Cytochromes are critical participants in the electron transport chains used in photosynthesis and cellular respiration. How do cytochromes donate and accept electrons?
1) Every cytochrome's iron‑containing heme group accepts electrons from oxygen and donates the electrons to the next cytochrome in the chain.
2) The cytochromes donate electrons excited by photons to other cytochromes that accept electrons as replacements for lost electrons.
3) Cytochromes donate electrons directly to the energy‑carrier molecules and and accept electrons from less electronegative substances.
4) Each cytochrome has an iron‑containing heme group that accepts electrons and then donates the electrons to a more electronegative substance.

The cytochromes are proteins that contain heme prosthetic groups. Cytochromes undergo oxidation and reduction through loss or gain of a single electron by the iron atom in the heme of the cytochrome:

[tex]Cytochrome-Fe²⁺ ⇄ cytochrome-Fe³⁺-e⁻[/tex]

The reduced form of ubiquinone (QH₂), an extraordinarily mobile transporter, transfers electrons to cytochrome reductase, a complex that contains cytochromes b and c₁, and a Fe-S center. This second complex reduces cytochrome c, a water-soluble membrane peripheral protein. Cytochrome c, like ubiquinone (Q), is a mobile electron transporter, which is transferred to cytochrome oxidase. This third complex contains the cytochromes a, a₃ and two copper ions. Heme iron and a copper ion of this oxidase transfer electrons to O₂, as the last acceptor, to form water.

Each transporter "downstream" is more electronegative than its neighbor "upstream"; oxygen is located in the inferior part of the chain. Thus, the electrons fall in an energetic gradient in the electron chain transport to a more stable localization in the electronegative oxygen atom.