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A2 · Practice questions · The light-dependent stage

Light in, ATP out.

Six original Cambridge-style questions on the light-dependent stage, from comparing the two pathways to explaining how a proton gradient makes ATP.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Two paths, one gradient.

01
Recall
[2 marks]

State the two types of photophosphorylation that occur in the light-dependent stage and the products of each.

  • Non-cyclic photophosphorylation: ATP and reduced NADP (with oxygen as a by-product). ✓
  • Cyclic photophosphorylation: ATP only. ✓
02
Explain
[4 marks]

Explain the roles of photosystem II and the oxygen-evolving complex in non-cyclic photophosphorylation.

  • Light absorbed by PSII causes photoactivation of chlorophyll: electrons are excited and leave the reaction centre. ✓
  • These electrons pass along the electron transport chain to PSI. ✓
  • The oxygen-evolving complex catalyses photolysis of water: 2H2O → 4H+ + 4e− + O2. ✓
  • The electrons replace those lost by PSII; the protons add to the gradient / help reduce NADP; oxygen is released. ✓
03
Explain
[4 marks]

Explain how the movement of electrons along the electron transport chain in the thylakoid membrane leads to the synthesis of ATP.

  • Energetic (excited) electrons release energy as they pass from carrier to carrier. ✓
  • This energy is used to transfer protons from the stroma across the thylakoid membrane into the thylakoid space. ✓
  • A proton concentration gradient forms (high in the thylakoid space). ✓
  • Protons return to the stroma by facilitated diffusion through ATP synthase, providing energy to make ATP from ADP and Pi. ✓
04
Analysis
[3 marks]

Compare cyclic and non-cyclic photophosphorylation, giving three differences.

  • Cyclic involves PSI only; non-cyclic involves PSI and PSII. ✓
  • Non-cyclic includes photolysis of water and releases oxygen; cyclic does not. ✓
  • Non-cyclic makes reduced NADP as well as ATP; cyclic makes only ATP (electrons return to PSI rather than going to NADP). ✓
05
Data
[3 marks]

Isolated chloroplasts are put in the light. The pH of the thylakoid space falls from about 7.5 to 4.5 within a minute, while the stroma rises to about pH 8. Explain these changes and their importance.

  • Energy released by electrons in the electron transport chain transfers protons from the stroma into the thylakoid space (and photolysis releases protons inside), so the thylakoid space becomes more acidic. ✓
  • Protons are removed from the stroma (transferred across the membrane and used to reduce NADP), so its pH rises. ✓
  • The steep gradient (about 1000 times more H+ inside) drives protons through ATP synthase, providing the energy to make ATP. ✓
06
Explain
[2 marks]

Suggest why a chloroplast whose PSII has been damaged can still make some ATP in the light, but cannot fix carbon dioxide for long.

  • PSI can still carry out cyclic photophosphorylation, pumping protons and making ATP. ✓
  • But no electrons flow from water through PSII to NADP, so no reduced NADP is made; GP cannot be reduced to TP, RuBP is not regenerated and fixation soon stops. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.