What is photolysis of water and what are its products?

• A. Water is split by light energy at PSII producing CO2 and glucose.
• B. Water is split by mitochondria to produce ATP.
• C. Water is split by light energy at PSII producing O2, electrons, and protons.
• D. Water is converted to water vapor during photosynthesis.

Answer: (C)

Photolysis of water is the light-driven splitting of water by photosystem II in the thylakoid membranes. When PSII absorbs light, the oxygen-evolving complex uses that energy to oxidize two water molecules, yielding oxygen gas, electrons, and protons. The overall reaction is 2 H2O → O2 + 4 H+ + 4 e−. The released oxygen is released to the atmosphere, the electrons replace those lost by chlorophyll in PSII and travel through the electron transport chain, and the protons accumulate in the thylakoid lumen to help build the proton gradient that powers ATP synthase. This provides the raw electrons and protons needed for the rest of the light reactions and ultimately for producing ATP and NADPH used in carbon fixation.

CO2 and glucose are produced later in the Calvin cycle, not by splitting water. Mitochondria do not perform photolysis of water; this process occurs in chloroplasts during photosynthesis. Water is not converted to water vapor in this step.