Water is one of the reactants in photosynthesis, it provides the hydrogen needed to form glucose (a hydrocarbon). Water and carbon dioxide enter the leaf through the stomata (small holes on the underside of the leaf that are controlled by gaurd cells) by diffusion.
Water is one of the most essential parts of photosynthesis. Six water molecules react with six carbon molecules to form one molecule of glucose. Water also creates a chemical potential across the cell membrane that results in the synthesis of ATP molecules that are required to provide energy to the plants.