Yes, photosynthesis does require water. The reason is a bit complicated, but the bottom line is that hydrogens and electrons from water are needed in the process that turns light energy into chemical energy. The oxygen from the H₂O is a waste product.

All cells need energy to do work. We take chemical energy in food and turn it into chemical energy in molecules like ATP and NADH. Plants do that too, but they can also take the energy in light and use it to make ATP and NADPH. They can “spend” the energy in these molecules to make sugar and do other work. You can think of these energy-carrying molecules like batteries. The low-energy forms are ADP and NADP+. The charged-up forms are ATP and NADPH.

When a pigment like chlorophyll absorbs light, it uses that energy to boost the energy of electrons. If the electrons just fall back to their lower-energy state, all that energy would turn into heat. Instead, the system in a chloroplast (an organelle inside a leaf cell) uses the energy to power up the ADP to ATP and the NADP+ to NADPH. Electrons from the water replace the ones in the pigments that got an energy boost. The “boosted” electron and the hydrogen from water will end up as part of the high energy NADPH.

This may all be pretty confusing, but there are a few key ideas to take away from it. One is the idea that things can’t appear from nothing or disappear into nothing. If H shows up in one place, it must have come from another place. Energy can’t be created or destroyed either. It can be transformed from one type to another, though. These ideas are the first law of thermodynamics. The second law of thermodynamics tells us that every time energy is converted from one form to another, some is given off as heat. Some of the energy in light goes to charge up the ADP into ATP and NADP+ to NADPH, but some is also “wasted” as heat.

Plants aren’t the only organisms that do photosynthesis. What else does it? How do you know that plants can also break down sugar to release energy? Here are a couple ofclues: maple syrup & trees without leaves in winter.

You may be interested in studying plant physiology.