FAD

FAD oxidized form

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

heme reduced form