How do the light-dependent reactions of photosynthesis capture light energy and convert it into ATP and reduced NADP?
Describe the light-dependent reactions of photosynthesis, including photophosphorylation and the photolysis of water
A focused answer to the H2 Biology Energy and Equilibrium outcome on the light-dependent stage of photosynthesis. Light absorption by chlorophyll, the photolysis of water, the electron transport chain and chemiosmosis in the thylakoid, and the production of ATP and reduced NADP.
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What this dot point is asking
SEAB wants you to describe the light-dependent reactions of photosynthesis on the thylakoid membranes: the absorption of light by chlorophyll, the photolysis of water, the electron transport chain and chemiosmosis producing ATP (photophosphorylation), and the reduction of NADP. The ATP and reduced NADP made here power the Calvin cycle.
The answer
Where and what
The light-dependent reactions occur on the thylakoid membranes inside the chloroplast. Their job is to use light energy to make ATP and reduced NADP, and to split water.
Light absorption and electron excitation
Chlorophyll absorbs light energy. This excites electrons in the chlorophyll to a higher energy level, so that they leave the chlorophyll and pass to an electron acceptor.
Electron transport and photophosphorylation
The excited electrons pass along an electron transport chain in the thylakoid membrane, releasing energy at each step. This energy pumps protons into the thylakoid space, creating a proton gradient (chemiosmosis). Protons then flow back through ATP synthase, driving ATP synthesis. Because the energy ultimately came from light, this is photophosphorylation. At the end of the chain the electrons, with protons, are accepted by NADP to form reduced NADP.
Photolysis of water
The electrons lost from chlorophyll are replaced by the photolysis of water: light energy splits water into protons, electrons and oxygen.
The electrons replace those lost by chlorophyll, the protons contribute to the gradient and to reduced NADP, and the oxygen is released as a by-product.
Examples in context
Example 1. The link to the Calvin cycle. The ATP and reduced NADP made in the light-dependent reactions are the energy and reducing power that the Calvin cycle uses to fix carbon dioxide into sugar. The two stages are inseparable parts of photosynthesis.
Example 2. Parallels with respiration. The light-dependent reactions and oxidative phosphorylation both use an electron transport chain, a proton gradient and ATP synthase. Recognising this shared chemiosmotic mechanism helps you remember both processes as variations on the same theme.
Try this
Q1. State where in the chloroplast the light-dependent reactions occur. [1 mark]
- Cue. On the thylakoid membranes.
Q2. Name the two useful products of the light-dependent reactions that are passed to the Calvin cycle. [1 mark]
- Cue. ATP and reduced NADP.
Q3. Explain why photolysis of water is necessary for the light-dependent reactions to continue. [2 marks]
- Cue. It replaces the electrons lost by chlorophyll when they are excited and leave, allowing electron flow and the process to continue; it also supplies protons and releases oxygen.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksDescribe the light-dependent reactions of photosynthesis, including the photolysis of water and the production of ATP and reduced NADP.Show worked answer →
Examiners want light absorption, electron flow, photolysis, chemiosmosis and the products.
The light-dependent reactions occur on the thylakoid membranes of the chloroplast. Light is absorbed by chlorophyll, which excites electrons to a higher energy level so that they leave the chlorophyll.
These high-energy electrons pass along an electron transport chain in the thylakoid membrane, releasing energy that is used to pump protons into the thylakoid space, creating a proton gradient. Protons flow back through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate; this is photophosphorylation. The electrons are finally accepted, with protons, by NADP to form reduced NADP.
The electrons lost from chlorophyll are replaced by the photolysis of water: light energy splits water into protons, electrons and oxygen. The oxygen is released as a by-product, and the electrons replace those lost by chlorophyll.
The products passed to the Calvin cycle are ATP and reduced NADP. Markers reward light absorption and electron excitation, the electron transport chain and proton gradient, ATP synthase and photophosphorylation, NADP reduction, the photolysis of water with oxygen released, and the two products.
Original3 marksExplain the role of the photolysis of water in the light-dependent reactions.Show worked answer →
The answer should give the splitting and its three contributions.
Photolysis is the splitting of water using light energy. It produces protons, electrons and oxygen.
The electrons from photolysis replace the electrons lost by chlorophyll when it was excited by light, allowing the process to continue. The protons contribute to the proton gradient used to make ATP and are accepted by NADP. The oxygen is a by-product and is released, accounting for the oxygen produced in photosynthesis.
Markers reward the splitting of water by light, the replacement of chlorophyll's lost electrons, the contribution of protons, and oxygen as the released by-product.
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