Questions: Multiple Choice Question In the light reactions of photosynthesis, what drives the synthesis of ATP by ATP synthase? the donation of H+ to O2 to form water the binding of oxygen to rubisco the transfer of a phosphate group from NADP+ to NADPH a proton gradient across the thylakoid membrane

The answer is the fourth one (or D): a proton gradient across the thylakoid membrane.

Explanation for each option:

1. The donation of $\mathrm{H}^{+}$ to $\mathrm{O}_{2}$ to form water:

   • This process is part of cellular respiration, not photosynthesis. In photosynthesis, water is split to release oxygen, not the other way around. Therefore, this option is incorrect.

2. The binding of oxygen to rubisco:

   • Rubisco is an enzyme involved in the Calvin cycle (the light-independent reactions of photosynthesis), where it catalyzes the fixation of carbon dioxide. It does not play a role in the light reactions or ATP synthesis. Therefore, this option is incorrect.

3. The transfer of a phosphate group from $\mathrm{NADP}^{+}$ to NADPH:

   • This describes the reduction of NADP+ to NADPH, which occurs in the light reactions, but it is not directly involved in the synthesis of ATP. Therefore, this option is incorrect.

4. A proton gradient across the thylakoid membrane:

   • During the light reactions of photosynthesis, light energy is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient. This gradient drives the synthesis of ATP by ATP synthase as protons flow back across the membrane through the enzyme. Therefore, this option is correct.

Summary: The synthesis of ATP by ATP synthase in the light reactions of photosynthesis is driven by a proton gradient across the thylakoid membrane.