Electron Transport Chain: Redox Ladder

Electron Entry Point

Chain Inhibitor

Electron pair supply rate 1.2 /s

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H⁺ pumped0

ΔG released0.0 kJ/mol

P/O ratio (this path)2.5

How it works

Each redox carrier in the chain has a standard reduction potential E°′. Electrons always flow from a couple with a lower (more negative) E°′ toward one with a higher (more positive) E°′ — this is "downhill" in free energy, even though the potential number itself is rising. The free energy released per electron pair is:

ΔG°′ = −n·F·ΔE°′
n = 2 electrons transferred
F = 96.485 kJ·mol⁻¹·V⁻¹ (Faraday constant)

Approximate carrier potentials used in this model:

NADH/NAD⁺        E°′ = −0.32 V
Succinate/FAD    E°′ = +0.03 V  (Complex II entry)
CoQ/CoQH₂        E°′ = +0.05 V
Cytochrome c     E°′ = +0.25 V
O₂/H₂O           E°′ = +0.82 V

Full NADH → O₂ transit releases ΔE°′ = 1.14 V → ΔG°′ ≈ −220 kJ/mol, enough to pump ~10 H⁺ across the inner membrane (4 at Complex I, 4 at Complex III, 2 at Complex IV). FADH₂ enters directly at Complex II, skipping Complex I's pump entirely, so it releases only ≈ −152 kJ/mol and moves ~6 H⁺ — which is exactly why NADH yields a higher P/O ratio (~2.5 ATP) than FADH₂ (~1.5 ATP) once that gradient later drives ATP synthase.

Entry point — routes electrons through Complex I (full drop) or straight into Complex II (bypasses the biggest energy release).
Inhibitors — rotenone, antimycin A and cyanide each block one real complex; electrons pile up behind the block and the chain stops releasing free energy or pumping H⁺ past that point.
H⁺ pumped / ΔG released — running totals accumulated as electron pairs actually complete each segment of the ladder.

Electron pair Proton pumped (H⁺) Inhibited complex

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Electron Transport Chain: Redox Potential Ladder