This is a specialized access route into chemical/redox free energy, not a wholly separate physical form of energy.
Operational definition
Energy/electron-acquisition mode; revised title.
The transfer of electrons between a living system and an electron donor or acceptor located outside its ordinary soluble intracellular metabolic pool. The extracellular partner may be a mineral, electrode, conductive matrix, another organism, or extracellular redox compound.
It includes:
- direct contact through conductive proteins or structures;
- transfer through soluble redox mediators;
- indirect transfer through hydrogen, formate, or another intermediate generated at the interface.
“Direct extracellular electron access” would be too narrow because mediated transfer is also included in the intended category. Electrotrophy and extracellular respiration are specialized manifestations of extracellular electron transfer. (PubMed Central (PMC))
4.1 Direct electron uptake
Electrons enter from:
- an electrode
- conductive mineral
- metallic surface
- another cell
- a conductive biological matrix
4.2 Mediated electron uptake
Electrons arrive through:
- redox mediators
- hydrogen generated at a surface
- formate
- soluble electron shuttles
- extracellular enzymes
4.3 Extracellular electron export
Electrons leave the organism toward:
- insoluble minerals
- anodes
- conductive partners
- extracellular electron acceptors
Microorganisms that take up extracellular electrons are commonly termed electrotrophs, while extracellular electron transfer can also operate in the opposite direction toward external acceptors. (PubMed Central (PMC))