definition of a heterotroph hypothesis

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Heterotroph

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Heterotroph Definition

A heterotroph is an organism that cannot manufacture its own food by carbon fixation and therefore derives its intake of nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain , heterotrophs are secondary and tertiary consumers.

Carbon fixation is the process of converting inorganic carbon (CO 2 ) into organic compounds such as carbohydrates, usually by photosynthesis. Organisms, which can use carbon fixation to manufacture their own nutrition, are called autotrophs .

There are two forms of heterotroph. Photoheterotrophs use light for energy, although are unable to use carbon dioxide as their sole carbon source and, therefore, use organic compounds from their environment. Heliobacteria and certain proteobacteria are photoheterotrophs. Alternatively, chemoheterotrophs obtain their energy from ingesting preformed organic energy sources such as lipids, carbohydrates and proteins which have been synthesized by other organisms.

By consuming reduced carbon compounds , heterotrophs are able to use all the energy that they consume for growth, reproduction and other biological functions.

Examples of Heterotroph

Heterotrophs that eat plants to obtain their nutrition are called herbivores , or primary consumers .

During photosynthesis, complex organic molecules (carbon dioxide) are converted into energy (ATP) through cellular respiration . The ATP is often in the form of simple carbohydrates ( monosaccharaides ), such as glucose, and more complex carbohydrates ( polysaccharides ), such as starch and cellulose.

Starch is easily broken down by most animals, due to the presence of an enzyme secreted from the salivary glands and pancreas called amylase .

Cellulose, which is a major component of plant cell walls and an abundant carbohydrate, converted from inorganic carbon, is harder to digest for many animals. Most herbivores have a symbiotic gut organism, which breaks down the cellulose into a usable form of energy.

Examples of herbivores include cows, sheep, deer and other ruminant animals, which ferment plant material in special chambers containing the symbiotic organisms, within their stomachs. Animals that eat only fruit, such as birds, bats and monkeys, are also herbivores, although they are called frugivores . Most plant material consists mostly of hard-to-digest cellulose, although plant nectar consists of mostly simple sugars, and is eaten by herbivores called nectarivores , such as hummingbirds, bees, butterflies and moths

The energy that is transferred through the food chain, initially from the inorganic compounds, converted to organic compounds that are used as energy by autotrophs, is stored within the body of the heterotrophs called primary consumers.

The energy carnivores can use as energy mainly comes from lipids (fats) that the herbivore has stored within its body. Small amounts of glycogen (a polysaccharide of glucose which serves as form of long term energy storage) is stored within the liver and in the muscles and can be used for energy intake by carnivores, although the supply is not abundant.

Carnivores are usually predators, such as secondary consumers : heterotrophs which eat herbivores, such as snakes, birds and frogs (often insectivores ) and marine organisms which consume zooplankton such as small fish, crabs and jellyfish. They may also be tertiary consumers , predators that eat other carnivores, such as lions, hawks, sharks, and wolves.

Carnivores may also be scavengers , animals such as vultures or cockroaches, which eat animals which are already dead; often this is the carrion (meat) of animals that has been left over from the kill of a predator.

Fungi are heterotrophic organisms, although they do not ingest their food as other animals do, but feed by absorption . Fungi have root-like structures called hyphae , that grow and form a network through the substrate on which the fungi is feeding. These hyphae secrete digestive enzymes , which break down the substrate, making digestion of the nutrients possible.

Fungi feed on a variety of different substrates, such as wood, cheese or flesh, although most of them specialize on a restricted range of food sources; some fungi are highly specialized, and are only able to obtain nutrition from a single species.

Many fungi are parasitic , which means they feed on a host without killing it. Although, most fungi are saprobic , meaning they feed from already dead or decaying material, such as leaf litter, animal carcasses and other debris. The saprobic fungi recycle the nutrients from the dead or decaying material, which becomes available as nutrients for animals that eat fungi. The role of decomposers that fungi have as recyclers at all trophic levels of the nutrient cycle is extremely important within ecosystems, although they are also highly valuable to humans economically. Many fungi are responsible for production of human food, such as yeast (Saccharomyces cerevisiae), which is used to make bread, beer and cheese. Fungi are also used as medicines, such as penicillin.

Related Biology Terms

• Autotroph – Also known as ‘primary producers’, these are organisms that can fix inorganic carbon as an energy source; most plants are autotrophs.
• Energy pyramid – The flow of energy through a food chain can be visualized as a pyramid, as energy is lost throughout each level.
• Trophic Level – One of the hierarchal levels of a food chain in an ecosystem.
• Nutrient Cycle – The movement or exchange of inorganic and organic material in the production of living organisms.

1. Heterotrophs obtain their energy from: A. Other animals B. Environmental chemicals C. Light D. All of the above

2. Photosynthesizing algae is: A. Heterotrophic B. Autotrophic C. Chemotrophic D. None of the above

3. By which process is inorganic carbon (CO 2 ) converted into organic carbon (C)? A. Digestion B. Absorption C. Cellular respiration D. Herbivory

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Heterotrophic Hypothesis

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Prebiotic soup hypothesis

According to the heterotrophic hypothesis for the origin of life, early organisms depended on abiotically synthesized organic molecules for their structural components and as an energy source. The hypothesis is usually considered in connection with abiotic organic matter of atmospheric origin, but also consistent with extraterrestrial inputs ( meteorites , micrometeorites, comets ) or hydrothermal synthesis. The dilution of organic matter in the ocean is considered sufficient reason to rule out its relevance by many opponents.

In 1871, the mention of a “warm little pond” by Darwin may be considered as the first mention of this hypothesis. Subsequently it was more clearly formulated by Oparin ( 1924 ), Haldane ( 1929 ), and Urey ( 1952 ), ideas which ultimately led to the Miller experiment ( 1953 ).

Abiotic Photosynthesis

Atmosphere, Escape

Atmosphere, Organic Synthesis

Chemical Evolution

Darwin’s Conception of Origins of Life ...

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References and Further Reading

Haldane JBS 1929/1967. “The origin of life”. The Rationalist Annual. Reprinted as an appendix in J.D. Bernal 1967, The Origin of Life. Weidenfeld & Nicolson, London

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Miller SL (1953) Production of Amino Acids Under Possible Primitive Earth Conditions. Science 117(3046):528

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Oparin AI (1924) The Origin of Life. Moscow Worker, Moscow

Urey HC (1952) On the early chemical history of the earth and the Origin of life. Proc Natl Acad Sci USA 38:351–363

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Université de Montpellier II, Institut des Biomolécules Max Mousseron CC1706, Place E. Bataillon, 34095, Montpellier Cedex, France

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Astrophysicist, Laboratoire d’Astrophysique de Bordeaux, BP 89, 33270, Floirac, France

Muriel Gargaud

Departamento de Planetología y Habitabilidad Centro de Astrobiología (CSIC-INTA), Universidad Autónoma de Madrid Campus Cantoblanco, Torrejón de Ardoz, 28049, Madrid, Spain

Ricardo Amils

Department of Astrophysics, Centro de Astrobiología (INTA-CSIC) Ctra de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

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Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC, 20015, USA

Henderson James (Jim) Cleaves II

Department of Astronomy, University of Massachusetts Lederle Graduate Research, 710 North Pleasant Street, Amherst, MA, 01003-9305, USA

William M. Irvine

GEOTOP & Départment des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal, CP 8888, succ. Centre-Ville, Montréal, QC, H3C 3P8, Canada

Daniele L. Pinti

Astrobiology, CNES/DSP/EU, 2 place Maurice-Quentin, 75039, Paris, France

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Pascal, R. (2011). Heterotrophic Hypothesis. In: Gargaud, M., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-11274-4_721

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heterotrophic hypothesis

Learn about this topic in these articles:, oparin’s theories on origin of life.

…hypothesis, the earliest organisms were heterotrophic; i.e., they obtained their nutrition ready-made from compounds that had already been formed in variety and profusion by what are in the laboratory quite ordinary means. Thus, at that early stage, these first organisms did not need to synthesize their own food materials in…

Lesson: Chapter - 8

Origin of life: the heterotroph hypothesis.

Life on Earth began about 3.5 billion years ago. At that point in the development of the Earth, the atmosphere was very different from what it is today. As opposed to the current atmosphere, which is mostly nitrogen and oxygen, the early Earth atmosphere contained mostly hydrogen, water, ammonia, and methane.

In experiments, scientists have showed that the electrical discharges of lightning, radioactivity, and ultraviolet light caused the elements in the early Earth atmosphere to form the basic molecules of biological chemistry, such as nucleotides, simple proteins, and ATP. It seems likely, then, that the Earth was covered in a hot, thin soup of water and organic materials. Over time, the molecules became more complex and began to collaborate to run metabolic processes. Eventually, the first cells came into being. These cells were  heterotrophs , which could not produce their own food and instead fed on the organic material from the primordial soup. (These heterotrophs give this theory its name.)

Video Lesson - Heterotroph Hypothesis

The anaerobic metabolic processes of the heterotrophs released carbon dioxide into the atmosphere, which allowed for the evolution of photosynthetic autotrophs, which could use light and CO2 to produce their own food. The autotrophs released oxygen into the atmosphere. For most of the original anaerobic heterotrophs, oxygen proved poisonous. The few heterotrophs that survived the change in environment generally evolved the capacity to carry out aerobic respiration. Over the subsequent billions of years, the aerobic autotrophs and heterotrophs became the dominant life-forms on the planet and evolved into all of the diversity of life now visible on Earth.

Evidence of Evolution

Humankind has always wondered about its origins and the origins of the life around it. Many cultures have ancient creation myths that explain the origin of the Earth and its life. In Western cultures, ideas about evolution were originally based on the Bible. The book of Genesis relates how God created all life on Earth about 6,000 years ago in a mass creation event. Proponents of creationism support the Genesis account and state that species were created exactly as they are currently found in nature. This oldest formal conception of the origin of life still has proponents today.

However, about 200 years ago, scientific evidence began to cast doubt on creationism. This evidence comes in a variety of forms.

Rock and Fossil Formation

Fossils provide the only direct evidence of the history of evolution. Fossil formation occurs when sediment covers some material or fills an impression. Very gradually, heat and pressure harden the sediment and surrounding minerals replace it, creating fossils. Fossils of prehistoric life can be bones, shells, or teeth that are buried in rock, and they can also be traces of leaves or footprints left behind by organisms.

Together, fossils can be used to construct a fossil record that offers a timeline of fossils reaching back through history. To puzzle together the fossil record, scientists have to be able to date the fossils to a certain time period. The strata of rock in which fossils are found give clues about their relative ages. If two fossils are found in the same geographic location, but one is found in a layer of sediment that is beneath the other layer, it is likely that the fossil in the lower layer is from an earlier era. After all, the first layer of sediment had to already be on the ground in order for the second layer to begin to build up on top of it. In addition to sediment layers, new techniques such as radioactive decay or carbon dating can also help determine a fossil’s age.

There are, however, limitations to the information fossils can supply. First of all, fossilization is an improbable event. Most often, remains and other traces of organisms are crushed or consumed before they can be fossilized. Additionally, fossils can only form in areas with sedimentary rock, such as ocean floors. Organisms that live in these environments are therefore more likely to become fossils. Finally, erosion of exposed surfaces or geological movements such as earthquakes can destroy already formed fossils. All of these conditions lead to large and numerous gaps in the fossil record.

Comparative Anatomy

Scientists often try to determine the relatedness of two organisms by comparing external and internal structures. The study of comparative anatomy is an extension of the logical reasoning that organisms with similar structures must have acquired these traits from a common ancestor. For example, the flipper of a whale and a human arm seem to be quite different when looked at on the outside. But the bone structure of each is surprisingly similar, suggesting that whales and humans have a common ancestor way back in prehistory. Anatomical features in different species that point to a common ancestor are called homologous structures .

However, comparative anatomists cannot just assume that every similar structure points to a common evolutionary origin. A hasty and reckless comparative anatomist might assume that bats and insects share a common ancestor, since both have wings. But a closer look at the structure of the wings shows that there is very little in common between them besides their function. In fact, the bat wing is much closer in structure to the arm of a man and the fin of a whale than it is to the wings of an insect. In other words, bats and insects evolved their ability to fly along two very separate evolutionary paths. These sorts of structures, which have superficial similarities because of similarity of function but do not result from a common ancestor, are called analogous structures .

In addition to homologous and analogous structures, vestigial structures , which serve no apparent modern function, can help determine how an organism may have evolved over time. In humans the appendix is useless, but in cows and other mammalian herbivores a similar structure is used to digest cellulose. The existence of the appendix suggests that humans share a common evolutionary ancestry with other mammalian herbivores. The fact that the appendix now serves no purpose in humans demonstrates that humans and mammalian herbivores long ago diverged in their evolutionary paths.

Comparative Embryology

Homologous structures not present in adult organisms often do appear in some form during embryonic development. Species that bear little resemblance to each other in their adult forms may have strikingly similar embryonic stages. In some ways, it is almost as if the embryo passes through many evolutionary stages to produce the mature organism. For example, for a large portion of its development, the human embryo possesses a tail, much like those of our close primate relatives. This tail is usually reabsorbed before birth, but occasionally children are born with the ancestral structure intact. Even though they are not generally present in the adult organism, tails could be considered homologous traits between humans and primates.

In general, the more closely related two species are, the more their embryological processes of development resemble each other.

Molecular Evolution

Just as comparative anatomy is used to determine the anatomical relatedness of species, molecular biology can be used to determine evolutionary relationships at the molecular level. Two species that are closely related will have fewer genetic or protein differences between them than two species that are distantly related and split in evolutionary development long in the past.

Certain genes or proteins in organisms change at a constant rate over time. These genes and proteins, called molecular clocks because they are so constant in their rate of change, are especially useful in comparing the molecular evolution of different species. Scientists can use the rate of change in the gene or protein to calculate the point at which two species last shared a common ancestor. For example, ribosomal RNA has a very slow rate of change, so it is commonly used as a molecular clock to determine relationships between extremely ancient species. Cytochrome c, a protein that plays an important role in aerobic respiration, is an example of a protein commonly used as a molecular clock.

Theories of Evolution

In the nineteenth century, as increasing evidence suggested that species changed over time, scientists began to develop theories to explain how these changes arise. During this time, there were two notable theories of evolution. The first, proposed by Lamarck, turned out to be incorrect. The second, developed by Darwin, is the basis of all evolutionary theory.

Lamarck: Use and Disuse

The first notable theory of evolution was proposed by Jean-Baptiste Lamarck (1744–1829). He described a two-part mechanism by which evolutionary change was gradually introduced into the species and passed down through generations. His theory is referred to as the theory of transformation or  Lamarckism.

The classic example used to explain Lamarckism is the elongated neck of the giraffe. According to Lamarck’s theory, a given giraffe could, over a lifetime of straining to reach high branches, develop an elongated neck. This vividly illustrates Lamarck’s belief that use could amplify or enhance a trait. Similarly, he believed that disuse would cause a trait to become reduced. According to Lamarck’s theory, the wings of penguins, for example, were understandably smaller than the wings of other birds because penguins did not use their wings to fly.

The second part of Lamarck’s mechanism for evolution involved the inheritance of acquired traits . He believed that if an organism’s traits changed over the course of its lifetime, the organism would pass these traits along to its offspring.

Lamarck’s theory has been proven wrong in both of its basic premises. First, an organism cannot fundamentally change its structure through use or disuse. A giraffe’s neck will not become longer or shorter by stretching for leaves. Second, modern genetics shows that it is impossible to pass on acquired traits; the traits that an organism can pass on are determined by the genotype of its sex cells, which does not change according to changes in phenotype.

Darwin: Natural Selection

While sailing aboard the HMS Beagle, the Englishman Charles Darwin had the opportunity to study the wildlife of the Galápagos Islands. On the islands, he was amazed by the great diversity of life. Most particularly, he took interest in the islands’ various finches, whose beaks were all highly adapted to their particular lifestyles. He hypothesized that there must be some process that created such diversity and adaptation, and he spent much of his time trying to puzzle out just what the process might be. In 1859, he published his theory of natural selection and the evolution it produced. Darwin explained his theory through four basic points:

• Each species produces more offspring than can survive.
• The individual organisms that make up a larger population are born with certain variations.
• The overabundance of offspring creates a competition for survival among individual organisms. The individuals that have the most favorable variations will survive and reproduce, while those with less favorable variations are less likely to survive and reproduce.
• Variations are passed down from parent to offspring.

Natural selection creates change within a species through competition, or the struggle for life. Members of a species compete with each other and with other species for resources. In this competition, the individuals that are the most fit—the individuals that have certain variations that make them better adapted to their environments—are the most able to survive, reproduce, and pass their traits on to their offspring. The competition that Darwin’s theory describes is sometimes called the survival of the fittest.

Natural Selection in Action

One of the best examples of natural selection is a true story that took place in England around the turn of the century. Near an agricultural town lived a species of moth. The moth spent much of its time perched on the lichen-covered bark of trees of the area. Most of the moths were of a pepper color, though a few were black. When the pepper-color moths were attached to the lichen-covered bark of the trees in the region, it was quite difficult for predators to see them. The black moths were easy to spot against the black-and-white speckled trunks.

The nearby city, however, slowly became industrialized. Smokestacks and foundries in the town puffed out soot and smoke into the air. In a fairly short time, the soot settled on everything, including the trees, and killed much of the lichen. As a result, the appearance of the trees became nearly black in color. Suddenly the pepper-color moths were obvious against the dark tree trunks, while the black moths that had been easy to spot now blended in against the trees. Over the course of years, residents of the town noticed that the population of the moths changed. Whereas about 90 percent of the moths used to be light, after the trees became black, the moth population became increasingly black.

Development of New Species

The scientific definition of a species is a discrete group of organisms that can only breed within its own confines. In other words, the members of one species cannot interbreed with the members of another species. Each species is said to experience reproductive isolation. If you think about evolution in terms of genetics, this definition of species makes a great deal of sense: if species could interbreed, they could share gene flow, and their evolution would not be separate. But since species cannot interbreed, each species exists on its own individual path.

As populations change, new species evolve. This process is known as speciation . Through speciation, the earliest simple organisms were able to branch out and populate the world with millions of different species. Speciation is also called divergent evolution, since when a new species develops, it diverges from a previous form. All homologous traits are produced by divergent evolution. Whales and humans share a distant common ancestor. Through speciation, that ancestor underwent divergent evolution and gave rise to new species, which in turn gave rise to new species, which over the course of millions of years resulted in whales and humans. The original ancestor had a limb structure that, over millions of years and successive occurrences of divergent evolution, evolved into the fin of the whale and the arm of the human.

Speciation occurs when two populations become reproductively isolated. Once reproductive isolation occurs for a new species, it will begin to evolve independently. There are two main ways in which speciation might occur. Allopatric speciation occurs when populations of a species become geographically isolated so that they cannot interbreed. Over time, the populations may become genetically different in response to the unique selection pressures operating in their different environments. Eventually the genetic differences between the two populations will become so extreme that the two populations would be unable to interbreed even if the geographic barrier disappeared.

A second, more common form of speciation is adaptive radiation, which is the creation of several new species from a single parent species. Think of a population of a given species, which we’ll imaginatively name population 1. The population moves into a new habitat and establishes itself in a niche, or role, in the habitat (we discuss niches in more detail in the chapter on Ecology). In so doing, it adapts to its new environment and becomes different from the parent species. If a new population of the parent species, population 2, moves into the area, it too will try to occupy the same niche as population 1. Competition between population 1 and population 2 ensues, placing pressure on both groups to adapt to separate niches, further distinguishing them from each other and the parent species. As this happens many times in a given habitat, several new species may be formed from a single parent species in a relatively short time. The immense diversity of finches that Darwin observed on the Galápagos Islands is an excellent example of the products of adaptive radiation.

Convergent Evolution

When different species inhabit similar environments, they face similar selection pressures, or use parts of their bodies to perform similar functions. These similarities can cause the species to evolve similar traits, in a process called convergent evolution. From living in the cold, watery, arctic regions, where most of the food exists underwater, penguins and killer whales have evolved some similar characteristics: both are streamlined to help them swim more quickly underwater, both have layers of fat to keep them warm, both have similar white-and-black coloration that helps them to avoid detection, and both have developed fins (or flippers) to propel them through the water. All of these similar traits are examples of analogous traits, which are the product of convergent evolution.

Convergent evolution sounds as if it is the opposite of divergent evolution, but that isn’t actually true. Convergent evolution is only superficial. From the outside, the fin of a whale may look like the flipper of a penguin, but the bone structure of a whale fin is still more similar to the limbs of other mammals than it is to the structure of penguin flippers. More importantly, convergent evolution never results in two species gaining the ability to interbreed; convergent evolution can’t take two species and turn them into one.

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 7.

• Earth formation
• Beginnings of life
• Origins of life

Hypotheses about the origins of life

• The RNA origin of life
• Origins of life on Earth

Key points:

• The Earth formed roughly 4.5 ‍   billion years ago, and life probably began between 3.5 ‍   and 3.9 ‍   billion years ago.
• The Oparin-Haldane hypothesis suggests that life arose gradually from inorganic molecules, with “building blocks” like amino acids forming first and then combining to make complex polymers.
• The Miller-Urey experiment provided the first evidence that organic molecules needed for life could be formed from inorganic components.
• Some scientists support the RNA world hypothesis , which suggests that the first life was self-replicating RNA. Others favor the metabolism-first hypothesis , placing metabolic networks before DNA or RNA.
• Simple organic compounds might have come to early Earth on meteorites.

Introduction

When did life appear on earth, the earliest fossil evidence of life, how might life have arisen.

• Simple inorganic molecules could have reacted (with energy from lightning or the sun) to form building blocks like amino acids and nucleotides, which could have accumulated in the oceans, making a "primordial soup." 3 ‍  
• The building blocks could have combined in further reactions, forming larger, more complex molecules (polymers) like proteins and nucleic acids, perhaps in pools at the water's edge.
• The polymers could have assembled into units or structures that were capable of sustaining and replicating themselves. Oparin thought these might have been “colonies” of proteins clustered together to carry out metabolism, while Haldane suggested that macromolecules became enclosed in membranes to make cell-like structures 4 , 5 ‍   .

From inorganic compounds to building blocks

Were miller and urey's results meaningful, from building blocks to polymers, what was the nature of the earliest life, the "genes-first" hypothesis, the "metabolism-first" hypothesis, what might early cells have looked like, another possibility: organic molecules from outer space.

• Miller, Urey, and others showed that simple inorganic molecules could combine to form the organic building blocks required for life as we know it.
• Once formed, these building blocks could have come together to form polymers such as proteins or RNA.
• Many scientists favor the RNA world hypothesis, in which RNA, not DNA, was the first genetic molecule of life on Earth. Other ideas include the pre-RNA world hypothesis and the metabolism-first hypothesis.
• Organic compounds could have been delivered to early Earth by meteorites and other celestial objects.

Works cited:

• Harwood, R. (2012). Patterns in palaeontology: The first 3 billion years of evolution. Palaeontology , 2(11), 1-22. Retrieved from http://www.palaeontologyonline.com/articles/2012/patterns-in-palaeontology-the-first-3-billion-years-of-evolution/ .
• Wacey, D., Kilburn, M. R., Saunders, M., Cliff, J., and Brasier, M. D. (2011). Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience , 4 , 698-702. http://dx.doi.org/10.1038/ngeo1238 .
• Primordial soup. (2016, January 20). Retrieved May 22, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Primordial_soup .
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2.18: Autotrophs and Heterotrophs

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Photosynthetic autotrophs, which make food using the energy in sunlight, include (a) plants, (b) algae, and (c) certain bacteria.

Photosynthesis provides over 99 percent of the energy for life on earth. A much smaller group of autotrophs - mostly bacteria in dark or low-oxygen environments - produce food using the chemical energy stored in inorganic molecules such as hydrogen sulfide, ammonia, or methane. While photosynthesis transforms light energy to chemical energy, this alternate method of making food transfers chemical energy from inorganic to organic molecules. It is therefore called chemosynthesis , and is characteristic of the tubeworms shown in Figure below . Some of the most recently discovered chemosynthetic bacteria inhabit deep ocean hot water vents or “black smokers.” There, they use the energy in gases from the Earth’s interior to produce food for a variety of unique heterotrophs: giant tube worms, blind shrimp, giant white crabs, and armored snails. Some scientists think that chemosynthesis may support life below the surface of Mars, Jupiter's moon, Europa, and other planets as well. Ecosystems based on chemosynthesis may seem rare and exotic, but they too illustrate the absolute dependence of heterotrophs on autotrophs for food.

A food chain shows how energy and matter flow from producers to consumers. Matter is recycled, but energy must keep flowing into the system. Where does this energy come from? Though this food chains "ends" with decomposers, do decomposers, in fact, digest matter from each level of the food chain? (see the "Flow of Energy" concept.)

Tubeworms deep in the Galapagos Rift get their energy from chemosynthetic bacteria living within their tissues. No digestive systems needed!

Making and Using Food

The flow of energy through living organisms begins with photosynthesis. This process stores energy from sunlight in the chemical bonds of glucose. By breaking the chemical bonds in glucose, cells release the stored energy and make the ATP they need. The process in which glucose is broken down and ATP is made is called cellular respiration .

Photosynthesis and cellular respiration are like two sides of the same coin. This is apparent from Figure below . The products of one process are the reactants of the other. Together, the two processes store and release energy in living organisms. The two processes also work together to recycle oxygen in Earth’s atmosphere.