# Chemiosmosis

Protons pumped across a membrane build a gradient, then flow back through ATP synthase, a molecular turbine that makes ATP. The gradient is called the proton-motive force.

Biology (university) · Part 03: Respiration and photosynthesis · Card 5 of 8 · Last updated 2026-10-19

## Analogy

A dam: water pumped up behind it spins a turbine as it flows back.

## In practice

About 26 to 28 of the roughly 30 ATP from one glucose come from this step.

## Practice

### Flashcards

- **What drives ATP synthase?** Protons flowing back down their gradient across the inner mitochondrial membrane.

### Interview questions

**Outline the stages of aerobic respiration and say where each occurs and what it produces.**

- Glycolysis in cytoplasm: 2 ATP, 2 NADH
- Citric acid cycle in matrix: NADH, FADH2, CO2
- Electron transport and chemiosmosis in inner membrane: most ATP
- Oxygen is final electron acceptor

**Explain how a proton gradient makes ATP in mitochondria.**

- Electron flow pumps protons into intermembrane space
- Gradient stores potential energy
- Protons flow through ATP synthase
- Flow drives ADP plus phosphate into ATP

## Questions people ask

### Where does the ATP synthase get its energy?

From the flow of protons down their gradient. The enzyme works like a turbine: protons passing through make part of it rotate, and the rotation changes the shape of binding sites so ADP and phosphate are pushed together to form ATP. About 3 or 4 protons are needed per ATP.

### Can uncoupling the gradient be useful?

Yes. Brown fat cells in babies and hibernating animals contain a protein that lets protons leak back across the membrane without making ATP. The energy of the gradient is released as heat instead. This helps keep newborn mammals warm without shivering.

## Related concepts

- [Electron transport chain](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-4)
- [Glycolysis](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-2)
- [Citric acid cycle](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-3)
- [Fermentation](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-6)

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