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An organism may be placed into one of the three major nutritional groups according to their carbon, energy, and electron sources.
- Carbon source refers to the source of carbon used by an organism for growth and development. An organism is defined as heterotrophic when using organic substrates to get its carbon for growth and development, whereas it is autotrophic, when its source of carbon is carbon dioxide(CO2).
- Reducing equivalent source refers to the necessity of deriving reducing equivalents (electrons) from environmental sources to be used in biosynthetic pathways (e. g. in the form of NADH or NADPH). An organism is defined as organotrophic when it uses organic compounds as source of electrons, whereas it is defined as lithotrophic when it inorganic compounds. Organotrophic organisms are often also heterotrophic, using organic compounds as sources of electrons and carbon at the same time. Similarly, lithotrophic organisms are often also autotrophic, using inorganic sources of electrons and (CO2) as inorganic carbon source.
- Energy source refers to the pathways used by the organism to produce ATP, which is required for fueling the anabolic pathways for biosynthesis of the constituents of the cell. An organism is defined phototrophic when it uses light as energy source, whereas it is chemotrophic when it conserves energy from reactions of chemical compounds.
The basis for energy metabolism of most chemotrophic organisms are oxidation-reduction reactions in which electrons move from an electron donor to an electron acceptor. Energy is released during the reaction. Therefore, compounds used as electron donors by chemotrophs must be diverted into both energy-yielding oxidative pathways and biosynthetic reductive pathways. The range of possible pairs of electron donors and acceptors for chemotrophs is limited to those whose reaction is exergonic enough to conserve enough energy for the transition of at least one proton over a membrane (equals to -15 to -20 kJ/mol). In contrast, phototrophs may use any electron donor and can even catalyse highly endergonic reactions (e. g. the photosynthetic production of starch from water and CO2).
It should be noted that the terms aerobic respiration, anaerobic respiration and fermentation are not referring to primary nutritional groups, but simply reflect the different use of possible electron acceptors in the energy metabolism of chemotrophic organisms, such as O2 (aerobic respiration), NO3-, SO42- (), or fumarate(anaerobic respiration), or intrinsic metabolic intermediates (fermentation).
Examples
All sort of combinations may exist in nature. For example a cyanobacteria is photolithotrophic, and fungi are chemo-organo-heterotrophic. Eucaryotes are generally easy to categorise. All animals are heterotrophic, as are fungi. Plants are photoautotrophic. However, some eucaryotic microorganisms are not limited to just one nutritional mode. For example, there are some algae that live photolithoautotrophically in the light, but shift to chemoorganoheterotophic life in the dark, and even higher plants retained their ability to respire heterotrophically on the starch at night, which has been synthesised phototrophically during the day.
On the contrary, procaryotes show a great diversity of nutritional categories. For example, purple S bacteria, or cyanobacteria are photolithoautotrophic, purple non-sulfur bacteria are photoorganoheterotrophic. Some bacteria are limited to only one nutritional group, whereas others are facultative and switch from one mode to the other, depending on the nutrient sources available. For example, archeobacteria are chemo-organo and/or chemo-litho autotrophic.
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