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[Biochemistry](https://durgical.com/en/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

- [Electron transport chain and oxidative phosphorylation](https://durgical.com/en/biochemistry/pathways/electron-transport-chain-and-oxidative-phosphorylation/)

## Related

- [ATP synthase (complex V)](https://durgical.com/en/biochemistry/enzymes/atp-synthase/)
- [Complex I (NADH dehydrogenase)](https://durgical.com/en/biochemistry/enzymes/complex-i/)
- [Complex III (cytochrome bc₁)](https://durgical.com/en/biochemistry/enzymes/complex-iii/)
- [Cytochrome c oxidase (complex IV)](https://durgical.com/en/biochemistry/enzymes/cytochrome-c-oxidase/)
- [Uncouplers of oxidative phosphorylation](https://durgical.com/en/biochemistry/topics/uncouplers-of-oxidative-phosphorylation/)
- [UCP1 (thermogenin)](https://durgical.com/en/biochemistry/enzymes/ucp1/)
- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)

## 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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Formation of ATP by chemiosmotic coupling](https://reactome.org/PathwayBrowser/#/R-HSA-163210) (Reactome, 2026); [Respiratory electron transport](https://reactome.org/PathwayBrowser/#/R-HSA-611105) (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.

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