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1. Complex I also known as NADH dehydrogenase or NADH-
coenzyme Q oxidoreductase.

2. Complex II also known as succinate dehydrogenase or
succinate-coenzyme Q reductase.
Coenzyme Q (Ubiquinone)

3. Complex III also known as cytochrome bc1 complex or
Coenzyme Q-cytochrome C oxidoreductase.
Cytochrome C

4. Complex IV also known as cytochrome C oxidase.
ATP synthase

➢ Electrons are carried to Complex I aboard NADH.
➢ Complex II directly receives electrons from FADH2, which does not

pass through complex I.
➢ Both complex I and complex II pass their electrons to coenzyme Q

(ubiquinone) which deliver the electrons to complex III.
➢ Complex III passes electrons to cytochrome C for transport to Complex

IV.
➢ Complex IV passes the electrons to O2, which splits into two oxygen

atoms and accepts protons from the matrix to form water.
➢ The movement of the electrons cause pumping of protons building a

proton gradient across the inner mitochondrial membrane, with a higher
concentration of H+ in the intermembrane space and a lower
concentration in the matrix which leads to activation of ATP synthesis.
(Oxidative phosphorylation)
➢ All of the electrons that enter the transport chain come from NADH and
FADH2 molecules produced during earlier stages of cellular respiration:
glycolysis, pyruvate oxidation, and citric acid cycle.
➢ NADH is very good at donating electrons (that is, its electrons are at a
high energy level), so it can transfer its electrons directly to complex I,

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