The proton gradient: Higher Level only: This statement is Higher Level only. It links the electron transport chain (3.2.13) to chemiosmosis (3.2.15).
Practise this as you read
- Explain how the proton gradient is generated.
- State the direction in which protons are pumped.
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The big idea: As electrons flow along the chain, the energy released is used by carrier proteins to pump protons (H⁺) from the matrix into the intermembrane space. Protons build up there, forming a proton gradient: a store of energy that ATP synthase will use.
The points to remember
- Electrons flow along the carriers, releasing energy at each step.
- The energy is used to pump protons across the inner membrane, by active transport.
- Protons go from the matrix into the intermembrane space.
- The intermembrane space is small, so the concentration there rises quickly.
- The proton gradient (high outside the inner membrane, low in the matrix) stores energy.
Remember it as: Electrons flow, protons go: a downhill flow powering an uphill pump.
Real example: a dam. Pumping protons is like pumping water up behind a dam: energy is stored in the height difference, ready to drive a turbine (ATP synthase) when the water flows back.
Trap: Protons are pumped into the intermembrane space, not into the matrix; they flow back into the matrix through ATP synthase.
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How it is asked: Expect an extended Explain on how the electron transport chain generates ATP, which needs the proton gradient step in the middle.
IB-style questionExplain[3 marks]
Explain how the flow of electrons along the electron transport chain produces a proton gradient.
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See the answer broken into clear points, with guidance on how each point earns marks.