles such as endoplasmic reticulum and Golgi apparatus play a role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by lysosomes, another specialized organelle. Plant cells have additional organelles that distinguish them from animal cells such as a cell wall, chloroplasts, and vacuole.

Metabolism

All cells require energy to sustain cellular processes. Energy is the capacity to do work, which, in thermodynamics, can be calculated using Gibbs free energy. According to the first law of thermodynamics, energy is conserved, i.e., cannot be created or destroyed. Hence, chemical reactions in a cell do not create new energy but are involved instead in the transformation and transfer of energy. [47] Nevertheless, all energy transfers lead to some loss of usable energy, which increases entropy (or state of disorder) as stated by the second law of thermodynamics. As a result, living organisms such as cells require continuous input of energy to maintain a low state of entropy. In cells, energy can be transferred as electrons during redox (reduction–oxidation) reactions, stored in covalent bonds, and generated by the movement of ions (e.g., hydrogen, sodium, potassium) across a membrane.

Metabolism is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as catabolic – the breaking down of compounds (for example, the breaking down of glucose to pyruvate by cellular respiration) or anabolic – the building up (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.

The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts – they allow a reaction to proceed more rapidly without being consumed by it – by reducing the amount of activation energy needed to convert reactants into products. Enzymes also allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.

Cellular respiration

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. [48] The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy because weak high-energy bonds, in particular in molecular oxygen, [49] are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow, controlled release of energy from the series of reactions.

Sugar in the form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: glycolysis, citric acid cycle (or Krebs cycle), electron transport chain, and oxidative phosphorylation. [50] Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates, with two net molecules of ATP being produced at the same time. [50] Each pyruvate is then oxidized into acetyl-CoA by the pyruvate dehydrogenase complex, which also generates NADH and carbon dioxide. Acetyl-Coa enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH2, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the mitochondrial cristae. Oxidative phosphorylation comprises the electron transport chain, which is a series of four protein complexes that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH2 that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane (chemiosmosis), which generates a proton motive force. [50] Energy from the proton motive force drives the enzyme ATP synthase to synthesize more ATPs by phosphorylating ADPs. The transfer of electrons terminates with molecular oxygen being the final electron acceptor.

If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD + so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD + for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD + regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD + attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate-level phosphorylation, which does not require oxygen.

Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. [51] [52] [53] In most cases, oxygen is also released as a waste product. Most plants, algae, and cyanobacteria perform photosynthesis, which is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth. [54]

Photosynthesis has four stages: Light absorption, electron transport, ATP synthesis, and carbon fixation. [50] Light absorption is the initial step of photosynthesis whereby light energy is absorbed by chlorophyll pigments attached to proteins in the thylakoid membranes. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a quinone designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP + , which is reduced to NADPH, a process that takes place in a protein complex called photosystem I (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration. [50]

During the third stage of photosynthesis, the movement of protons down their concentration gradients from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase. [50] The NADPH and ATPs generated by the light-dependent reactions in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle. [55]

Cell signaling

Cell communication (or signaling) is the ability of cells to receive, process, and transmit signals with its environment and with itself. [56] [57] Signals can be non-chemical such as light, electrical impulses, and heat, or chemical signals (or ligands) that interact with receptors, which can be found embedded in the cell membrane of another cell or located deep inside a cell. [58] [57] There are generally four types of chemical signals: autocrine, paracrine, juxtacrine, and hormones. [58] In autocrine signaling, the ligand affects the same cell that releases it. Tumor cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affect them. For example, brain cells called neurons release ligands called neurotransmitters that diffuse across a synaptic cleft to bind with a receptor on an adjacent cell such as another neuron or muscle cell. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through the circulatory systems of animals or vascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an inotropic receptor can alter the excitability of a target cell. Other types of receptors include protein kinase receptors (e.g., receptor for the hormone insulin) and G protein-coupled receptors. Activation of G protein-coupled receptors can initiate second messenger cascades. The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events is called signal transduction

Cell cycle

The cell cycle is a series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA and some of its organelles, and the subsequent partitioning of its cytoplasm into two daughter cells in a process called cell division. [59] In eukaryotes (i.e., animal, plant, fungal, and protist cells), there are two distinct types of cell division: mitosis and meiosis. [60] Mitosis is part of the cell cycle, in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often followed by telophase and cytokinesis which divides the cytoplasm, organelles and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells. [61] The cell cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed. After cell division, each of the daughter cells begin the interphase of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. [62] Homologous chromosomes are separated in the first division (meiosis I), and sister chromatids are separated in the second division (meiosis II). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.

Prokaryotes (i.e., archaea and bacteria) can also undergo cell division (or binary fission). Unlike the processes of mitosis and meiosis in eukaryotes, binary fission takes in prokaryotes takes place without the formation of a spindle apparatus on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by FtsZ polymerization and "Z-ring" formation) [63] The new cell wall (septum) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes, and plasmids.

Genetics

Inheritance

Genetics is the scientific study of inheritance. [64] [65] [66] Mendelian inheritance, specifically, is the process by which genes and traits are passed on from parents to offspring. [31] It was formulated by Gregor Mendel, based on his work with pea plants in the mid-nineteenth century. Mendel established several principles of inheritance. The first is that genetic characteristics, which are now called alleles, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on his law of dominance and uniformity, which states that some alleles are dominant while others are recessive an organism with at least one dominant allele will display the phenotype of that dominant allele. [67] Exceptions to this rule include penetrance and expressivity. [31] Mendel noted that during gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene, which is stated by his law of segregation. Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, Mendel formulated the law of independent assortment, which states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are sex-linked. Test crosses can be performed to experimentally determine the underlying genotype of an organism with a dominant phenotype. [68] A Punnett square can be used to predict the results of a test cross. The chromosome theory of inheritance, which states that genes are found on chromosomes, was supported by Thomas Morgans's experiments with fruit flies, which established the sex linkage between eye color and sex in these insects. [69] In humans and other mammals (e.g., dogs), it is not feasible or practical to conduct test cross experiments. Instead, pedigrees, which are genetic representations of family trees, [70] are used instead to trace the inheritance of a specific trait or disease through multiple generations. [71]

Deoxyribonucleic acid (DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic hereditary information. The two DNA strands are known as polynucleotides as they are composed of monomers called nucleotides. [72] [73] Each nucleotide is composed of one of four nitrogenous bases (cytosine [C], guanine [G], adenine [A] or thymine [T]), a sugar called deoxyribose, and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. It is the sequence of these four bases along the backbone that encodes genetic information. Bases of the two polynucleotide strands are bound together by hydrogen bonds, according to base pairing rules (A with T and C with G), to make double-stranded DNA. The bases are divided into two groups: pyrimidines and purines. In DNA, the pyrimidines are thymine and cytosine whereas the purines are adenine and guanine. The two strands of DNA run in opposite directions to each other and are thus antiparallel. DNA is replicated once the two strands separate.

A gene is a unit of heredity that corresponds to a region of DNA that influences the form or function of an organism in specific ways. DNA is found as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. A chromosome is an organized structure consisting of DNA and histones. The set of chromosomes in a cell and any other hereditary information found in the mitochondria, chloroplasts, or other locations is collectively known as a cell's genome. In eukaryotes, genomic DNA is localized in the cell nucleus, or with small amounts in mitochondria and chloroplasts. [74] In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. [75] The genetic information in a genome is held within genes, and the complete assemblage of this information in an organism is called its genotype. [76] Genes encode the information needed by cells for the synthesis of proteins, which in turn play a central role in influencing the final phenotype of the organism.

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The process is summarized in the central dogma of molecular biology first formulated by Francis Crick in 1958. [77] [78] [79] Gene expression is the most fundamental level at which a genotype gives rise to a phenotype, i.e., observable trait. The genetic information stored in DNA represents the genotype, whereas the phenotype results from the synthesis of proteins that control an organism's structure and development, or that act as enzymes catalyzing specific metabolic pathways. A large part of DNA (e.g., >98% in humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences. Messenger RNA (mRNA) strands are created using DNA strands as a template in a process called transcription, where DNA bases are exchanged for their corresponding bases except in the case of thymine (T), for which RNA substitutes uracil (U). [80] Under the genetic code, these mRNA strands specify the sequence of amino acids within proteins in a process called translation, which occurs in ribosomes. This process is used by all life—eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea), and utilized by viruses—to generate the macromolecular machinery for life. Gene products are often proteins, but in non-protein-coding genes such as transfer RNA (tRNA) and small nuclear RNA (snRNA), the product is a functional non-coding RNA. [81] [82] All steps in the gene expression process can be regulated, including the transcription, RNA splicing, translation, and post-translational modification of a protein. Regulation of gene expression gives control over the timing, location, and amount of a given gene product (protein or ncRNA) present in a cell and can have a profound effect on cellular structure and function.

Genomes

A genome is an organism's complete set of DNA, including all of its genes. [83] Sequencing and analysis of genomes can be done using high throughput DNA sequencing and bioinformatics to assemble and analyze the function and structure of entire genomes. [84] [85] [86] Many genes encode more than one protein, with posttranslational modifications increasing the diversity of proteins within a cell. A cell's proteome is its entire set of proteins expressed by its genome. [87] The genomes of prokaryotes are small, compact, and diverse. In contrast, the genomes of eukaryotes are larger and more complex such as having more regulatory sequences and much of its genome are made up of non-coding DNA sequences for functional RNA (rRNA, tRNA, and mRNA) or regulatory sequences. The genomes of various model organisms such as arabidopsis, fruit fly, mice, nematodes, and yeast have been sequenced. The sequencing of the entire human genome has yielded practical applications such as DNA fingerprinting, which can be used for paternity testing and forensics. In medicine, sequencing of the entire human genome has allowed for the identification mutations that cause tumors as well as genes that cause a specific genetic disorder. [87]

Biotechnology

Biotechnology is the use of cells or living organisms to develop products for humans. [88] It includes tools such as recombinant DNA, which are DNA molecules formed by laboratory methods of genetic recombination such as molecular cloning, which bring together genetic material from multiple sources, creating sequences that would otherwise not be found in a genome. Other tools include the use of genomic libraries, DNA microarrays, expression vectors, synthetic genomics, and CRISPR gene editing. [88] [89] Many of these tools have wide applications such as creating medically useful proteins, or improving plant cultivation and animal husbandry. [88] Human insulin, for example, was the first medicine to be made using recombinant DNA technology. Other approaches such as pharming can produce large quantities of medically useful products through the use of genetically modified organisms. [88]

Genes, development, and evolution

Development is the process by which a multicellular organism (plant or animal) goes through a series of a changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle. [90] There are four key processes that underlie development: Determination, differentiation, morphogenesis, and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells from less specialized cells such as stem cells. [91] [92] Stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. [93] Cellular differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals, which are largely due to highly controlled modifications in gene expression and epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. [94] Thus, different cells can have very different physical characteristics despite having the same genome. Morphogenesis, or development of body form, is the result of spatial differences in gene expression. [90] Specially, the organization of differentiated tissues into specific structures such as arms or wings, which is known as pattern formation, is governed by morphogens, signaling molecules that move from one group of cells to surrounding cells, creating a morphogen gradient as described by the French flag model. Apoptosis, or programmed cell death, also occurs during morphogenesis, such as the death of cells between digits in human embryonic development, which frees up individual fingers and toes. Expression of transcription factor genes can determine organ placement in a plant and a cascade of transcription factors themselves can establish body segmentation in a fruit fly. [90]

A small fraction of the genes in an organism's genome called the developmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved among phyla, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox genes. Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva. [95] Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's large ground-finch was enlarged by the BMP gene, [96] or when snakes lost their legs as Distal-less (Dlx) genes became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs. [97] Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, distal-less, which controls such diverse structures as the mandible in vertebrates, [98] [99] legs and antennae in the fruit fly, [100] and eyespot pattern in butterfly wings. [101] Given that small changes in toolbox genes can cause significant changes in body structures, they have often enabled convergent or parallel evolution.

Evolution

Evolutionary processes

A central organizing concept in biology is that life changes and develops through evolution, which is the change in heritable characteristics of populations over successive generations. [102] [103] Evolution is now used to explain the great variations of life on Earth. The term evolution was introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809, [104] and fifty years later Charles Darwin and Alfred Russel Wallace formulated the theory of evolution by natural selection. [105] [106] [107] [108] According to this theory, individuals differ from each other with respect to their heritable traits, resulting in different rates of survival and reproduction. As a results, traits that are better adapted to their environment are more likely to be passed on to subsequent generations. [109] [110] Darwin was not aware of Mendel's work of inheritance and so the exact mechanism of inheritance that underlie natural selection was not well-understood [111] until the early 20th century when the modern synthesis reconciled Darwinian evolution with classical genetics, which established a neo-Darwinian perspective of evolution by natural selection. [112] This perspective holds that evolution occurs when there are changes in the allele frequencies within a population of interbreeding organisms. In the absence of any evolutionary process acting on a large random mating population, the allele frequencies will remain constant across generations as described by the Hardy–Weinberg principle. [113]

Another process that drives evolution is genetic drift, which is the random fluctuations of allele frequencies within a population from one generation to the next. [114] When selective forces are absent or relatively weak, allele frequencies are equally likely to drift upward or downward at each successive generation because the alleles are subject to sampling error. [115] This drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone.

Speciation

Speciation is the process of splitting one lineage into two lineages that evolve independently from each other. [116] For speciation to occur, there has to be reproductive isolation. [116] Reproductive isolation can result from incompatibilities between genes as described by Bateson–Dobzhansky–Muller model. Reproductive isolation also tends to increase with genetic divergence. Speciation can occur when there are physical barriers that divide an ancestral species, a process known as allopatric speciation. [116] In contrast, sympatric speciation occurs in the absence of physical barriers.

Pre-zygotic isolation such as mechanical, temporal, behavioral, habitat, and gametic isolations can prevent different species from hybridizing. [116] Similarly, post-zygotic isolations can result in hybridization being selected against due to the lower viability of hybrids or hybrid infertility (e.g., mule). Hybrid zones can emerge if there were to be incomplete reproductive isolation between two closely related species.

Phylogenies

A phylogeny is an evolutionary history of a specific group of organisms or their genes. [117] A phylogeny can be represented using a phylogenetic tree, which is a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a lineage of descendents of a particular species or population. When a lineage divides into two, it is represented as a node (or split) on the phylogenetic tree. The more splits there are over time, the more branches there will be on the tree, with the common ancestor of all the organisms in that tree being represented by the root of that tree. Phylogenetic trees may portray the evolutionary history of all life forms, a major evolutionary group (e.g., insects), or an even smaller group of closely related species. Within a tree, any group of species designated by a name is a taxon (e.g., humans, primates, mammals, or vertebrates) and a taxon that consists of all its evolutionary descendants is a clade. Closely related species are referred to as sister species and closely related clades are sister clades.

Phylogenetic trees are the basis for comparing and grouping different species. [117] Different species that share a feature inherited from a common ancestor are described as having homologous features. Homologous features may be any heritable traits such as DNA sequence, protein structures, anatomical features, and behavior patterns. A vertebral column is an example of a homologous feature shared by all vertebrate animals. Traits that have a similar form or function but were not derived from a common ancestor are described as analogous features. Phylogenies can be reconstructed for a group of organisms of primary interests, which are called the ingroup. A species or group that is closely related to the ingroup but is phylogenetically outside of it is called the outgroup, which serves a reference point in the tree. The root of the tree is located between the ingroup and the outgroup. [117] When phylogenetic trees are reconstructed, multiple trees with different evolutionary histories can be generated. Based on the principle of Parsimony (or Occam's razor), the tree that is favored is the one with the fewest evolutionary changes needed to be assumed over all traits in all groups. Computational algorithms can be used to determine how a tree might have evolved given the evidence. [117]

Phylogeny provides the basis of biological classification, which is based on Linnaean taxonomy that was developed by Carl Linnaeus in the 18th century. [117] This classification system is rank-based, with the highest rank being the domain followed by kingdom, phylum, class, order, family, genus, and species. [117] All living organisms can be classified as belonging to one of three domains: Archaea (originally Archaebacteria) bacteria (originally eubacteria), or eukarya (includes the protist, fungi, plant, and animal kingdoms). [118] A binomial nomenclature is used to classify different species. Based on this system, each species is given two names, one for its genus and another for its species. [117] For example, humans are Homo sapiens, with Homo being the genus and sapiens being the species. By convention, the scientific names of organisms are italicized, with only the first letter of the genus capitalized. [119] [120]

History of life

The history of life on Earth traces the processes by which organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct, descended from a last universal common ancestor that lived about 3.5 billion years ago. [121] [122] The similarities among all known present-day species indicate that they have diverged through the process of evolution from their common ancestor. [123] Biologists regard the ubiquity of the genetic code as evidence of universal common descent for all bacteria, archaea, and eukaryotes. [124] [10] [125] [126]

Microbal mats of coexisting bacteria and archaea were the dominant form of life in the early Archean Epoch and many of the major steps in early evolution are thought to have taken place in this environment. [127] The earliest evidence of eukaryotes dates from 1.85 billion years ago, [128] [129] and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions. [130]

Algae-like multicellular land plants are dated back even to about 1 billion years ago, [131] although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 billion years ago. [132] Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event. [133]

Ediacara biota appear during the Ediacaran period, [134] while vertebrates, along with most other modern phyla originated about 525 million years ago during the Cambrian explosion. [135] During the Permian period, synapsids, including the ancestors of mammals, dominated the land, [136] but most of this group became extinct in the Permian–Triassic extinction event 252 million years ago. [137] During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates [138] one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. [139] After the Cretaceous–Paleogene extinction event 66 million years ago killed off the non-avian dinosaurs, [140] mammals increased rapidly in size and diversity. [141] Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. [142]

Diversity

Bacteria and Archaea

Bacteria are a type of cell that constitute a large domain of prokaryotic microorganisms. Typically a few micrometers in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, [143] and the deep biosphere of the earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory. [144]

Archaea constitute the other domain of prokaryotic cells and were initially classified as bacteria, receiving the name archaebacteria (in the Archaebacteria kingdom), a term that has fallen out of use. [145] Archaeal cells have unique properties separating them from the other two domains, Bacteria and Eukaryota. Archaea are further divided into multiple recognized phyla. Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat and square cells of Haloquadratum walsbyi. [146] Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably for the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, [147] including archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon, but unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding unlike bacteria, no known species of Archaea form endospores.

The first observed archaea were extremophiles, living in extreme environments, such as hot springs and salt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every habitat, including soil, oceans, and marshlands. Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet.

Archaea are a major part of Earth's life. They are part of the microbiota of all organisms. In the human microbiome, they are important in the gut, mouth, and on the skin. [148] Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation nitrogen cycling organic compound turnover and maintaining microbial symbiotic and syntrophic communities, for example. [149]

Protists

Protists are eukaryotic organism that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), [150] the exclusion of other eukaryotes means that protists do not form a natural group, or clade. [a] So some protists may be more closely related to animals, plants, or fungi than they are to other protists however, like algae, invertebrates, or protozoans, the grouping is used for convenience. [151]

The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on morphological (especially ultrastructural), [152] [153] [154] biochemical (chemotaxonomy) [155] [156] and DNA sequence (molecular research) information. [157] [158] Because protists as a whole are paraphyletic, new systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes.

Plant diversity

Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Botany is the study of plant life, which would exclude fungi and some algae. Botanists have studied approximately 410,000 species of land plants of which some 391,000 species are vascular plants (including approximately 369,000 species of flowering plants), [159] and approximately 20,000 are bryophytes. [160]

Algae is a large and diverse group of photosynthetic eukaryotic organisms. Included organisms range from unicellular microalgae, such as Chlorella, Prototheca and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem, which are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta.

Nonvascular plants are plants without a vascular system consisting of xylem and phloem. Instead, they may possess simpler tissues that have specialized functions for the internal transport of water. Vascular plants, on the other hand, are a large group of plants (c. 300,000 accepted known species) [161] that are defined as land plants with lignified tissues (the xylem) for conducting water and minerals throughout the plant. [162] They also have a specialized non-lignified tissue (the phloem) to conduct products of photosynthesis. Vascular plants include the clubmosses, horsetails, ferns, gymnosperms (including conifers) and angiosperms (flowering plants).

Seed plants (or spermatophyte) comprise five divisions, four of which are grouped as gymnosperms and one is angiosperms. Gymnosperms includes conifers, cycads, Ginkgo, and gnetophytes. Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form cones, or solitary as in yew, Torreya, Ginkgo. [163] Angiosperms are the most diverse group of land plants, with 64 orders, 416 families, approximately 13,000 known genera and 300,000 known species. [161] Like gymnosperms, angiosperms are seed-producing plants. They are distinguished from gymnosperms by having characteristics such as flowers, endosperm within their seeds, and production of fruits that contain the seeds.

Fungi

Fungi are eukaryotic organisms that include microorganisms such as yeasts and molds, as well as the more familiar mushrooms. A characteristic that places fungi in a different kingdom from plants, bacteria, and some protists is chitin in their cell walls. Fungi, like animals, are heterotrophs they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes into their environment. Fungi do not photosynthesize. Growth is their means of mobility, except for spores (a few of which are flagellated), which may travel through the air or water. Fungi are the principal decomposers in ecological systems. These and other differences place fungi in a single group of related organisms, named the Eumycota (true fungi or Eumycetes), which share a common ancestor (from a monophyletic group). This fungal group is distinct from the structurally similar myxomycetes (slime molds) and oomycetes (water molds).

Most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and also parasites. They may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment.

The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 2.2 million to 3.8 million species. [164] Of these, only about 148,000 have been described, [165] with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. [166]

Animal diversity

Animals are multicellular eukaryotic organisms that form the kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. They have complex interactions with each other and their environments, forming intricate food webs.

Sponges, the members of the phylum Porifera, are a basal Metazoa (animal) clade as a sister of the Diploblasts. [167] [168] [169] [170] [171] They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells.

97%) of animal species are invertebrates, [172] which are animals that neither possess nor develop a vertebral column (commonly known as a backbone or spine), derived from the notochord. This includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include arthropods (insects, arachnids, crustaceans, and myriapods), mollusks (chitons, snail, bivalves, squids, and octopuses), annelid (earthworms and leeches), and cnidarians (hydras, jellyfishes, sea anemones, and corals). Many invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata. [173]

In contrast, vertebrates comprise all species of animals within the subphylum Vertebrata (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. [174] Vertebrates include such groups as jawless fishes, jawed vertebrates such as cartilaginous fishes (sharks, rays, and ratfish), bony fishes, tetrapods such as amphibians, reptiles, birds and mammals.

Viruses

Viruses are submicroscopic infectious agents that replicate inside the living cells of organisms. [175] Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. [176] [177] More than 6,000 virus species have been described in detail. [178] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. [179] [180]

When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of the genetic material (DNA or RNA), a protein coat called capsid, and in some cases an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope, as they are one-hundredth the size of most bacteria.

The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. [181] Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life", [182] and as self-replicators. [183]

Viruses can spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids and viruses in animals can be carried by blood-sucking insects. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection.

Plant form and function

Plant body

The plant body is made up of organs that can be organized into two major organ systems: a root system and a shoot system. [184] The root system anchors the plants into place. The roots themselves absorb water and minerals and store photosynthetic products. The shoot system is composed of stem, leaves, and flowers. The stems hold and orient the leaves to the sun, which allow the leaves to conduct photosynthesis. The flowers are shoots that have been modified for reproduction. Shoots are composed of phytomers, which are functional units that consist of a node carrying one or more leaves, internode, and one or more buds.

A plant body has two basic patterns (apical–basal and radial axes) that been established during embryogenesis. [184] Cells and tissues are arranged along the apical-basal axis from root to shoot whereas the three tissue systems (dermal, ground, and vascular) that make up a plant's body are arranged concentrically around its radial axis. [184] The dermal tissue system forms the epidermis (or outer covering) of a plant, which is usually a single cell layer that consists of cells that have differentiated into three specialized structures: stomata for gas exchange in leaves, trichomes (or leaf hair) for protection against insects and solar radiation, and root hairs for increased surface areas and absorption of water and nutrients. The ground tissue makes up virtually all the tissue that lies between the dermal and vascular tissues in the shoots and roots. It consists of three cell types: Parenchyma, collenchyma, and sclerenchyma cells. Finally, the vascular tissues are made up of two constituent tissues: xylem and phloem. The xylem is made up two of conducting cells called tracheids and vessel elements whereas the phloem is characterized by the presence of sieve tube elements and companion cells. [184]

Plant nutrition and transport

Like all other organisms, plants are primarily made up of water and other molecules containing elements that are essential to life. [185] The absence of specific nutrients (or essential elements), many of which have been identified in hydroponic experiments, can disrupt plant growth and reproduction. The majority of plants are able to obtain these nutrients from solutions that surrounds their roots in the soil. [185] Continuous leaching and harvesting of crops can deplete the soil of its nutrients, which can be restored with the use of fertilizers. Carnivorous plants such as Venus flytraps are able to obtain nutrients by digesting other arthropods whereas parasitic plants such as mistletoes can parasitize other plants for water and nutrients.

Plants need water to conduct photosynthesis, transport solutes between organs, cool their leaves by evaporation, and maintain internal pressures that support their bodies. [185] Water is able to diffuse in and out of plant cells by osmosis. The direction of water movement across a semipermeable membrane is determined by the water potential across that membrane. [185] Water is able to diffuse across a root cell's membrane through aquaporins whereas solutes are transported across by the membrane by ion channels and pumps. In vascular plants, water and solutes are able to enter the xylem, a vascular tissue, by way of an apoplast and symplast. Once in the xylem, the water and minerals are distributed upward by transpiration from the soil to the aerial parts of the plant. [162] [185] In contrast, the phloem, another vascular tissue, distributes carbohydrates (e.g., sucrose) and other solutes such as hormones by translocation from a source (e.g., mature leaf or root) in which they were produced to a sink (e.g., root, flower, or developing fruit) in which they will be used and stored. [185] Sources and sinks can switch roles, depending on the amount of carbohydrates accumulated or mobilized for the nourishment of other organs.

Plant development

Plant development is regulated by environmental cues and the plant's own receptors, hormones, and genome. [186] Morever, they have several characteristics that allow them to obtain resources for growth and reproduction such as meristems, post-embryonic organ formation, and differential growth.

Development begins with a seed, which is an embryonic plant enclosed in a protective outer covering. Most plant seeds are usually dormant, a condition in which the seed's normal activity is suspended. [186] Seed dormancy may last may last weeks, months, years, and even centuries. Dormancy is broken once conditions are favorable for growth, and the seed will begin to sprout, a process called germination. Imbibition is the first step in germination, whereby water is absorbed by the seed. Once water is absorbed, the seed undergoes metabolic changes whereby enzymes are activated and RNA and proteins are synthesized. Once the seed germinates, it obtains carbohydrates, amino acids, and small lipids that serve as building blocks for its development. These monomers are obtained from the hydrolysis of starch, proteins, and lipids that are stored in either the cotyledons or endosperm. Germination is completed once embryonic roots called radicle have emerged from the seed coat. At this point, the developing plant is called a seedling and its growth is regulated by its own photoreceptor proteins and hormones. [186]

Unlike animals in which growth is determinate, i.e., ceases when the adult state is reached, plant growth is indeterminate as it is an open-ended process that could potentially be lifelong. [184] Plants grow in two ways: primary and secondary. In primary growth, the shoots and roots are formed and lengthened. The apical meristem produces the primary plant body, which can be found in all seed plants. During secondary growth, the thickness of the plant increases as the lateral meristem produces the secondary plant body, which can be found in woody eudicots such as trees and shrubs. Monocots do not go through secondary growth. [184] The plant body is generated by a hierarchy of meristems. The apical meristems in the root and shoot systems give rise to primary meristems (protoderm, ground meristem, and procambium), which in turn, give rise to the three tissue systems (dermal, ground, and vascular).

Plant reproduction

Most angiosperms (or flowering plants) engage in sexual reproduction. [187] Their flowers are organs that facilitate reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate two types of pollination: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination happened in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower’s stigma. This pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators. [188]

Plant responses

Like animals, plants produce hormones in one part of its body to signal cells in another part to respond. The ripening of fruit and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.

To function and survive, plants produce a wide array of chemical compounds not found in other organisms. Because they cannot move, plants must also defend themselves chemically from herbivores, pathogens and competition from other plants. They do this by producing toxins and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract pollinators or herbivores to spread ripe seeds.

Many plant organs contain different types of photoreceptor proteins, each of which reacts very specifically to certain wavelengths of light. [189] The photoreceptor proteins relay information such as whether it is day or night, duration of the day, intensity of light available, and the source of light. Shoots generally grow towards light, while roots grow away from it, responses known as phototropism and skototropism, respectively. They are brought about by light-sensitive pigments like phototropins and phytochromes and the plant hormone auxin. [190] Many flowering plants bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism.

In addition to light, plants can respond to other types of stimuli. For instance, plants can sense the direction of gravity to orient themselves correctly. They can respond to mechanical stimulation. [191]

Animal form and function

Principles

The cells in each animal body are bathed in interstitial fluid, which make up the cell's environment. This fluid and all its characteristics (e.g., temperature, ionic composition) can be described as the animal's internal environment, which is in contrast to the external environment that encompasses the animal's outside world. [192] Animals can be classified as either regulators or conformers. Animals such as mammals and birds are regulators as they are able to maintain a constant internal environment such as body temperature despite their environments changing. These animals are also described as homeotherms as they exhibit thermoregulation by keeping their internal body temperature constant. In contrast, animals such as fishes and frogs are conformers as they adapt their internal environment (e.g., body temperature) to match their external environments. These animals are also described as poikilotherms or ectotherms as they allow their body temperatures to match their external environments. In terms of energy, regulation is more costly than conformity as an animal expands more energy to maintain a constant internal environment such as increasing its basal metabolic rate, which is the rate of energy consumption. [192] Similarly, homeothermy is more costly than poikilothermy. Homeostasis is the stability of an animal's internal environment, which is maintained by negative feedback loops. [192] [193]

The body size of terrestrial animals vary across different species but their use of energy does not scale linearly according to their size. [192] Mice, for example, are able to consume three times more food than rabbits in proportion to their weights as the basal metabolic rate per unit weight in mice is greater than in rabbits. [192] Physical activity can also increase an animal's metabolic rate. When an animal runs, its metabolic rate increases linearly with speed. [192] However, the relationship is non-linear in animals that swim or fly. When a fish swims faster, it encounters greater water resistance and so its metabolic rates increases exponential. [192] Alternatively, the relationship of flight speeds and metabolic rates is U-shaped in birds. [192] At low flight speeds, a bird must maintain a high metabolic rates to remain airborne. As it speeds up its flight, its metabolic rate decreases with the aid of air rapidly flows over its wings. However, as it increases in its speed even further, its high metabolic rates rises again due to the increased effort associated with rapid flight speeds. Basal metabolic rates can be measured based on an animal's rate of heat production.

Water and salt balance

An animal's body fluids have three properties: osmotic pressure, ionic composition, and volume. [194] Osmotic pressures determine the direction of the diffusion of water (or osmosis), which moves from a region where osmotic pressure (total solute concentration) is low to a region where osmotic pressure (total solute concentration) is high. Aquatic animals are diverse with respect to their body fluid compositions and their environments. For example, most invertebrate animals in the ocean have body fluids that are isosmotic with seawater. In contrast, ocean bony fishes have body fluids that are hyposmotic to seawater. Finally, freshwater animals have body fluids that are hyperosmotic to fresh water. Typical ions that can be found in an animal's body fluids are sodium, potassium, calcium, and chloride. The volume of body fluids can be regulated by excretion. Vertebrate animals have kidneys, which are excretory organs made up of tiny tubular structures called nephrons, which make urine from blood plasma. The kidneys' primary function is to regulate the composition and volume of blood plasma by selectively removing material from the blood plasma itself. The ability of xeric animals such as kangaroo rats to minimize water loss by producing urine that is 10-20 times concentrated than their blood plasma allows them to adapt in desert environments that receive very little precipitation. [194]

Nutrition and digestion

Animals are heterotrophs as they feed on other living organisms to obtain energy and organic compounds. [195] They are able to obtain food in three major ways such as targeting visible food objects, collecting tiny food particles, or depending on microbes for critical food needs. The amount of energy stored in food can be quantified based on the amount of heat (measured in calories or kilojoules) emitted when the food is burnt in the presence of oxygen. If an animal were to consume food that contains an excess amount of chemical energy, it will store most of that energy in the form of lipids for future use and some of that energy as glycogen for more immediate use (e.g., meeting the brain's energy needs). [195] The molecules in food are chemical building blocks that are needed for growth and development. These molecules include nutrients such as carbohydrates, fats, and proteins. Vitamins and minerals (e.g., calcium, magnesium, sodium, and phosphorus) are also essential. The digestive system, which typically consist of a tubular tract that extends from the mouth to the anus, is involved in the breakdown (or digestion) of food into small molecules as it travels down peristaltically through the gut lumen shortly after it has been ingested. These small food molecules are then absorbed into the blood from the lumen, where they are then distributed to the rest of the body as building blocks (e.g., amino acids) or sources of energy (e.g., glucose). [195]

In addition to their digestive tracts, vertebrate animals have accessory glands such as a liver and pancreas as part of their digestive systems. [195] The processing of food in these animals begins in the foregut, which includes the mouth, esophagus, and stomach. Mechanical digestion of food starts in the mouth with the esophagus serving as a passageway for food to reach the stomach, where it is stored and disintegrated (by the stomach's acid) for further processing. Upon leaving the stomach, food enters into the midgut, which is the first part of the intestine (or small intestine in mammals) and is the principal site of digestion and absorption. Food that does not get absorbed are stored as indigestible waste (or feces) in the hindgut, which is the second part of the intestine (or large intestine in mammals). The hindgut then completes the reabsorption of needed water and salt prior to eliminating the feces from the rectum. [195]

Breathing

The respiratory system consists of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. [196] Gas exchange in the lungs occurs in millions of small air sacs in mammals and reptiles these are called alveoli, and in birds they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. [197] These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

Circulation

A circulatory system usually consists of a muscular pump such as a heart, a fluid (blood), and system of blood vessels that deliver it. [198] [199] Its principal function is to transport blood and other substances to and from cell (biology)s and tissues. There are two types of circulatory systems: open and closed. In open circulatory systems, blood exits blood vessels as it circulates throughout the body whereas in closed circulatory system, blood is contained within the blood vessels as it circulates. Open circulatory systems can be observed in invertebrate animals such as arthropods (e.g., insects, spiders, and lobsters) whereas closed circulatory systems can be found in vertebrate animals such as fishes, amphibians, and mammals. Circulation in animals occur between two types of tissues: systemic tissues and breathing (or pulmonary) organs. [198] Systemic tissues are all the tissues and organs that make up an animal's body other than its breathing organs. Systemic tissues take up oxygen but adds carbon dioxide to the blood whereas a breathing organs takes up carbon dioxide but add oxygen to the blood. [200] In birds and mammals, the systemic and pulmonary systems are connected in series.

In the circulatory system, blood is important because it is the means by which oxygen, carbon dioxide, nutrients, hormones, agents of immune system, heat, wastes, and other commodities are transported. [198] In annelids such as earthworms and leeches, blood is propelled by peristaltic waves of contractions of the heart muscles that make up the blood vessels. Other animals such as crustaceans (e.g., crayfish and lobsters), have more than one heart to propel blood throughout their bodies. Vertebrate hearts are multichambered and are able to pump blood when their ventricles contract at each cardiac cycle, which propels blood through the blood vessels. [198] Although vertebrate hearts are myogenic, their rate of contraction (or heart rate) can be modulated by neural input from the body's autonomic nervous system.

Muscle and movement

In vertebrates, the muscular system consists of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture and circulates blood throughout the body. [201] Together with the skeletal system, it forms the musculoskeletal system, which is responsible for the movement of vertebrate animals. [202] Skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the sliding filament theory. The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. Unlike skeletal muscles, contractions of smooth and cardiac muscles are myogenic as they are initiated by the smooth or heart muscle cells themselves instead of a motor neuron. Nevertheless, the strength of their contractions can be modulated by input from the autonomic nervous system. The mechanisms of contraction are similar in all three muscle tissues.

In invertebrates such as earthworms and leeches, circular and longitudinal muscles cells form the body wall of these animals and are responsible for their movement. [203] In an earthworm that is moving through a soil, for example, contractions of circular and longitudinal muscles occur reciprocally while the coelomic fluid serves as a hydroskeleton by maintaining turgidity of the earthworm. [204] Other animals such as mollusks, and nematodes, possess obliquely striated muscles, which contain bands of thick and thin filaments that are arranged helically rather than transversely, like in vertebrate skeletal or cardiac muscles. [205] Advanced insects such as wasps, flies, bees, and beetles possess asynchronous muscles that constitute the flight muscles in these animals. [205] These flight muscles are often called fibrillar muscles because they contain myofibrils that are thick and conspicuous. [206]

Nervous system

The nervous system is a network of cells that processes sensory information and generates behaviors. At the cellular level, the nervous system is defined by the presence of neurons, which are cells specialized to handle information. [208] They can transmit or receive information at sites of contacts called synapses. [208] More specifically, neurons can conduct nerve impulses (or action potentials) that travel along their thin fibers called axons, which can then be transmitted directly to a neighboring cell through electrical synapses or cause chemicals called neurotransmitters to be released at chemical synapses. According to the sodium theory, these action potentials can be generated by the increased permeability of the neuron's cell membrane to sodium ions. [209] Cells such as neurons or muscle cells may be excited or inhibited upon receiving a signal from another neuron. The connections between neurons can form neural pathways, neural circuits, and larger networks that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glia or glial cells, which provide structural and metabolic support.

Nervous systems are found in most multicellular animals, but vary greatly in complexity. [210] In vertebrates, the nervous system consists of the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which consists of nerves that connect the CNS to every other part of the body. Nerves that transmit signals from the CNS are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory nerves or afferent nerves. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit directly from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

Many animals have sense organs that can detect their environment. These sense organs contain sensory receptors, which are sensory neurons that convert stimuli into electrical signals. [211] Mechanoreceptors, for example, which can be found in skin, muscle, and hearing organs, generate action potentials in response to changes in pressures. [211] [212] Photoreceptor cells such as rods and cones, which are part of the vertebrate retina, can respond to specific wavelengths of light. [211] [212] Chemoreceptors detect chemicals in the mouth (taste) or in the air (smell). [212]

Hormonal control

Hormones are signaling molecules transported in the blood to distant organs to regulate their function. [213] [214] Hormones are secreted by internal glands that are part of an animal's endocrine system. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In humans specifically, the major endocrine glands are the thyroid gland and the adrenal glands. Many other organs that are part of other body systems have secondary endocrine functions, including bone, kidneys, liver, heart and gonads. For example, kidneys secrete the endocrine hormone erythropoietin. Hormones can be amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins. [215] The endocrine system can be contrasted to both exocrine glands, which secrete hormones to the outside of the body, and paracrine signaling between cells over a relatively short distance. Endocrine glands have no ducts, are vascular, and commonly have intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.

Animal reproduction

Animals can reproduce in one of two ways: asexual and sexual. Nearly all animals engage in some form of sexual reproduction. [216] They produce haploid gametes by meiosis. The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. [217] These fuse to form zygotes, [218] which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. [219] In most other groups, the blastula undergoes more complicated rearrangement. [220] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. [221] In most cases, a third germ layer, the mesoderm, also develops between them. [222] These germ layers then differentiate to form tissues and organs. [223] Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation budding, such as in Hydra and other cnidarians or parthenogenesis, where fertile eggs are produced without mating, such as in aphids. [224] [225]

Animal development

Animal development begins with the formation of a zygote that results from the fusion of a sperm and egg during fertilization. [226] The zygote undergoes a rapid multiple rounds of mitotic cell period of cell divisions called cleavage, which forms a ball of similar cells called a blastula. Gastrulation occurs, whereby morphogenetic movements convert the cell mass into a three germ layers that comprise the ectoderm, mesoderm and endoderm.

The end of gastrulation signals the beginning of organogenesis, whereby the three germ layers form the internal organs of the organism. [227] The cells of each of the three germ layers undergo differentiation, a process where less-specialized cells become more-specialized through the expression of a specific set of genes. Cellular differentiation is influenced by extracellular signals such as growth factors that are exchanged to adjacent cells, which is called juxtracrine signaling, or to neighboring cells over short distances, which is called paracrine signaling. [228] [229] Intracellular signals consist of a cell signaling itself (autocrine signaling), also play a role in organ formation. These signaling pathways allows for cell rearrangement and ensures that organs form at specific sites within the organism. [227] [230]

Immune system

The immune system is a network of biological processes that detects and responds to a wide variety of pathogens. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions.

Nearly all organisms have some kind of immune system. Bacteria have a rudimentary immune system in the form of enzymes that protect against virus infections. Other basic immune mechanisms evolved in ancient plants and animals and remain in their modern descendants. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms, including the ability to adapt to recognize pathogens more efficiently. Adaptive (or acquired) immunity creates an immunological memory leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination.

Animal behavior

Behaviors play a central a role in animals' interaction with each other and with their environment. [231] They are able to use their muscles to approach one another, vocalize, seek shelter, and migrate. An animal's nervous system activates and coordinates its behaviors. Fixed action patterns, for instance, are genetically determined and stereotyped behaviors that occur without learning. [231] [232] These behaviors are under the control of the nervous system and can be quite elaborate. [231] Examples include the pecking of kelp gull chicks at the red dot on their mother's beak. Other behaviors that have emerged as a result of natural selection include foraging, mating, and altruism. [233] In addition to evolved behavior, animals have evolved the ability to learn by modifying their behaviors as a result of early individual experiences. [231]

Ecology

Ecosystems

Ecology is the study of the distribution and abundance of living organisms, the interaction between them and their environment. [234] The community of living (biotic) organisms in conjunction with the nonliving (abiotic) components (e.g., water, light, radiation, temperature, humidity, atmosphere, acidity, and soil) of their environment is called an ecosystem. [235] [236] [237] These biotic and abiotic components are linked together through nutrient cycles and energy flows. [238] Energy from the sun enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes. [239]

The Earth's physical environment is shaped by solar energy and topography. [237] The amount of solar energy input varies in space and time due to the spherical shape of the Earth and its axial tilt. Variation in solar energy input drives weather and climate patterns. Weather is the day-to-day temperature and precipitation activity, whereas climate is the long-term average of weather, typically averaged over a period of 30 years. [240] [241] Variation in topography also produces environmental heterogeneity. On the windward side of a mountain, for example, air rises and cools, with water changing from gaseous to liquid or solid form, resulting in precipitation such as rain or snow. [237] As a result, wet environments allow for lush vegetation to grow. In contrast, conditions tend to be dry on the leeward side of a mountain due to the lack of precipitation as air descends and warms, and moisture remains as water vapor in the atmosphere. Temperature and precipitation are the main factors that shape terrestrial biomes.

Populations

A population is the number of organisms of the same species that occupy an area and reproduce from generation to generation. [242] [243] [244] [245] [246] Its abundance can be measured using population density, which is the number of individuals per unit area (e.g., land or tree) or volume (e.g., sea or air). [242] Given that it is usually impractical to count every individual within a large population to determine its size, population size can be estimated by multiplying population density by the area or volume. Population growth during short-term intervals can be determined using the population growth rate equation, which takes into consideration birth, death, and immigration rates. In the longer term, the exponential growth of a population tends to slow down as it reaches its carrying capacity, which can be modeled using the logistic equation. [243] The carrying capacity of an environment is the maximum population size of a species that can be sustained by that specific environment, given the food, habitat, water, and other resources that are available. [247] The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability resources and the cost of maintaining them. In human populations, new technologies such as the Green revolution have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the famous of which was by Thomas Malthus in the 18th century. [242]

Communities

A community is a group of populations of two or more different species occupying the same geographical area at the same time. A biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, like pollination and predation, or long-term both often strongly influence the evolution of the species involved. A long-term interaction is called a symbiosis. Symbioses range from mutualism, beneficial to both partners, to competition, harmful to both partners. [249]

Every species participates as a consumer, resource, or both in consumer–resource interactions, which form the core of food chains or food webs. [250] There are different trophic levels within any food web, with the lowest level being the primary producers (or autotrophs) such as plants and algae that convert energy and inorganic material into organic compounds, which can then be used by the rest of the community. [54] [251] [252] At the next level are the heterotrophs, which are the species that obtain energy by breaking apart organic compounds from other organisms. [250] Heterotrophs that consume plants are primary consumers (or herbivores) whereas heterotrophs that consume herbivores are secondary consumers (or carnivores). And those that eat secondary consumers are tertiary consumers and so on. Omnivorous heterotrophs are able to consume at multiple levels. Finally, there are decomposers that feed on the waste products or dead bodies of organisms. [250]

On average, the total amount of energy incorporated into the biomass of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level. [253]

Biosphere

In the global ecosystem (or biosphere), matter exist as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations. [255] For example, matter from terrestrial autotrophs are both biotic and accessible to other living organisms whereas the matter in rocks and minerals are abiotic and inaccessible to living organisms. A biogeochemical cycle is a pathway by which specific elements of matter are turned over or moved through the biotic (biosphere) and the abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. There are biogeochemical cycles for nitrogen, carbon, and water. In some cycles there are reservoirs where a substance remains or is sequestered for a long period of time.

Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale. [256] The largest driver of warming is the emission of greenhouse gases, of which more than 90% are carbon dioxide and methane. [257] Fossil fuel burning (coal, oil, and natural gas) for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and manufacturing. [258] Temperature rise is accelerated or tempered by climate feedbacks, such as loss of sunlight-reflecting snow and ice cover, increased water vapor (a greenhouse gas itself), and changes to land and ocean carbon sinks.

Conservation

Conservation biology is the study of the conservation of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. [259] [260] [261] It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engender genetic, population, species, and ecosystem diversity. [262] [263] [264] [265] The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years, [266] which has contributed to poverty, starvation, and will reset the course of evolution on this planet. [267] [268] Biodiversity affects the functioning of ecosystems, which provide a variety of services upon which people depend.

Conservation biologists research and educate on the trends of biodiversity loss, species extinctions, and the negative effect these are having on our capabilities to sustain the well-being of human society. Organizations and citizens are responding to the current biodiversity crisis through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales. [269] [262] [263] [264]


Questions multiply

Nevertheless, the development of technology with the potential to support sentient, disembodied organs has broad ethical implications for the welfare of animals and people. “There isn’t really an oversight mechanism in place for worrying about the possible ethical consequences of creating consciousness in something that isn’t a living animal,” says Stephen Latham, a bioethicist at Yale who worked with Sestan’s team. He says that doing so might be ethically justifiable in some cases — for instance, if it enable scientists to test drugs for degenerative brain diseases on the organs, rather than people.

Gauging awareness in a brain outside a body would probably be difficult, given that the organ’s surroundings would differ so radically from its natural environment. “We could imagine that brain could be capable of consciousness,” says George Mashour, a neuroscientist at the University of Michigan in Ann Arbor who studies near-death experiences. “But it’s very interesting to think about what kind of consciousness, in the absence of organs and peripheral stimulation.”

The latest study also raises questions about whether brain damage and death are permanent. Lance Becker, an emergency-medicine specialist at the Feinstein Institute for Medical Research in Manhasset, New York, says that many physicians assume that even minutes without oxygen can cause irreversible harm. But the pig experiments suggest that the brain might stay viable for much longer than previously thought, even without outside support. “This paper throws a hand grenade into the middle of what the common beliefs are,” says Becker. “We may have vastly underestimated the ability of the brain to recover.”

Part-revived pig brains raise slew of ethical quandaries

That could have practical and ethical consequences for organ donation. In some European countries, emergency responders who cannot resuscitate a person after a heart attack will sometimes use a system that preserves organs for transplantation by pumping oxygenated blood through the body — but not the brain. If a technology such as BrainEx becomes widely available, the ability to extend the window for resuscitation could shrink the pool of eligible organ donors, says Stuart Youngner, a bioethicist at Case Western Reserve University in Cleveland, Ohio.

“There’s a potential conflict here between the interests of potential donors — who might not even be donors — and people who are waiting for organs,” he adds.


A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

T he life sciences focus on patterns, processes, and relationships of living organisms. Life is self-contained, self-sustaining, self-replicating, and evolving, operating according to laws of the physical world, as well as genetic programming. Life scientists use observations, experiments, hypotheses, tests, models, theory, and technology to explore how life works. The study of life ranges over scales from single molecules, through organisms and ecosystems, to the entire biosphere, that is all life on Earth. It examines processes that occur on time scales from the blink of an eye to those that happen over billions of years. Living systems are interconnected and interacting. Although living organisms respond to the physical environment or geosphere, they have also fundamentally changed Earth over evolutionary time. Rapid advances in life sciences are helping to provide biological solutions to societal problems related to food, energy, health, and environment.

From viruses and bacteria to plants to fungi to animals, the diversity of the millions of life forms on Earth is astonishing. Without unifying principles, it would be difficult to make sense of the living world and apply those understandings to solving problems. A core principle of the life sciences is that all organisms are related by evolution and that evolutionary processes have led to the tremendous diversity of the biosphere. There is diversity within species as well as between species. Yet what is learned about the function of a gene or a cell or a process in one organism is relevant to other organisms because of their ecological interactions and evolutionary relatedness. Evolution and its underlying genetic

mechanisms of inheritance and variability are key to understanding both the unity and the diversity of life on Earth.

The committee developed four core ideas reflecting unifying principles in life sciences. These core ideas are essential for a conceptual understanding of the life sciences and will enable students to make sense of emerging research findings. We begin at the level of organisms, delving into the many processes and structures, at scales ranging from components as small as individual atoms to organ systems that are necessary for life to be sustained. Our focus then broadens to consider organisms in their environment&mdashhow they interact with the environment&rsquos living (biotic) and physical (abiotic) features. Next the chapter considers how organisms reproduce, passing genetic information to their offspring, and how these mechanisms lead to variability and hence diversity within species. Finally, the core ideas in the life sciences culminate with the principle that evolution can explain how the diversity that is observed within species has led to the diversity of life across species through a process of descent with adaptive modification. Evolution also accounts for the remarkable similarity of the fundamental characteristics of all species.

The first core idea, LS1: From Molecules to Organisms: Structures and Processes, addresses how individual organisms are configured and how these structures function to support life, growth, behavior, and reproduction. The first core idea hinges on the unifying principle that cells are the basic unit of life.

The second core idea, LS2: Ecosystems: Interactions, Energy, and Dynamics, explores organisms&rsquo interactions with each other and their physical environment. This includes how organisms obtain resources, how they change their environment, how changing environmental factors affect organisms and ecosystems, how social interactions and group behavior play out within and between species, and how these factors all combine to determine ecosystem functioning.

The third core idea, LS3: Heredity: Inheritance and Variation of Traits across generations, focuses on the flow of genetic information between generations. This idea explains the mechanisms of genetic inheritance and describes the environmental and genetic causes of gene mutation and the alteration of gene expression.

The fourth core idea, LS4: Biological Evolution: Unity and Diversity, explores &ldquochanges in the traits of populations of organisms over time&rdquo [1] and the factors that account for species&rsquo unity and diversity alike. The section

Evolution and its underlying genetic mechanisms of inheritance and variability are key to understanding both the unity and the diversity of life on Earth.

begins with a discussion of the converging evidence for shared ancestry that has emerged from a variety of sources (e.g., comparative anatomy and embryology, molecular biology and genetics). It describes how variation of genetically determined traits in a population may give some members a reproductive advantage in a given environment. This natural selection can lead to adaptation, that is, to a distribution of traits in the population that is matched to and can change with environmental conditions. Such adaptations can eventually lead to the development of separate species in separated populations. Finally, the idea describes the factors, including human activity, that affect biodiversity in an ecosystem, and the value of biodiversity in ecosystem resilience. See Box 6-1 for a summary of these four core ideas and their components.

These four core ideas, which represent basic life sciences fields of investigation&mdashstructures and processes in organisms, ecology, heredity, and evolution&mdashhave a long history and solid foundation based on the research evidence established by many scientists working across multiple fields. The role of unifying principles in advancing modern life sciences is articulated in The Role of Theory in Advancing 21st-Century Biology and A New Biology for the 21st Century [2, 3]. In developing these core ideas, the committee also drew on the established K-12 science education literature, including National Science Education Standards and Benchmarks for Science Literacy [4, 5]. The ideas also incorporate contemporary documents, such as the Science College Board Standards for College Success [6], and the ideas are consistent with frameworks for national and international assessments, such as those of the National Assessment of Educational Progress (NAEP), the Programme for International Student Assessment (PISA), and the Trends in International Mathematics and Science Study (TIMSS) [7-9]. Furthermore, the ideas align with the core concepts for biological literacy for undergraduates to build on as described in the American Association for the Advancement of Science (AAAS) report Vision and Change in Undergraduate Biology Education [10].

CORE AND COMPONENT IDEAS IN THE LIFE SCIENCES

Core Idea LS1: From Molecules to Organisms: Structures and Processes

LS1.A: Structure and Function

LS1.B: Growth and Development of Organisms

LS1.C: Organization for Matter and Energy Flow in Organisms

LS1.D: Information Processing

Core Idea LS2: Ecosystems: Interactions, Energy, and Dynamics

LS2.A: Interdependent Relationships in Ecosystems

LS2.B: Cycles of Matter and Energy Transfer in Ecosystems

LS2.C: Ecosystem Dynamics, Functioning, and Resilience

LS2.D: Social Interactions and Group Behavior

Core Idea LS3: Heredity: Inheritance and Variation of Traits

LS3.A: Inheritance of Traits

Core Idea LS4: Biological Evolution: Unity and Diversity

LS4.A: Evidence of Common Ancestry and Diversity

LS4.D: Biodiversity and Humans

From Molecules to Organisms: Structures and Processes

How do organisms live, grow, respond to their environment, and reproduce?

All living organisms are made of cells. Life is the quality that distinguishes living things&mdashcomposed of living cells&mdashfrom nonliving objects or those that have died. While a simple definition of life can be difficult to capture, all living things&mdashthat is to say all organisms&mdashcan be characterized by common aspects of their structure and functioning. Organisms are complex, organized, and built on a hierarchical structure, with each level providing the foundation for the next, from the chemical foundation of elements and atoms, to the cells and systems of individual organisms, to species and populations living and interacting in complex ecosystems. Organisms can be made of a single cell or millions of cells working together and include animals, plants, algae, fungi, bacteria, and all other microorganisms.

Organisms respond to stimuli from their environment and actively maintain their internal environment through homeostasis. They grow and reproduce, transferring their genetic information to their offspring. While individual organisms carry the same genetic information over their lifetime, mutation and the transfer from parent to offspring produce new combinations of genes. Over generations natural selection can lead to changes in a species overall hence, species evolve over time. To maintain all of these processes and functions, organisms require materials and energy from their environment nearly all energy that sustains life ultimately comes from the sun.

LS1.A: STRUCTURE AND FUNCTION

How do the structures of organisms enable life&rsquos functions?

A central feature of life is that organisms grow, reproduce, and die. They have characteristic structures (anatomy and morphology), functions (molecular-scale processes to organism-level physiology), and behaviors (neurobiology and, for some animal species, psychology). Organisms and their parts are made of cells, which are the structural units of life and which themselves have molecular substructures that support their functioning. Organisms range in composition from a single cell (unicellular microorganisms) to multicellular organisms, in which different groups of large numbers of cells work together to form systems

of tissues and organs (e.g., circulatory, respiratory, nervous, musculoskeletal), that are specialized for particular functions.

Special structures within cells are also responsible for specific cellular functions. The essential functions of a cell involve chemical reactions between many types of molecules, including water, proteins, carbohydrates, lipids, and nucleic acids. All cells contain genetic information, in the form of DNA. Genes are specific regions within the extremely large DNA molecules that form the chromosomes. Genes contain the instructions that code for the formation of molecules called proteins, which carry out most of the work of cells to perform the essential functions of life. That is, proteins provide structural components, serve as signaling devices, regulate cell activities, and determine the performance of cells through their enzymatic actions.

Grade Band Endpoints for LS1.A

By the end of grade 2. All organisms have external parts. Different animals use their body parts in different ways to see, hear, grasp objects, protect themselves, move from place to place, and seek, find, and take in food, water and air. Plants also have different parts (roots, stems, leaves, flowers, fruits) that help them survive, grow, and produce more plants.

By the end of grade 5. Plants and animals have both internal and external structures that serve various functions in growth, survival, behavior, and reproduction. (Boundary: Stress at this grade level is on understanding the macroscale systems and their function, not microscopic processes.)

By the end of grade 8. All living things are made up of cells, which is the smallest unit that can be said to be alive. An organism may consist of one single cell (unicellular) or many different numbers and types of cells (multicellular). Unicellular organisms (microorganisms), like multicellular organisms, need food, water, a way to dispose of waste, and an environment in which they can live.

Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell. In multicellular organisms, the body is a system of multiple interacting subsystems. These subsystems are groups of cells that work together to form tissues or organs that are specialized for particular body functions. (Boundary: At this grade level, only a few major cell structures should be introduced.)

By the end of grade 12. Systems of specialized cells within organisms help them perform the essential functions of life, which involve chemical reactions that take place between different types of molecules, such as water, proteins, carbohydrates, lipids, and nucleic acids. All cells contain genetic information in the form of DNA molecules. Genes are regions in the DNA that contain the instructions that code for the formation of proteins, which carry out most of the work of cells.

Multicellular organisms have a hierarchical structural organization, in which any one system is made up of numerous parts and is itself a component of the next level. Feedback mechanisms maintain a living system&rsquos internal conditions within certain limits and mediate behaviors, allowing it to remain alive and functional even as external conditions change within some range. Outside that range (e.g., at a too high or too low external temperature, with too little food or water available), the organism cannot survive. Feedback mechanisms can encourage (through positive feedback) or discourage (negative feedback) what is going on inside the living system.

LS1.B: GROWTH AND DEVELOPMENT OF ORGANISMS

How do organisms grow and develop?

The characteristic structures, functions, and behaviors of organisms change in predictable ways as they progress from birth to old age. For example, upon reaching adulthood, organisms can reproduce and transfer their genetic information to their offspring. Animals engage in behaviors that increase their chances for reproduction, and plants may develop specialized structures and/or depend on animal behavior to accomplish reproduction.

Understanding how a single cell can give rise to a complex, multicellular organism builds on the concepts of cell division and gene expression. In multi-cellular organisms, cell division is an essential component of growth, development, and repair. Cell division occurs via a process called mitosis: when a cell divides in two, it passes identical genetic material to two daughter cells. Successive divisions produce many cells. Although the genetic material in each of the cells is identical, small differences in the immediate environments activate or inactivate different genes, which can cause the cells to develop slightly differently. This process of differentiation allows the body to form specialized cells that perform diverse functions, even though they are all descended from a single cell, the fertilized egg. Cell growth and differentiation are the mechanisms by which a fertilized egg develops into a complex organism. In sexual reproduction, a specialized type of cell division

called meiosis occurs and results in the production of sex cells, such as gametes (sperm and eggs) or spores, which contain only one member from each chromosome pair in the parent cell.

Grade Band Endpoints for LS1.B

By the end of grade 2. Plants and animals have predictable characteristics at different stages of development. Plants and animals grow and change. Adult plants and animals can have young. In many kinds of animals, parents and the offspring themselves engage in behaviors that help the offspring to survive.

By the end of grade 5. Reproduction is essential to the continued existence of every kind of organism. Plants and animals have unique and diverse life cycles that include being born (sprouting in plants), growing, developing into adults, reproducing, and eventually dying.

By the end of grade 8. Organisms reproduce, either sexually or asexually, and transfer their genetic information to their offspring. Animals engage in characteristic behaviors that increase the odds of reproduction. Plants reproduce in a variety of ways, sometimes depending on animal behavior and specialized features (such as attractively colored flowers) for reproduction. Plant growth can continue throughout the plant&rsquos life through production of plant matter in photosynthesis. Genetic factors as well as local conditions affect the size of the adult plant. The growth of an animal is controlled by genetic factors, food intake, and interactions with other organisms, and each species has a typical adult size range. (Boundary: Reproduction is not treated in any detail here for more specifics about grade level, see LS3.A.)

By the end of grade 12. In multicellular organisms individual cells grow and then divide via a process called mitosis, thereby allowing the organism to grow. The organism begins as a single cell (fertilized egg) that divides successively to produce many cells, with each parent cell passing identical genetic material (two variants

of each chromosome pair) to both daughter cells. As successive subdivisions of an embryo&rsquos cells occur, programmed genetic instructions and small differences in their immediate environments activate or inactivate different genes, which cause the cells to develop differently&mdasha process called differentiation. Cellular division and differentiation produce and maintain a complex organism, composed of systems of tissues and organs that work together to meet the needs of the whole organism. In sexual reproduction, a specialized type of cell division called meiosis occurs that results in the production of sex cells, such as gametes in animals (sperm and eggs), which contain only one member from each chromosome pair in the parent cell.

LS1.C: ORGANIZATION FOR MATTER AND ENERGY FLOW IN ORGANISMS

How do organisms obtain and use the matter and energy they need to live and grow?

Sustaining life requires substantial energy and matter inputs. The complex structural organization of organisms accommodates the capture, transformation, transport, release, and elimination of the matter and energy needed to sustain them. As matter and energy flow through different organizational levels&mdashcells, tissues, organs, organisms, populations, communities, and ecosystems&mdashof living systems, chemical elements are recombined in different ways to form different products. The result of these chemical reactions is that energy is transferred from one system of interacting molecules to another.

In most cases, the energy needed for life is ultimately derived from the sun through photosynthesis (although in some ecologically important cases, energy is derived from reactions involving inorganic chemicals in the absence of sunlight&mdashe.g., chemosynthesis). Plants, algae (including phytoplankton), and other energy-fixing microorganisms use sunlight, water, and carbon dioxide to facilitate photosynthesis, which stores energy, forms plant matter, releases oxygen, and maintains plants&rsquo activities. Plants and algae&mdashbeing the resource base for animals, the animals that feed on animals, and the decomposers&mdashare energy-fixing organisms that sustain the rest of the food web.

Grade Band Endpoints for LS1.C

By the end of grade 2. All animals need food in order to live and grow. They obtain their food from plants or from other animals. Plants need water and light to live and grow.

By the end of grade 5. Animals and plants alike generally need to take in air and water, animals must take in food, and plants need light and minerals anaerobic life, such as bacteria in the gut, functions without air. Food provides animals with the materials they need for body repair and growth and is digested to release the energy they need to maintain body warmth and for motion. Plants acquire their material for growth chiefly from air and water and process matter they have formed to maintain their internal conditions (e.g., at night).

By the end of grade 8. Plants, algae (including phytoplankton), and many microorganisms use the energy from light to make sugars (food) from carbon dioxide from the atmosphere and water through the process of photosynthesis, which also releases oxygen. These sugars can be used immediately or stored for growth or later use. Animals obtain food from eating plants or eating other animals. Within individual organisms, food moves through a series of chemical reactions in which it is broken down and rearranged to form new molecules, to support growth, or to release energy. In most animals and plants, oxygen reacts with carbon-containing molecules (sugars) to provide energy and produce carbon dioxide anaerobic bacteria achieve their energy needs in other chemical processes that do not require oxygen.

By the end of grade 12. The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen. The sugar molecules thus formed contain carbon, hydrogen, and oxygen their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA), used for example to form new cells. As matter and energy flow through different organizational levels of living systems, chemical elements are recombined in different ways to form different products. As a result of these chemical reactions, energy is transferred from one system of interacting molecules to another. For example, aerobic (in the presence of oxygen) cellular respiration is a chemical process in which the bonds of food molecules and oxygen molecules are broken and new compounds are formed that can transport energy to muscles. Anaerobic (without oxygen) cellular respiration follows a different and less efficient chemical pathway to provide energy in cells. Cellular respiration also releases the energy needed to maintain body temperature despite ongoing energy loss to the surrounding environment. Matter and energy are conserved in each change. This is true of all biological systems, from individual cells to ecosystems.

LS1.D: INFORMATION PROCESSING

How do organisms detect, process, and use information about the environment?

An organism&rsquos ability to sense and respond to its environment enhances its chance of surviving and reproducing. Animals have external and internal sensory receptors that detect different kinds of information, and they use internal mechanisms for processing and storing it. Each receptor can respond to different inputs (electromagnetic, mechanical, chemical), some receptors respond by transmitting impulses that travel along nerve cells. In complex organisms, most such inputs travel to the brain, which is divided into several distinct regions and circuits that serve primary roles, in particular functions such as visual perception, auditory perception, interpretation of perceptual information, guidance of motor movement, and decision making. In addition, some of the brain&rsquos circuits give rise to emotions and store memories. Brain function also involves multiple interactions between the various regions to form an integrated sense of self and the surrounding world.

Grade Band Endpoints for LS1.D

By the end of grade 2. Animals have body parts that capture and convey different kinds of information needed for growth and survival&mdashfor example, eyes for light, ears for sounds, and skin for temperature or touch. Animals respond to these inputs with behaviors that help them survive (e.g., find food, run from a predator). Plants also respond to some external inputs (e.g., turn leaves toward the sun).

By the end of grade 5. Different sense receptors are specialized for particular kinds of information, which may then be processed and integrated by an animal&rsquos brain, with some information stored as memories. Animals are able to use their perceptions and memories to guide their actions. Some responses to information are instinctive&mdashthat is, animals&rsquo brains are organized so that they do not have to think about how to respond to certain stimuli.

By the end of grade 8. Each sense receptor responds to different inputs (electromagnetic, mechanical, chemical), transmitting them as signals that travel along nerve cells to the brain. The signals are then processed in the brain, resulting in immediate behaviors or memories. Changes in the structure and functioning of many millions of interconnected nerve cells allow combined inputs to be stored as memories for long periods of time.

By the end of grade 12. In complex animals, the brain is divided into several distinct regions and circuits, each of which primarily serves dedicated functions, such as visual perception, auditory perception, interpretation of perceptual information, guidance of motor movement, and decision making about actions to take in the event of certain inputs. In addition, some circuits give rise to emotions and memories that motivate organisms to seek rewards, avoid punishments, develop fears, or form attachments to members of their own species and, in some cases, to individuals of other species (e.g., mixed herds of mammals, mixed flocks of birds). The integrated functioning of all parts of the brain is important for successful interpretation of inputs and generation of behaviors in response to them.

Ecosystems: Interactions, Energy, and Dynamics

How and why do organisms interact with their environment and what are the effects of these interactions?

Ecosystems are complex, interactive systems that include both biological communities (biotic) and physical (abiotic) components of the environment. As with individual organisms, a hierarchal structure exists groups of the same organisms (species) form populations, different populations interact to form communities, communities live within an ecosystem, and all of the ecosystems on Earth make up the biosphere. Organisms grow, reproduce, and perpetuate their species by obtaining necessary resources through interdependent relationships with other organisms and the physical environment. These same interactions can facilitate or restrain growth and enhance or limit the size of populations, maintaining the balance between available resources and those who consume them. These interactions can also change both biotic and abiotic characteristics of the environment. Like individual organisms, ecosystems are sustained by the continuous flow of energy, originating primarily from the sun, and the recycling of matter and nutrients within the system. Ecosystems are dynamic, experiencing shifts in population composition and abundance and changes in the physical environment over time, which ultimately affects the stability and resilience of the entire system.

LS2.A: INTERDEPENDENT RELATIONSHIPS IN ECOSYSTEMS

How do organisms interact with the living and nonliving environments to obtain matter and energy?

Ecosystems are ever changing because of the interdependence of organisms of the same or different species and the nonliving (physical) elements of the environment. Seeking matter and energy resources to sustain life, organisms in an ecosystem interact with one another in complex feeding hierarchies of producers, consumers, and decomposers, which together represent a food web. Interactions between organisms may be predatory, competitive, or mutually beneficial. Ecosystems have carrying capacities that limit the number of organisms (within populations) they can support. Individual survival and population sizes depend on such factors as predation, disease, availability of resources, and parameters of the physical environment. Organisms rely on physical factors, such as light, temperature, water, soil, and space for shelter and reproduction. Earth&rsquos varied combinations of these factors provide the physical environments in which its ecosystems (e.g., deserts, grasslands, rain forests, and coral reefs) develop and in which the diverse species of the planet live. Within any one ecosystem, the biotic interactions between organisms (e.g., competition, predation, and various types of facilitation, such as pollination) further influence their growth, survival, and reproduction, both individually and in terms of their populations.

Grade Band Endpoints for LS2.A

By the end of grade 2. Animals depend on their surroundings to get what they need, including food, water, shelter, and a favorable temperature. Animals depend on plants or other animals for food. They use their senses to find food and water, and they use their body parts to gather, catch, eat, and chew the food. Plants depend on air, water, minerals (in the soil), and light to grow. Animals can move around, but plants cannot, and they often depend on animals for pollination or to move their seeds around. Different plants survive better in different settings because they have varied needs for water, minerals, and sunlight.

By the end of grade 5. The food of almost any kind of animal can be traced back to plants. Organisms are related in food webs in which some animals eat plants

for food and other animals eat the animals that eat plants. Either way, they are &ldquoconsumers.&rdquo Some organisms, such as fungi and bacteria, break down dead organisms (both plants or plants parts and animals) and therefore operate as &ldquodecomposers.&rdquo Decomposition eventually restores (recycles) some materials back to the soil for plants to use. Organisms can survive only in environments in which their particular needs are met. A healthy ecosystem is one in which multiple species of different types are each able to meet their needs in a relatively stable web of life. Newly introduced species can damage the balance of an ecosystem.

By the end of grade 8. Organisms and populations of organisms are dependent on their environmental interactions both with other living things and with nonliving factors. Growth of organisms and population increases are limited by access to resources. In any ecosystem, organisms and populations with similar requirements for food, water, oxygen, or other resources may compete with each other for limited resources, access to which consequently constrains their growth and reproduction. Similarly, predatory interactions may reduce the number of organisms or eliminate whole populations of organisms. Mutually beneficial interactions, in contrast, may become so interdependent that each organism requires the other for survival. Although the species involved in these competitive, predatory, and mutually beneficial interactions vary across ecosystems, the patterns of interactions of organisms with their environments, both living and nonliving, are shared.

By the end of grade 12. Ecosystems have carrying capacities, which are limits to the numbers of organisms and populations they can support. These limits result from such factors as the availability of living and nonliving resources and from such challenges as predation, competition, and disease. Organisms would have the capacity to produce populations of great size were it not for the fact that environments and resources are finite. This fundamental tension affects the abundance (number of individuals) of species in any given ecosystem.

LS2.B: CYCLES OF MATTER AND ENERGY TRANSFER IN ECOSYSTEMS

How do matter and energy move through an ecosystem?

The cycling of matter and the flow of energy within ecosystems occur through interactions among different organisms and between organisms and the physical environment. All living systems need matter and energy. Matter fuels the energy-releasing chemical reactions that provide energy for life functions and provides the

material for growth and repair of tissue. Energy from light is needed for plants because the chemical reaction that produces plant matter from air and water requires an energy input to occur. Animals acquire matter from food, that is, from plants or other animals. The chemical elements that make up the molecules of organisms pass through food webs and the environment and are combined and recombined in different ways. At each level in a food web, some matter provides energy for life functions, some is stored in newly made structures, and much is discarded to the surrounding environment. Only a small fraction of the matter consumed at one level is captured by the next level up. As matter cycles and energy flows through living systems and between living systems and the physical environment, matter and energy are conserved in each change.

The carbon cycle provides an example of matter cycling and energy flow in ecosystems. Photosynthesis, digestion of plant matter, respiration, and decomposition are important components of the carbon cycle, in which carbon is exchanged between the biosphere, atmosphere, oceans, and geosphere through chemical, physical, geological, and biological processes.

Grade Band Endpoints for LS2.B

By the end of grade 2. Organisms obtain the materials they need to grow and survive from the environment. Many of these materials come from organisms and are used again by other organisms.

By the end of grade 5. Matter cycles between the air and soil and among plants, animals, and microbes as these organisms live and die. Organisms obtain gases, water, and minerals from the environment and release waste matter (gas, liquid, or solid) back into the environment.

By the end of grade 8. Food webs are models that demonstrate how matter and energy is transferred between producers (generally plants and other organisms that engage in photosynthesis), consumers, and decomposers as the three groups interact&mdashprimarily for food&mdashwithin an ecosystem. Transfers of matter into and out of the physical environment occur at every level&mdashfor example, when molecules from food react with oxygen captured from the environment, the carbon dioxide and water thus produced are transferred back to the environment, and ultimately so are waste products, such as fecal material. Decomposers recycle nutrients from dead plant or animal matter back to the soil in terrestrial environments or to the water in aquatic environments. The atoms that make up the

Ecosystems are sustained by the continuous flow of energy, originating primarily from the sun, and the recycling of matter and nutrients within the system.

organisms in an ecosystem are cycled repeatedly between the living and nonliving parts of the ecosystem.

By the end of grade 12. Photosynthesis and cellular respiration (including anaerobic processes) provide most of the energy for life processes. Plants or algae form the lowest level of the food web. At each link upward in a food web, only a small fraction of the matter consumed at the lower level is transferred upward, to produce growth and release energy in cellular respiration at the higher level. Given this inefficiency, there are generally fewer organisms at higher levels of a food web, and there is a limit to the number of organisms that an ecosystem can sustain.

The chemical elements that make up the molecules of organisms pass through food webs and into and out of the atmosphere and soil and are combined and recombined in different ways. At each link in an ecosystem, matter and energy are conserved some matter reacts to release energy for life functions, some matter is stored in newly made structures, and much is discarded. Competition among species is ultimately competition for the matter and energy needed for life.

Photosynthesis and cellular respiration are important components of the carbon cycle, in which carbon is exchanged between the biosphere, atmosphere, oceans, and geosphere through chemical, physical, geological, and biological processes.

LS2.C: ECOSYSTEM DYNAMICS, FUNCTIONING, AND RESILIENCE

What happens to ecosystems when the environment changes?

Ecosystems are dynamic in nature their characteristics fluctuate over time, depending on changes in the environment and in the populations of various species. Disruptions in the physical and biological components of an ecosystem&mdashwhich can lead to shifts in the types and numbers of the ecosystem&rsquos organisms, to the maintenance or the extinction of species, to the migration of species into or out of the region, or to the formation of new species (speciation)&mdashoccur for a

variety of natural reasons. Changes may derive from the fall of canopy trees in a forest, for example, or from cataclysmic events, such as volcanic eruptions. But many changes are induced by human activity, such as resource extraction, adverse land use patterns, pollution, introduction of nonnative species, and global climate change. Extinction of species or evolution of new species may occur in response to significant ecosystem disruptions.

Species in an environment develop behavioral and physiological patterns that facilitate their survival under the prevailing conditions, but these patterns may be maladapted when conditions change or new species are introduced. Ecosystems with a wide variety of species&mdashthat is, greater biodiversity&mdashtend to be more resilient to change than those with few species.

Grade Band Endpoints for LS2.C

By the end of grade 2. The places where plants and animals live often change, sometimes slowly and sometimes rapidly. When animals and plants get too hot or too cold, they may die. If they cannot find enough food, water, or air, they may die.

By the end of grade 5. When the environment changes in ways that affect a place&rsquos physical characteristics, temperature, or availability of resources, some organisms survive and reproduce, others move to new locations, yet others move into the transformed environment, and some die.

By the end of grade 8. Ecosystems are dynamic in nature their characteristics can vary over time. Disruptions to any physical or biological component of an ecosystem can lead to shifts in all of its populations.

Biodiversity describes the variety of species found in Earth&rsquos terrestrial and oceanic ecosystems. The completeness or integrity of an ecosystem&rsquos biodiversity is often used as a measure of its health.


Tardigrades return from the dead

If you go into outer space without protection, you'll die.

The lack of pressure would force the air in your lungs to rush out. Gases dissolved in your body fluids would expand, pushing the skin apart and forcing it to inflate like a balloon. Your eardrums and capillaries would rupture, and your blood would start to bubble and boil. Even if you survived all that, ionising radiation would rip apart the DNA in your cells. Mercifully, you would be unconscious in 15 seconds.

How do these seemingly insignificant creatures survive in such extreme conditions?

But one group of animals can survive this: tiny creatures called tardigrades about 1mm long. In 2007, thousands of tardigrades were attached to a satellite and blasted into space. After the satellite had returned to Earth, scientists examined them and found that many of them had survived. Some of the females had even laid eggs in space, and the newly-hatched young were healthy.

It's not just the harsh environs of outer space that tardigrades can survive in. The little critters seem adept at living in some of the harshest regions of Earth. They have been discovered 5546m (18,196ft) up a mountain in the Himalayas, in Japanese hot springs, at the bottom of the ocean and in Antarctica. They can withstand huge amounts of radiation, being heated to 150 °C, and being frozen almost to absolute zero.

How do these seemingly insignificant creatures survive in such extreme conditions, and why have they evolved these superpowers? It turns out that tardigrades have a host of tricks up their sleeves, which would put most organisms to shame.

Tardigrades, at first glance, are intimidating. They have podgy faces with folds of flesh, a bit like a Doctor Who monster. They have eight legs, with ferocious claws resembling those of great bears. Their mouth is also a serious weapon, with dagger-like teeth that can spear prey.

Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago

But there's no need to worry. Tardigrades are one of nature's smallest animals. They are never more than 1.5 mm long, and can only be seen with a microscope. They are commonly known as "water bears".

There are 900 known species. Most feed by sucking the juices from moss, lichens and algae. Others are carnivores, and can even prey on other tardigrades.

They are truly ancient. Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago, when the first complex animals were evolving. And ever since they were discovered, it has been clear that they are special.

Tardigrades were discovered in 1773 by a German pastor named Johann August Ephraim Goeze. Three years later, the Italian clergyman and scientist Lazzaro Spallanzani discovered that they had superpowers.

Spallanzani added water to sediment from a rain gutter, and looked under a microscope. He found hundreds of little bear-shaped creatures swimming around. In his book "Opuscoli di Fisica Animale, e Vegetabile", he named them "il Tardigrado", meaning "slow-stepper", because they moved so slowly.

In 1995, dried tardigrades were brought back to life after 8 years

In truth, this wasn't a first. Back in 1702, the Dutch scientist Anton van Leeuwenhoek sent a letter to the Royal Society in London, entitled "On certain animalcules found in the sediment in gutters on the roofs of houses". He took dry, apparently lifeless dust from a gutter and added water. Using a microscope of his own devising, Leeuwenhoek found that within an hour many small "animalcules" became active, and began swimming and crawling around.

These animals were rotifers, tiny aquatic creatures that look like they have wheels on their heads. They could seemingly survive months without water.

However, tardigrades may be able to survive without it for decades. In 1948, the Italian zoologist Tina Franceschi claimed that tardigrades found in dried moss from museum samples over 120 years old could be reanimated. After rehydrating a tardigrade, she observed one of its front legs moving.

This finding has never been replicated. But it does not seem impossible. In 1995, dried tardigrades were brought back to life after 8 years.

For most animals, life without water is completely impossible.

"When a typical cell dries out its membranes rupture and leak, and its proteins unfold and aggregate together, making them useless," says extremophile researcher Thomas Boothby of the University of North Carolina in Chapel Hill. "DNA will also start to fragment the longer it is dry."

The tardigrade curls up into a dry husk

Somehow tardigrades avoid all this. "Since water bears can survive drying, they must have tricks for preventing or fixing the damage that cells like ours would die from," says Boothby.

How do they do it? One of the key discoveries came in 1922, courtesy of a German scientist named H. Baumann. He found that when a tardigrade dries out it retracts its head and its eight legs. It then enters a deep state of suspended animation that closely resembles death.

Shedding almost all the water in its body, the tardigrade curls up into a dry husk. Baumann called this a "Tönnchenform", but it is now commonly known as a "tun". Its metabolism slows to 0.01% of the normal rate. It can stay in this state for decades, only reanimating when it comes into contact with water.

Besides tardigrades, some nematode worms, yeast and bacteria can also survive desiccation. They do this by making a lot of a particular sugar called trehalose. This sugar forms a glass-like state inside their cells that stabilises key components, such as proteins and membranes, which would otherwise be destroyed.

Tardigrades might have unique tricks for surviving desiccation

Trehalose can also wrap itself around any remaining water molecules, stopping them from rapidly expanding if the temperature rises. Rapidly expanding water molecules are dangerous because they can rupture cells, which can be fatal.

You might expect that tardigrades would use this trick to survive drying, but according to Boothby, only some species seem to make trehalose. "Some species do not appear to contain trehalose, or make it at such low levels that the sugar is undetectable," he says.

"This suggests that tardigrades might have unique tricks for surviving desiccation," says Boothby. "We know that, as they start to dry out, tardigrades make protectants that allow them to survive becoming completely dry. But what exactly these protectants are is still a mystery."

When tardigrades start to dry out, they seem to make a lot of antioxidants. These are chemicals, like vitamins C and E, that soak up dangerously reactive chemicals. This may mop up harmful chemicals in the tardigrades' cells.

The tun state is key to tardigrades' ability to cope with being dried out

Tardigrades face a particular threat from "reactive oxygen species". These substances are produced as by-products of normal cell function, but can break down the main components of a cell, including its DNA. Animals exposed to environmental stress often have lots of them floating around.

The antioxidants may explain one of tardigrades' neatest abilities. If a tardigrade stays in its dry tun state for a long time, its DNA gets damaged. But after it reawakens it is able to quickly fix it.

It's clear that the tun state is key to tardigrades' ability to cope with being dried out. But long before Baumann discovered it, tardigrades had revealed other superpowers.

For starters, they seem not to care what temperature it is. In 1842 a French scientist named Doyère showed that a tardigrade in its tun state could survive being heated to temperatures of 125 °C for several minutes. In the 1920s, a Benedictine friar named Gilbert Franz Rahm brought tardigrades back to life after heating them to 151 °C for 15 minutes.

Rahm also tested them in the cold. He immersed them in liquid air at -200 °C for 21 months, in liquid nitrogen at -253 °C for 26 hours, and in liquid helium at -272 °C for 8 hours. Afterwards the tardigrades sprang back to life as soon as they came into contact with water.

We now know that some tardigrades can tolerate being frozen to -272.8 °C, just above absolute zero. To put that into perspective, the lowest temperature ever recorded on Earth was a balmy -89.2 °C in central Antarctica in 1983. The tardigrades coped with a profound chill that does not occur naturally and must be created in the lab, at which atoms come to a virtual standstill.

The biggest hazard tardigrades face in the cold is ice. If ice crystals form inside their cells, they can tear apart crucial molecules like DNA.

Tardigrades can actually tolerate ice forming within their cells

Some animals, including some fish, make antifreeze proteins that lower the freezing point of their cells, ensuring that ice doesn't form. But these proteins haven't been found in tardigrades.

Instead it seems tardigrades can actually tolerate ice forming within their cells. Either they can protect themselves from the damage caused by ice crystals, or they can repair it.

Tardigrades may produce chemicals called ice nucleating agents. These encourage ice crystals to form outside their cells rather than inside, protecting the vital molecules. Trehalose sugar may also protect those that produce it, as it prevents the formation of large ice crystals that would perforate the cell membranes.

But while we have some idea of how tardigrades cope with the cold, we have no idea how they cope with heat. At scorching temperatures like 150 °C, proteins and cell membranes should unravel, and the chemical reactions that sustain life cease to happen.

The most heat-tolerant organisms known are bacteria that live around the edges of hydrothermal vents in the deep sea. They can still grow at 122 °C. If Rahm is to be believed, tardigrades can survive even higher temperatures.

Many animals that have evolved to live in hot places, like hot springs and scorching deserts, produce chemicals called heat shock proteins. These act as chaperones for proteins inside cells, helping them keep their shape. They also repair heat-damaged proteins.

That's all well and good, but there is no conclusive evidence that tardigrades produce these chemicals. Factor in the other things they can survive, and the picture becomes even more baffling.

In 1964, scientists exposed tardigrades to lethal doses of X-rays and found that they could survive. Later experiments showed they can also cope with excessive amounts of alpha, gamma and ultraviolet radiation &ndash even if they're not in the tun state.

Radiation was one of the biggest threats facing the tardigrades sent into space in 2007. Those exposed to higher levels of radiation fared worse than those protected, but the mortality rate was not 100%.

They can also cope with extreme pressure that would squash most animals flat, according to a study published in 1998 by Kunihiro Seki and Masato Toyoshima of Kanagawa University in Hiratsuka, Japan. They found that tardigrades in the tun state could survive a pressure of 600 megapascals (MPa).

At these crushing pressures, proteins and DNA are ripped apart

This is beyond anything they might encounter in nature. The deepest part of the sea is the Challenger Deep in the Mariana Trench in the Pacific Ocean, which goes down 10,994 m. There, the water pressure is around 100 MPa. Somehow the tardigrades survived six times that.

At these crushing pressures, proteins and DNA are ripped apart. Cell membranes, which are composed of fat, become solid like butter in a fridge. Most microorganisms stop metabolising at 30 MPa, and bacteria can't survive much beyond 300 MPa.

The sheer variety of stressors that tardigrades can survive is almost dizzying. But maybe the explanation is surprisingly simple.

Extreme heat and cold, radiation and high pressures all have one thing in common: they damage DNA and other bits of the tardigrades' cells. Heat and cold both cause proteins to unfold, stick together and stop working. Radiation tears up DNA and other crucial molecules. High pressures solidify the fatty membranes around cells.

So if all the stressors cause similar problems, maybe the tardigrades only need a handful of tricks to survive them. "Nobody knows for sure," says Boothby. But "there are certainly some good reasons to think that overlapping strategies might be used to cope with some of these extremes."

Freezing a tardigrade and drying it out both cause the same problem

For instance, being dried out and being exposed to radiation both damage the tardigrades' DNA. "So it would make sense that tardigrades response to these two conditions in a similar way," says Boothby: by making antioxidants and repairing the damaged DNA.

If that's true, tardigrades' resistance to radiation is a happy accident: a side-effect of their adaptation to sudden drought. Similarly, freezing a tardigrade and drying it out both cause the same problem: not enough liquid water in the animal's cells.

Oddly enough the tun state, their most famous trick, is also the least versatile. "Tardigrades can survive freezing, radiation, and low-oxygen conditions without forming a tun," says Boothby. "So the tun state is probably a specific adaptation for dealing with or slowing water loss." However it does also allow them to survive extreme pressure.

This idea, that tardigrades are only using one or two survival tricks, might help explain the other big question about them: why do they bother?

They have evolved to cope with environments so extreme, they don't even exist on Earth

Unlike bacteria that live in boiling hot springs or other extreme sites, most tardigrades live in relatively unremarkable places. They tend to live in or near water, and there's nothing a tardigrade likes more than a good chunk of moss and lichen. Their lives aren't even that exciting: while most creatures their size dart about frantically, tardigrades are sluggish.

Yet despite their rather tedious lifestyle, they have evolved to cope with environments so extreme, they don't even exist on Earth.

Or rather, some of them have. The oldest and most primitive group of tardigrades, the Arthrotardigrada, cannot survive extreme conditions or suspend their metabolism. These more vulnerable creatures offer a clue to why the other tardigrades got so tough.

Arthrotardigrada only live in the ocean. It's only land-dwelling and fresh-water species that have the extreme survival skills. That suggests leaving the ocean was the key.

Today they can be found in some of the driest places on Earth

"One reason that marine tardigrades aren't as good at surviving extremes is that they just don't need to be," says Boothby. "Oceans are so big that they don't undergo rapid changes in temperature or salinity, and they certainly don't dry up overnight."

By contrast, the land is dangerously changeable. Tardigrades need a thin layer of water around their bodies to breath, eat, mate and move around. But in many parts of the land, drought is a risk. "The tardigrades that live in these places need to be able to cope when their environments suddenly change," says Boothby.

So it makes sense that land-dwelling tardigrades would evolve a way to survive suddenly being dried out. It was a matter of survival. What's more, once they had it, the land tardigrades could exploit new habitats. Today they can be found in some of the driest places on Earth, where other animals cannot survive.

But this idea just raises another question. If being able to survive drying out is so useful for land animals, why don't they all do it? Why didn't frogs, earthworms and humans evolve the same ability?

Similarly, why can't other animals survive the heat, cold and radiation that tardigrades can? Perhaps the question isn't why tardigrades are so tough, but why other animals are so vulnerable.

Going into the tun state is a risky decision

"There are probably several reasons why more animals and plants haven't evolved the tardigrades' abilities," says Boothby. "Many animals probably just don't need to. They either don't live in environments that can quickly dry out, or they can develop ways of avoiding drying out, like the camel."

But beyond that, there are surely costs to the tardigrades' abilities &ndash costs that other animals have avoided paying. In particular, going into the tun state is a risky decision.

"When a tardigrade completely dries out, it becomes inactive and is unable to actively avoid dangers in its surroundings," says Boothby. An inactive tardigrade might not die of thirst, but it could get eaten. "We know that a lot of desiccation-tolerant organisms have to make xenoprotectants: molecules that keep bacteria and fungi from basically eating them while they are in their inactive state."

It may be that becoming as tough as a tardigrade wouldn&rsquot pay off for other animals. But it has worked for them. They are 500 million years old and live all over the planet, so they aren't going anywhere.


Flatworm

It has been observed for centuries that if you cut a worm in half it will regenerate to become two worms. Biologist Thomas Hunt Morgan illustrated the regenerative powers of the planarian worm in a 1901 book but despaired of ever understanding regeneration.

How blown away he would have been by what’s available to scientists today.

“That’s what’s exciting,” Monaghan said. “We’re getting the molecular tools to ask what genes are regulating these regenerating phenomena.”

And planarians are still a star subject. In 2011, MIT researchers transplanted a special cell into a dying, irradiated planarian and the animal was able to fully regenerate. This year, researchers at the Max Planck Institute of Molecular Cell Biology and Genetics in Germany found a molecular switch in a flatworm that enabled it to grow a new head. Also this year, scientists at Tufts University showed that a decapitated planarian will not only regrow a new head, it will retain learned information as well as planarians who never lost their heads.

Guarantee: Next time you lose your keys, you’ll remember that a worm using a replacement head would probably have remembered where he/she put them.



Comments:

  1. Macgowan

    Bravo, what words ... wonderful thought

  2. Garran

    I agree with everything above per said.



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