All living things undergo metabolism and may be classified according to three key conditions:
- The method by which they obtain their carbon
- The method by which they derive their energy
- The method by which they generate reducing equivalents
1. Carbon Source
Organisms can obtain carbon for synthesising organic molecules by own of three means:
- Autotrophic – Carbon is obtained from inorganic compounds (e.g. CO2)
- Heterotrophic – Carbon is obtained from organic compounds (e.g. sugars)
- Mixotrophic – Carbon may be obtained from either inorganic or organic sources
2. Energy Source
Organisms can derive the energy they need for living and growing by one of two means:
- Phototrophic – The energy is obtained from sunlight
- Chemotrophic – The energy is obtained from externally derived chemical compounds
3. Reducing Equivalents
Reducing equivalents are chemical species that can transfer electrons needed for energy conversions and biosynthesis
Organisms can obtain reducing equivalents from one of two sources:
- Lithotrophic – Electrons are derived from inorganic compounds (e.g. H2, NH3, CO, H2S, SO42-, Fe)
- Organotrophic – Electrons are derived from organic compounds (e.g. carbohydrates, proteins, lipids)
These terms can be combined to describe the organism – e.g. photolithoautotrophs (plants), chemoorganoheterotrophs (animals)
Types of Metabolism
