Photosynthesis: Question 3

Worked solution

Part (a): Non-cyclic photophosphorylation

1. Light energy is absorbed by chlorophyll and accessory pigments within photosystem II (PSII). This energy excites an electron in the reaction-centre chlorophyll, raising it to a higher energy level, and the electron is emitted from PSII.
2. This high-energy electron passes along a chain of carrier proteins in the thylakoid membrane, the electron transport chain, travelling towards photosystem I. As it passes from carrier to carrier, it loses energy in a series of redox reactions.
3. The energy released is used to actively pump protons (H⁺) across the thylakoid membrane, from the stroma into the thylakoid lumen, building up a proton concentration gradient.
4. Protons then diffuse back down this gradient, out of the lumen and into the stroma, through the enzyme ATP synthase embedded in the membrane. This flow of protons (chemiosmosis) provides the energy to synthesise ATP from ADP and inorganic phosphate. The overall process of making ATP this way is called photophosphorylation.
5. At the same time, the electrons lost from PSII are replaced: light energy also splits water molecules at PSII, in a reaction called photolysis, producing protons, electrons and oxygen. The electrons produced replace those emitted from PSII, and the oxygen is released as a by-product (it can diffuse out of the leaf or be used in respiration).
6. Meanwhile, light energy also excites electrons within photosystem I (PSI), raising them to an even higher energy level. These excited electrons, together with protons from the stroma, are used (with the enzyme NADP reductase) to reduce NADP, forming reduced NADP. The electrons lost from PSI are replaced by the electrons that have arrived via the electron transport chain from PSII.

So the overall products of non-cyclic photophosphorylation are ATP, reduced NADP, and oxygen.

Part (b): How cyclic photophosphorylation differs

Cyclic photophosphorylation involves only photosystem I, photosystem II plays no part. Electrons excited from PSI pass along a short electron transport chain, and the energy released as they do so is still used to pump protons across the thylakoid membrane and generate ATP by chemiosmosis, exactly as before.

The key difference is where the electrons end up: instead of being used to reduce NADP, they are passed back around the short chain and return to photosystem I, hence “cyclic”. Because PSII is not involved, there is no photolysis of water and therefore no oxygen is released; and because the electrons return to PSI rather than reducing NADP, no reduced NADP is produced. Cyclic photophosphorylation therefore yields ATP only.

Part (c): Why cyclic photophosphorylation is also needed

The Calvin cycle, which uses the products of the light-dependent stage, requires more ATP than reduced NADP overall: ATP is used both to reduce GP to triose phosphate (alongside reduced NADP) and, separately, to regenerate RuBP from triose phosphate, while reduced NADP is only used in the reduction of GP. Non-cyclic photophosphorylation on its own produces ATP and reduced NADP in roughly matched amounts, which is not quite enough ATP relative to the reduced NADP for the Calvin cycle’s needs.

By running cyclic photophosphorylation alongside non-cyclic photophosphorylation, the chloroplast can generate the additional ATP that the Calvin cycle requires, without producing surplus reduced NADP that would not be needed. This helps balance the supply of ATP and reduced NADP to match what the light-independent reactions actually demand.

Final answers

• (a) Non-cyclic photophosphorylation: PSII loses an electron along the electron transport chain to PSI (pumping protons, driving ATP synthesis by chemiosmosis); PSII’s electron is replaced by photolysis of water (releasing oxygen); PSI’s excited electron (with a proton) reduces NADP. Products: ATP, reduced NADP, oxygen.
• (b) Cyclic photophosphorylation uses only PSI; its electron returns to PSI after passing round a short chain, still generating ATP by chemiosmosis, but with no photolysis, no oxygen and no reduced NADP produced.
• (c) Cyclic photophosphorylation supplies the extra ATP the Calvin cycle needs (beyond what non-cyclic photophosphorylation provides), without making unneeded extra reduced NADP.