What happens to the hydrogen released when water is split?

• A. It combines with CO2 to form glucose
• B. It is released as hydrogen gas
• C. It becomes part of oxygen gas
• D. It is converted into water

Answer: (B)

When water is split in the light-dependent reactions, the hydrogen from the water ends up as protons (H+) in the thylakoid lumen and as part of the NADPH-pool later. These protons create the proton gradient across the thylakoid membrane that drives ATP synthase to make ATP, and some of the hydrogen ions participate in reducing NADP+ to NADPH. The gas released from splitting water is oxygen, not hydrogen gas. So the hydrogen isn’t released as H2 gas; it serves to power ATP formation and NADPH production in the chloroplast.