The light reaction in photosynthesis is directly responsible for splitting water into hydrogen ions and oxygen:
2 H[sub]2[/sub]O + 2 NADP[sup]+[/sup] + 2 ADP + 2 Pi + light → 2 NADPH + 2 H[sup]+[/sup] + 2 ATP + O[sub]2[/sub]
The chemical energy stored in ATP and NADPH is then used for carbon fixation in the Calvin cycle:
3 CO[sub]2[/sub] + 9 ATP + 6 NADPH + 6 H[sup]+[/sup] → C[sub]3[/sub]H[sub]6[/sub]O[sub]3[/sub]-phosphate + 9 ADP + 8 Pi + 6 NADP[sup]+ [/sup]+ 3 H[sub]2[/sub]O
The extra oxygen on CO[sub]2[/sub] goes to water. The three carbon sugar-phosphate is then used by the plant to build higher-carbon sugars, including six-carbon hexoses.
A full summary equation of photosynthesis to carbon dioxide to hexose would more properly be written as:
6 CO[sub]2[/sub] + 12 H[sub]2[/sub]O → C[sub]6[/sub]H[sub]12[/sub]O[sub]6[/sub] + 6 O[sub]2[/sub] + 6 H[sub]2[/sub]O
This emphasizes that the water produced during the reaction is different than the water used during the reaction.
One could argue that the water produced during the Calvin cycle is fed back into the light reaction to produce oxygen, but that doesn’t necessarily have to happen. Many other reactions in the cell require water, and a lot of water is simply lost due to transpiration.