When a water molecule enters the granum in the process of photosynthesis, it is likely to undergo a series of chemical reactions to produce energy-rich molecules such as ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are used in the subsequent light-independent reactions to produce glucose and other organic compounds.
Specifically, the water molecule will undergo a process called photolysis, which involves breaking the water molecule into its component parts, hydrogen ions (H+) and oxygen (O2), using the energy from sunlight. This process occurs in the thylakoid membrane of the granum and is facilitated by a complex of proteins called the Photosystem II (PSII). The oxygen produced during photolysis is released into the atmosphere, while the hydrogen ions are used to produce ATP and NADPH in a series of electron transport chain reactions. These energy-rich molecules are then used in the Calvin cycle, a series of biochemical reactions that occur in the stroma of the chloroplast, to produce glucose and other organic compounds