Chemiosmosis, explained: how cells make ATP
The idea IB Biology HL students find hardest to put into words, in six steps, with the mistake the examiners keep flagging.
Chemiosmosis makes ATP from a proton gradient. In a mitochondrion, the electron transport chain uses energy from electrons to pump protons out of the matrix into the intermembrane space. Protons then flow back through ATP synthase, and that flow provides the energy to join ADP and phosphate. Always state which way the protons move.
Why this one is hard
Chemiosmosis asks you to hold a whole chain in your head: electrons, protons, a membrane, a gradient and an enzyme. It is also heavily examined. How cells make ATP was a 7-mark part of Paper 2 in May 2025 and again in May 2026, and respiration and photosynthesis together carry 71 marks across the 16 HL papers since May 2025 (our count of Papers 1B and 2).
The idea: a dam inside your cells
A dam holds water high up. When the water falls back through a turbine, the turbine spins and makes electricity. A mitochondrion does the same thing with protons. The electron transport chain pumps protons into a small space, the protons flow back through ATP synthase, and that flow makes ATP.
The six steps in a mitochondrion
- Reduced NAD hands electrons to the electron transport chain in the inner membrane.
- As the electrons pass along the chain, they release energy.
- That energy pumps protons out of the matrix and into the intermembrane space.
- Protons build up there. This is the proton gradient, and the membrane stops them leaking back.
- The way back is through ATP synthase. Protons flow down the gradient and back into the matrix, and that flow provides the energy to join ADP and phosphate into ATP. This step is chemiosmosis.
- At the end of the chain, oxygen accepts the electrons and protons from the matrix and forms water, so electrons keep flowing.
The same machine in a chloroplast
| Mitochondrion | Chloroplast | |
|---|---|---|
| Energy comes from | oxidising food | light |
| Electrons come from | reduced NAD | water, by photolysis at photosystem II |
| Protons pumped from | the matrix | the stroma |
| Protons pumped into | the intermembrane space | the thylakoid space |
| Protons flow back through | ATP synthase | ATP synthase |
| Electrons end up on | oxygen | NADP |
The two mistakes to stop making
Leaving out the direction. A May 2025 subject report flags answers where the direction of the protons is wrong or missing, both when the gradient forms and when ATP synthase uses it. Write where the protons start and where they end, both times: out of the matrix through the chain, back in through ATP synthase.
Saying energy is stored in bonds. The same report calls this a widespread misunderstanding. Breaking a bond takes energy and forming one releases it. Write that ATP hydrolysis releases energy, and that the proton flow provides the energy to make ATP.
Check yourself
Cyanide stops electrons moving along the chain. Explain why ATP production by chemiosmosis stops.
Show the answer
With no electrons moving, no energy is released to pump protons. The gradient runs down, protons stop flowing through ATP synthase, and nothing provides the energy to join ADP and phosphate.
Go through it with us, free
We run a free 45-minute live session on chemiosmosis: the steps, the two mistakes and one 7-mark answer written out live. SL students get the ATP basics first.
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Sources
Examiner evidence: May 2025 Biology subject reports, time zones 2 and 3, in our words. Mark counts: our own, from the HL papers since May 2025. Syllabus: guide statements C1.2.13 to C1.2.16 and C1.3.12. This page was developed independently from and is not endorsed by the International Baccalaureate Organization. Examples and wording are ours.