electron donor

An electron donor passes an electron from one molecule to a second chemical compound, an electron acceptor. An electron donor is a reducing agent that is itself oxidized in the process of donating an electron

.............Donor –e^-→ Acceptor
...reducing agent –e^-→ oxidizing agent
..........oxidized –e^-→ reduced

Often the organic molecule that provides a source of electrons (donor molecule) will also serve as a source of cell carbon.

Oxidation Involves Loss of electrons, and Reduction Involves Gain of electrons (mnemonic 'oilrig').

Carbon dioxide is the most abundant form of carbon on earth and many microbes are capable, provided with enough ATP and NADH, of incorporating CO2 into cell carbon. This process is termed CO2 fixation. Organisms capable of fixing CO2 are classified as autotrophs and these include phototrophs and lithotrophs. The bacterium Pseudomonas cepacia is capable of growth on benzene (an organic molecule) alone, generating energy, via respiration and synthesizing all needed carbon molecules from it.

There are four pathways that are used for the fixation of CO2:
1. the Calvin cycle or ribulose bisphosphate pathway (RuBP),
2. the reverse tricarboxyclic acid cycle or reductive tricarboxylic acid pathway (rTCA),
3. the reductive acetyl CoA pathway (rACA),
4. the 3-Hydroxypropionate cycle.

Electron donors transfer electrons to electron acceptors during cellular respiration, resulting in the release of energy for utilization in metabolic pathways. Microorganisms, such as bacteria, obtain energy by transferring electrons from an electron donor to an electron acceptor, releasing energy for use by cellular machinery.