Biochemistry · Topic
Chemiosmosis and the proton gradient !
Chemiosmosis explains how the respiratory chain and ATP synthase are linked: through a proton gradient across the inner membrane.
Updated 2026-11-01
Key facts
- Peter Mitchell proposed it in 1961 (Nobel 1978): the coupling is not a direct chemical one but an H⁺ gradient.
- The proton-motive force has two parts: the electrical potential (about 150–180 mV, matrix negative) and the pH difference (matrix more alkaline).
- The inner membrane is impermeable to H⁺: they return only through ATP synthase or through uncouplers.
- Coupling: without ADP the gradient is not used and the chain slows; if the gradient is broken, the chain speeds up but no ATP is made.
In detail
- Complexes I, III and IV move H⁺ from the matrix into the intermembrane space as they pass electrons.
- A chemical gradient (more H⁺ outside) and an electrical one (more positive charge outside) build up.
- H⁺ flow back through the Fo channel of ATP synthase and spin it.
- The rotation drives ATP synthesis in the F1 head.
- Part of the gradient also exports ATP (ADP/ATP translocase), brings in phosphate and pyruvate and imports Ca²⁺.
- Also called
- Mitchell's chemiosmotic theory, Proton-motive force
Clinical notes
- Mitochondrial membrane potential is measured with dyes in research; its fall is an early sign of apoptosis.
- Chemical (2,4-dinitrophenol) and physiological (UCP1) uncouplers discharge the gradient and produce heat.
Pathways
Related
- ATP synthase (complex V)
- Complex I (NADH dehydrogenase)
- Complex III (cytochrome bc₁)
- Cytochrome c oxidase (complex IV)
- Uncouplers of oxidative phosphorylation
- UCP1 (thermogenin)
- ATP (adenosine triphosphate)
Test yourself
Think of the answer, then tap the question to see it.
Which two components make up the proton-motive force?
The electrical membrane potential (matrix negative) and the pH gradient (matrix alkaline).
What happens to O₂ consumption if oligomycin is added to respiring mitochondria?
It falls: the gradient cannot discharge through ATP synthase and slows the chain (respiratory control).
What happens to O₂ consumption if an uncoupler is then added?
It rises: H⁺ return another way, the chain runs freely and the energy leaves as heat, without ATP.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Formation of ATP by chemiosmotic coupling (Reactome, 2026); Respiratory electron transport (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.