Figure 2. The “light” reaction of photosynthesis in generalized outline showing chlorophyll fluorescence emanating from Photosystem II. Photosystem I is weakly fluorescent (not shown) but has no variable component. Photosystem I and Photosystem II are designated by the order in which they were discovered, not the order of the reaction. Note that two molecules of water enter the reaction on the left side. Water is split to resupply electrons that are lost from Chlα in the PSII reaction center, generating free oxygen and a constant stream of electrons that feed into PSII. Source: adapted and re-drawn from Govindjee and Wilbert Veit.