Describe cyclic photophosphorylation and when it occurs.

Describe cyclic photophosphorylation and when it occurs.

• A. Electrons excited in PSI return to the electron transport chain, generating ATP without NADPH or O2; occurs when more ATP is required
• B. Electrons from PSII go to NADP+ to form NADPH; yields O2
• C. Water is split to release O2; energy stored as NADPH
• D. The Calvin cycle uses ATP from cyclic photophosphorylation to fix CO2

Answer: (A)

Cyclic photophosphorylation uses only Photosystem I. When light excites electrons in PSI, they’re redirected back into the electron transport chain rather than being transferred to NADP+. The electrons travel from PSI to ferredoxin, then to the cytochrome b6f complex, and return to PSI via plastocyanin, forming a cycle. This flow pumps protons across the thylakoid membrane, and ATP synthase uses that proton motive force to make ATP. Because the electrons don’t reach NADP+ or involve water splitting at PSII, no NADPH is produced and no O2 is released. This occurs when the cell needs more ATP than NADPH, such as to meet the ATP demand of the Calvin cycle’s regeneration steps.

Cyclic photophosphorylation uses only Photosystem I. When light excites electrons in PSI, they’re redirected back into the electron transport chain rather than being transferred to NADP+. The electrons travel from PSI to ferredoxin, then to the cytochrome b6f complex, and return to PSI via plastocyanin, forming a cycle. This flow pumps protons across the thylakoid membrane, and ATP synthase uses that proton motive force to make ATP. Because the electrons don’t reach NADP+ or involve water splitting at PSII, no NADPH is produced and no O2 is released. This occurs when the cell needs more ATP than NADPH, such as to meet the ATP demand of the Calvin cycle’s regeneration steps.