In photosynthetic cells, synthesis of ATP by the chemiosmotic mechanism occurs during which of the following processes?

• A. the Calvin cycle
• B. glycolysis
• C. photorespiration
• D. the light reactions of photosynthesis

Answer: (D)

Verified step by step guidance

1. Understand that ATP synthesis by the chemiosmotic mechanism involves the movement of protons (H⁺ ions) across a membrane, creating a proton gradient that drives ATP production via ATP synthase.
2. Recall that in photosynthesis, the light reactions occur in the thylakoid membranes of the chloroplasts, where light energy is used to excite electrons in photosystems II and I.
3. Recognize that during the light reactions, water molecules are split (photolysis), releasing electrons, protons (H⁺), and oxygen. The electrons move through the electron transport chain, and the energy released is used to pump protons into the thylakoid lumen, creating a proton gradient.
4. Understand that the proton gradient established in the thylakoid lumen is used by ATP synthase to synthesize ATP as protons flow back into the stroma through the enzyme. This process is known as photophosphorylation and is a chemiosmotic mechanism.
5. Conclude that ATP synthesis by the chemiosmotic mechanism occurs during the light reactions of photosynthesis, not during the Calvin cycle, glycolysis, or photorespiration.