Cyclic photophosphorylation is a pathway of electron flow in the light-dependent reactions that involves only photosystem I.
2. The excited electrons pass to an electron acceptor.
3. Instead of being used to reduce NADP, they pass back along an electron transport chain to photosystem I, losing energy at each carrier.
4. The energy released pumps protons into the thylakoid space, and ATP is made by chemiosmosis as the protons flow back through ATP synthase.Because the electrons return to the same photosystem, the flow is a cycle. This means:
• ATP is the only useful product;
• no reduced NADP is made, because the electrons do not reach NADP;
• no photolysis of water is needed, so no oxygen is released.Why is it useful? The Calvin cycle needs 3 ATP for every 2 reduced NADP. Non-cyclic photophosphorylation does not make quite enough ATP for this ratio, so cyclic photophosphorylation tops up the ATP supply. It also provides ATP for other processes in the chloroplast.Cyclic photophosphorylation happens mainly on the intergranal lamellae, where photosystem I is concentrated. It increases when reduced NADP builds up and little NADP is available to accept electrons.Worked example: fixing 6 CO2 needs 18 ATP and 12 reduced NADP. If non-cyclic flow made the 12 reduced NADP and 15 ATP, cyclic flow would need to supply 18 − 15 = 3 ATP.
2. The excited electrons pass to an electron acceptor.
3. Instead of being used to reduce NADP, they pass back along an electron transport chain to photosystem I, losing energy at each carrier.
4. The energy released pumps protons into the thylakoid space, and ATP is made by chemiosmosis as the protons flow back through ATP synthase.Because the electrons return to the same photosystem, the flow is a cycle. This means:
• ATP is the only useful product;
• no reduced NADP is made, because the electrons do not reach NADP;
• no photolysis of water is needed, so no oxygen is released.Why is it useful? The Calvin cycle needs 3 ATP for every 2 reduced NADP. Non-cyclic photophosphorylation does not make quite enough ATP for this ratio, so cyclic photophosphorylation tops up the ATP supply. It also provides ATP for other processes in the chloroplast.Cyclic photophosphorylation happens mainly on the intergranal lamellae, where photosystem I is concentrated. It increases when reduced NADP builds up and little NADP is available to accept electrons.Worked example: fixing 6 CO2 needs 18 ATP and 12 reduced NADP. If non-cyclic flow made the 12 reduced NADP and 15 ATP, cyclic flow would need to supply 18 − 15 = 3 ATP.