Question

Experiments show that h2o is essential for photosynthesis

Answer 1

Stable isotopes of carbon and hydrogen are useful indicators of the photosynthetic pathway. 

Use a hydrogen isotope labelled water to see that the water is split in the photolysis reaction. Then track the hydrogen ions to see where they end up in the plants. 

2 (H2O) --- photolysis reaction --> O2 + ATP + NADPH (carrier molecule will transfer H isotope to calvin cycle if water provides the H to build glucose) 

NADPH transfers to the Calvin cycle step of photosynthesis and provides the hydrogen to build the CO2 into an organic molecule. 

Photosynthesis 
CO2 + H2O → → O2 + CH2O