Final answer:
ATP synthase is the protein system that phosphorylates ADP to make ATP in the light-dependent ETC of photosynthesis. It utilizes the proton gradient across the thylakoid membrane to produce ATP, which powers the Calvin cycle along with NADPH, also produced in the light-dependent reactions.
Step-by-step explanation:
In the light-dependent electron transport chain (ETC) of photosynthesis, the protein system that phosphorylates ADP to make ATP is ATP synthase. This occurs during a process known as photophosphorylation. ATP synthase uses the energy from an electrochemical gradient, established by the movement of protons across the thylakoid membrane, to catalyze the conversion of ADP into ATP. This is essential for providing the energy required for subsequent stages of photosynthesis, such as the Calvin cycle.
During cyclic photophosphorylation in cyanobacteria, NADPH formation does not occur. This is different from non-cyclic photophosphorylation, where both ATP and NADPH are produced.
Furthermore, in photosystem II’s reaction center, sunlight energy is used to remove electrons from water, which then enter the ETC, leading to ATP production and the reduction of NADP+ to NADPH by photosystem I. In this context, the electrons that ultimately pass through the photosynthetic ETC come from water molecules. The cytochrome complex involved in photosynthesis plays a role by transferring protons across the thylakoid membrane and facilitating electron transport from photosystem II to photosystem I.