A-Level Biology May/June 2025 Q4(c)(ii): Some of the protein components of the thylakoid membrane have a role in the light-depen…

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  Proteins in the thylakoid membrane have several distinct roles in the light-dependent stage of photosynthesis:

  1. Electron Transport: A series of protein carriers form an electron transport chain (ETC). These proteins accept and pass on high-energy electrons that have been excited by light energy in the photosystems.

  2. Proton Pumping: As electrons move along the ETC, they release energy. This energy is used by the protein carriers to actively pump protons (H⁺) from the stroma into the thylakoid space. This establishes a proton concentration gradient across the thylakoid membrane.

  3. ATP Synthesis: The protein complex ATP synthase allows protons to flow down their electrochemical gradient, from the thylakoid space back into the stroma. The kinetic energy from this proton motive force is used to catalyse the synthesis of ATP from ADP and inorganic phosphate (Pi).

  4. Photolysis: An enzyme, the oxygen-evolving complex, which is a protein associated with Photosystem II, catalyses the splitting (photolysis) of water molecules into electrons, protons, and oxygen.

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  How the marks are awarded

  • M1 — The model answer identifies that proteins form an electron transport chain to carry electrons.
  • M1 — The answer explains that proteins use energy to pump protons into the thylakoid space, generating a proton gradient.
  • M1 — The role of the protein ATP synthase in making ATP is correctly stated.
  • M1 — The answer identifies the role of the oxygen-evolving complex enzyme in catalysing the photolysis of water.

  Common mistakes

  • Confusing the location of processes, for example, stating that protons are pumped out of the thylakoid space or that ATP is made in the thylakoid lumen.
  • Mentioning proteins from the light-independent stage, such as RuBisCO, which is located in the stroma, not the thylakoid membrane.
  • Providing vague answers like 'proteins carry energy' without specifying they are electron carriers or that they pump protons.
  • Forgetting to mention the role of ATP synthase, focusing only on the photosystems and the electron transport chain.

  Examiner tip: For questions about biological processes, always link each specific structure to its precise function and explain how it contributes to the overall outcome.

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