A Water Molecule can be Split by a Leaf!
One of the many remarkable things about water is that the molecule is very stable and it is hard to get the oxygen away from those two hydrogens. Yet it can be and is done routinely every day by ordinary leaves in the process of photosynthesis.
Gerald Karp waxes eloquent as he describes how difficult this achievement is. "In fact, the splitting of water is the most thermodynamically challenging (endergonic) reaction known to occur in living organisms. Splitting water in the laboratory requires the use of a strong electric current or temperatures approaching 2000°C. Yet a plant cell can accomplish this feat on a snowy mountainside using only the energy of visible light."
This splitting of water is part of the process of photophosphorylation to change ADP to ATP, the universal energy currency for the processes of life. By extracting an oxygen from water in a two-stage process of non-cyclic electron transport, the cell can also produce enough energy for the production of carbohydrates for cellular respiration.