Heterotroph
From EvoWiki
A heterotroph or is an organism which gets its carbon from organic compounds, usually by consuming or breaking down other organisms. Many heterotrophs are also organotrophs, meaning they also get their energy from organic compounds. These include all animals and fungi and many bacteria and protists.
Interestingly, there are photosynthetic bacteria that are heterotrophs, notably the purple nonsulfur bacteria (Rhodospirillaceae).
Organisms have a wide range of degrees of heterotrophy. Many bacteria, like pseudomonads, can use any of a variety of organic compounds as their sole carbon source, while on the opposite extreme, intracellular symbionts and parasites are often so dependent on their host cells for nutrition that they are very difficult to grow in lab conditions.
The animal kingdom is somewhat intermediate, sharing the need to consume a variety of biomolecules, but still able to make many others. The "essential" amino acids, familiar from human nutrition as those that cannot be made by our bodies and therefore must be consumed, are a nutritional need across the animal kingdom. Likewise, the B vitamins are enzyme cofactors that must be consumed, and for the same reason.
However, many animals have workarounds that enable them to survive on diets that are short on EAA's and vitamins. A common one is gut symbionts, present in many land vertebrates and termites. Ruminants have extra stomachs before the usual vertebrate stomach, with the first one becoming a fermentation vat for the grass and leaves that they eat. A more common solution among land vertebrates, however, is making a pouch toward the rear of the gut for this purpose, the cecum. Also used are intracellular symbionts, like Buchnera bacteria in aphids. These insects consume plant sap, which is nutritionally very limited; the bacteria supply what the aphids themselves cannot. Sequencing the bacterium's genome reveals that though Buchnera is closely related to enteric bacteria like Escherichia coli, it only has biosynthesis enzymes for what its hosts cannot produce, like the "essential" amino acids.
The degree of heterotrophy is correlated with the type of nutrition. The weakest heterotrophy is found in organisms of decay, which live in soil, gut contents, decomposing dead organisms, etc.; these organisms cannot afford to be very choosy. It is stronger in organisms that consume other organisms, because those others can have ready-made biomolecules, though they are broken down into small ones before they get absorbed. And it is strongest in intracellular organisms, because they can acquire the full gamut of biomolecules from their hosts.
See also: autotroph, chemotroph, lithotroph, phototroph.
