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LearnYBI11WBI14 · Unit 4: Energy, Environment, Microbiology and ImmunityEnergy flow, ecosystems and the environmentLight-dependent reactions
Energy flow, ecosystems and the environment · Subtopic 2 of 5

Light-Dependent Reactions

Chloroplasts have thylakoids (stacked into grana) where the light-dependent reactions occur, surrounded by stroma (light-independent). Photosystems contain chlorophyll a and b and accessory pigments (carotenoids) that absorb different wavelengths; the absorption spectrum matches the action spectrum. Non-cyclic photophosphorylation: light excites electrons in chlorophyll (photoionisation) of PSII; they pass along an electron transport chain to PSI, then to NADP, forming reduced NADP; energy pumps H⁺ into the thylakoid space and ATP synthase makes ATP by chemiosmosis. Photolysis of water replaces PSII's electrons: 2H2O→4H++4e−+O2\mathrm{2H_2O\rightarrow4H^++4e^-+O_2}. Cyclic photophosphorylation (PSI only) makes ATP but no reduced NADP or O₂. Pigments are separated by chromatography (RfR_f).

Cambridge 9700 Paper 4Edexcel YBI11 WBI14AQA 7402 Paper 2≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Relate chloroplast structure to function.
  • Describe pigments and spectra.
  • Describe cyclic and non-cyclic photophosphorylation and photolysis.
  • Separate pigments by chromatography.
Pause and think

Leaves look green. What does that tell you about which light chlorophyll uses?

Section 2 of 14

Core idea

Start hereSplitting water with light

Every breath you take contains oxygen released when plants split water using sunlight.

The big idea

Light excites electrons in chlorophyll; their energy makes ATP and reduced NADP, and water is split to replace them.

Non-cyclic photophosphorylation

Electrons flow from water to NADP.

Worked examplePigment Rf

A pigment moves 3.6 cm; solvent 8.0 cm.

Goal
Rf.
Strategy
Divide.
  1. Rf=3.68.0R_f=\frac{3.6}{8.0}
    Pigment ÷ solvent front.
  2. =0.45=0.45
    Between 0 and 1.
Answer0.45.
✓ CheckLower than carotene, which runs near the front ✓.

Summary

  • Thylakoids; pigments.
  • ATP + reduced NADP + O₂.
  • Cyclic: ATP only.
Key words
Photolysis
Splitting of water by light energy.
Photosystem
Cluster of pigments around a reaction centre.
HistoryHill reaction. Robert Hill showed in 1937 that isolated chloroplasts release oxygen when given an electron acceptor, proving O₂ comes from water.
Fun factAutumn colours. In autumn chlorophyll breaks down first, revealing yellow and orange carotenoids that were there all along.
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