reactants → lower-free-energy products
Operational definition
Primary energy-source category.
The capacity for useful work arising from differences in chemical potential between reactants and products. In living systems, it is most often accessed through oxidation–reduction reactions in which electrons move from a donor to an acceptor, producing lower-free-energy products.
Chemical free energy is not a material substance contained in a molecule. It is a property of a possible transformation under specified concentrations, temperature, pressure, pH, and redox conditions.
3.1 Organic or biogenic chemical substrates
Chemical free energy previously concentrated in biomass or biological products:
- carbohydrates
- lipids
- amino acids
- organic acids
- hydrocarbons
- methane
- alcohols
- dead organic matter
- host-derived compounds
- partner-derived metabolites
3.2 Inorganic or geochemical substrates
- molecular hydrogen
- reduced sulfur compounds
- ferrous iron
- ammonia
- nitrite
- carbon monoxide
- reduced minerals
- other inorganic electron donors
3.3 Mixed chemical substrate use
An organism uses combinations of organic and inorganic donors according to environmental conditions.
3.4 Chemical disproportionation
One compound or oxidation state is divided into both more oxidized and more reduced products, allowing the organism to obtain free energy without the simple donor–acceptor arrangement used in conventional respiration.