Two photosystems are involved in oxygenic photosynthesis: photosystem I and photosystem II. They operate together in the light-dependent reactions of plants, algae, and cyanobacteria.
Photosystem II acts first in the familiar linear electron-flow pathway. It uses light energy to remove electrons from water, producing oxygen, protons, and replacement electrons. Those electrons pass through an electron-transport chain, helping establish a proton gradient used to make ATP.
Photosystem I absorbs light again and raises the electrons to a higher energy state. They are ultimately used to reduce NADP+ to NADPH, another energy-rich product needed by the Calvin cycle to help convert carbon dioxide into carbohydrates. The names P680 and P700 refer to the approximate reaction-center absorption wavelengths of photosystems II and I, respectively. Some anoxygenic photosynthetic bacteria use only one photosystem because they do not split water to release oxygen.