In the process of photosynthesis, which of the following chemical reactions…

Concept: Photolysis means a splitting (lysis) brought about by light (photo). In the light reaction of photosynthesis it is the light-driven oxidation of water at the oxygen-evolving complex of photosystem II — the step that supplies replacement electrons to the reaction centre, releases protons into the thylakoid lumen and sets molecular oxygen free.

Application: Among the equations given, look for the one in which a molecule is being taken apart rather than built up or handed on. Water is the molecule split during photolysis:

2H2O → 4H+ + 4e− + O2

1. Light energy absorbed by photosystem II drives the manganese-bearing oxygen-evolving complex to oxidise two molecules of water.

2. The four electrons released replace the electrons that the P680 reaction centre lost when it absorbed light.

3. The four protons are set free inside the thylakoid lumen and add to the proton gradient that later drives ATP synthase.

4. One molecule of O2 is released — the source of practically all the oxygen in the atmosphere.

Cross-check: the other equations belong to different stages of photosynthesis:

• NADP + e− → reduced NADP is the terminal electron-acceptance step of photosystem I, catalysed by ferredoxin–NADP+ reductase on the stromal face.

• RuBP + O2 → 3-PGA + phosphoglycolate under RuBisCO is the oxygenase (photorespiration) route carried out in the stroma.

• ADP + Pi → ATP is photophosphorylation by ATP synthase, powered by the proton gradient across the thylakoid membrane.

Splitting of a molecule by light energy happens only in the water-oxidation equation, so 2H2O → 4H+ + 4e− + O2 is the reaction that takes place during photolysis.