The roots suck up water from the ground, into the chloroplasts.  In there, the water is sun-heated and gets split into hydrogen, oxygen and ATP.  The oxygen then gets released for us to breath and the hydrogen is put to a different part of the leaf.

Dark Reaction (doesn't involve the sun)

After the hydrogen gets to the other part of the leaf, the CO2 (carbon dioxide), that we breath out, mixes with the hydrogen and gets fused using ATP, making sugar/starch.