Here is what I’ve found:
Photosynthesis
Photosynthesis uses the energy obtained from light to free electrons from the chlorophyll pigments that collect the light. Chlorophyll molecules do not have an infinite supply of electrons, so they regain the lost electron from a molecule of water. What remains are electrons and hydrogen ions (electrically charged particles of hydrogen). Oxygen is created as a byproduct, which is why it is expelled into the atmosphere.
Cellular respiration
In cellular respiration the electron transport chain occurs after glucose has already been broken down. Eight molecules of NADPH and two molecules of FADH2 remain. These molecules are intended to donate electrons and hydrogen ions to the electron transport chain. The movement of electrons galvanizes hydrogen ions across the membrane of the mitochondrion. Because this forms a concentration of hydrogen ions on one side, they are compelled to move back to the inside of the mitochondrion, which galvanizes the synthesis of ATP. At the very end of the process, electrons are accepted by oxygen, which then binds to the hydrogen ions in order to produce water.