OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
Photosynthesis: (d) the light-dependent stage of photosynthesis
The light-dependent stage occurs on the thylakoid membranes of chloroplasts.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 5 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The light-dependent stage occurs on the thylakoid membranes of chloroplasts. Light absorbed by photosystems excites electrons in chlorophyll, which are passed along an electron transport chain, generating ATP by chemiosmosis as protons are pumped into the thylakoid space and flow back through ATP synthase. Water is split by photolysis, releasing electrons to replace those lost, protons and oxygen as a by-product. NADP is reduced to NADPH. The ATP and NADPH produced are used in the light-independent stage. Learners should link each step to its location and to the fate of the products.
What earns the marks
- The light-dependent stage takes place on the thylakoid membranes of chloroplasts.
- Light excites electrons in chlorophyll in the photosystems, and these electrons pass along an electron transport chain.
- Electron transport pumps protons into the thylakoid space, and their flow back through ATP synthase produces ATP by chemiosmosis.
- Photolysis of water supplies electrons to replace those lost, releasing protons and oxygen.
- NADP is reduced to NADPH, and ATP and NADPH are passed to the light-independent stage.
Common mistakes and how to put them right
Placing the light-dependent stage in the stroma.
Put it right: It occurs on the thylakoid membranes; the stroma hosts the light-independent stage.
Saying oxygen is used rather than released.
Put it right: Oxygen is a by-product of photolysis of water.
Confusing ATP production with substrate-level phosphorylation only.
Put it right: ATP is made by chemiosmosis via ATP synthase.
Forgetting that water supplies replacement electrons.
Put it right: Photolysis replaces electrons lost from chlorophyll.
Exam tips
- Name the location: thylakoid membranes.
- Sequence the steps: light absorption, electron excitation, electron transport, proton pumping, chemiosmosis, photolysis, NADP reduction.
- State the products and their destinations: ATP and NADPH to the light-independent stage, oxygen released.
- Use precise terms: chemiosmosis, photolysis, electron transport chain, ATP synthase.
Try the lesson
The start of the Photosynthesis: (d) the light-dependent stage of photosynthesis lesson, made from checked teaching notes for this specification point.
Have a go first
In the light-dependent stage of photosynthesis, which process produces ATP?
Listen
The light-dependent stage occurs on the thylakoid membranes of chloroplasts. Light absorbed by photosystems excites electrons in chlorophyll, which are passed along an electron transport chain, generating ATP by chemiosmosis as protons are pumped into the thylakoid space and flow back through ATP synthase. Water is split by photolysis, releasing electrons to replace those lost, protons and oxygen as a by-product. NADP is reduced to NADPH. The ATP and NADPH produced are used in the light-independent stage. Learners should link each step to its location and to the fate of the products. A strong answer earns marks for these points. The light-dependent stage takes place on the thylakoid membranes of chloroplasts. Light excites electrons in chlorophyll in the photosystems, and these electrons pass along an electron transport chain. Electron transport pumps protons into the thylakoid space, and their flow back through ATP synthase produces ATP by chemiosmosis. Photolysis of water supplies electrons to replace those lost, releasing protons and oxygen. A common slip: Placing the light-dependent stage in the stroma. Instead, it occurs on the thylakoid membranes; the stroma hosts the light-independent stage. Exam tip: Name the location: thylakoid membranes.
See it
Key words
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Light absorption and electron excitation
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Electron transport and chemiosmosis
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Photolysis of water
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Products ATP and NADPH
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See every definition
- Light absorption and electron excitation
- Light absorbed by photosystems excites electrons in chlorophyll, which are passed along an electron transport chain.
- Electron transport and chemiosmosis
- Electron transport pumps protons into the thylakoid space, and their flow back through ATP synthase produces ATP by chemiosmosis.
- Photolysis of water
- Water is split, releasing electrons to replace those lost, protons and oxygen as a by-product.
- Products ATP and NADPH
- NADP is reduced to NADPH, and the ATP and NADPH produced are used in the light-independent stage.
Label the diagram
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1Box 1
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3Box 3
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