Operational definition
Primary energy-source category.
The capacity for biological work supplied by electromagnetic radiation whose photons can be absorbed by a biological photochemical system. It becomes biologically usable when photon absorption produces molecular excitation, charge separation, electron transfer, or ion pumping.
Radiant free energy counts as a metabolic source only when it contributes to energy conservation or biosynthesis. Light used solely for vision, orientation, or signaling is photoreception, not phototrophy.
2.1 Photonic input
hν → molecular excitation
Light is converted into a chemically or electrochemically conserved state.
2.2 Reaction-center phototrophy
- oxygenic photosynthesis
- anoxygenic photosynthesis
- cyclic photophosphorylation
- noncyclic electron flow
2.3 Retinal-based phototrophy
- proton-pumping rhodopsins
- sodium-pumping rhodopsins
- chloride-pumping systems
- supplementary light-driven energy capture
The light-driven system may supply ATP, reducing power, or merely reduce the organism’s chemical-energy demand. Not all phototrophs fix carbon.