Two different variations of electron transport are used during photosynthesis:
*Noncyclic electron transport or non-cyclic photophosphorylation produces NADPH + H and ATP.
*Cyclic electron transport or cyclic photophosphorylation produces only ATP.
The noncyclic variety involves the participation of both photosystems, while the cyclic electron flow is dependent on only photosystem I.
*Photosystem I uses light energy to reduce NADP to NADPH + H, and is active in both noncyclic and cyclic electron transport. In cyclic mode, the energized electron is passed down a chain that ultimately returns it (in its base state) to the chlorophyll that energized it.
*Photosystem II uses light energy to oxidize water molecules, producing electrons (e), protons (H), and molecular oxygen (O), and is only active in noncyclic transport. Electrons in this system are not conserved, but are rather continually entering from oxidized 2HO (O + 4 H + 4 e) and exiting with NADP when it is finally reduced to NADPH.