FAD

FAD oxidized form


When FAD (oxidized form) is reduced, two H atoms are added directly to produce FADH2 (reduced form)
FAD reduced form

Heme


heme oxidized form


When heme groups (oxidized form) in cytochrome molecules are reduced, single electrons are added directly the central iron atom, converting Fe+++ (oxidized form) to Fe++(reduced form). Heme groups do not accept protons.

heme reduced form