Answer: The light-dependent reactions of photosynthesis set up the proton gradient that drives ATP synthsis.
Explanation: The light reactions of photosynthesis is called photolysis reactions. It converts solar energy into chemical energy in the form of NADPH and ATP.
There are two types of photosystems: Photosystem I and Photosystem II. Each has one primary photochemical reaction center (either chlorophyll P700 or P680) and a set of accessory pigments to absorb additional light.
Photosynthesis is initiated with the simultaneous excitation of pairs of special reaction center made of chlorophyll(a) molecules, and is called P680 (in photosystem II, or PSII) and P700 (in photosystem I, or PSI)
The Z-scheme represents the steps in the light reactions. It shows the pathway of electron transport from water to NADP+ (nicotinamide adenine dinucleotide phosphate). The process involves the release of oxygen, the "reduction" of NADP+ to NADPH and the build up of hydrogen ions within the thylakoid lumen.
This proton gradient drives ATP production.