Figure 14–48 The conversion of light energy to chemical energy in the photosystem II complex. (a) Schematic diagram of the photosystem II reaction center, whose special pair of chlorophyll molecules is designated as p680 based on the wavelength of its absorbance maximum (680 nm). (B) Cofactors and pigments at the core of the reaction center. Shown are the manganese (Mn) cluster, the tyrosine side chain that links it to the p680 special pair, four chlorophylls (green), two pheophytins (light blue), two plastoquinones (pink), and an iron atom (red). the path of electrons is shown by blue arrows. In the manganese cluster, four manganese atoms (light blue), one calcium atom (purple), and five oxygen atoms (red) work together to catalyze the oxidation of water. the water-splitting reaction occurs in four successive steps, each requiring the energy of one photon. Each photon turns a p680 reaction-center chlorophyll into a positively charged chlorophyll ion. through an ionized tyrosine side chain (yellow), this chlorophyll ion pulls an electron away from a water molecule bound at the manganese cluster. In this way, a total of four electrons are withdrawn from two water molecules to generate molecular oxygen, which is released into the atmosphere.
Each electron that is energized by light passes from the special pair along an electron-transfer chain inside the complex, along the indicated path to the permanently bound plastoquinone Qa and then to plastoquinone QB as electron acceptors. Once QB has picked up two electrons (plus two protons; see Figure 14–17), it dissociates from its binding site in the complex and enters the lipid bilayer as a mobile electron carrier, being immediately replaced by a new, nonreduced molecule of plastoquinone. Note that the chlorophylls and pheophytins form two symmetrical branches of a potential electron-transport chain. Only one branch is active, thus ensuring that the plastoquinones become fully reduced in minimum time.
Figure 14–49 The structure of the complete photosystem II complex. this photosystem contains at least 16 protein subunits, along with 36 chlorophylls, two pheophytins, two hemes, and a number of protective carotenoids (colored). Most of these pigments and cofactors are deeply buried, tightly complexed to protein (gray). the path of electrons is indicated by the blue arrows, and is explained in Figure 14–48B. the photosystem II complex presented here is the cyanobacterial complex, which is simpler and more stable than the plant complex, which works in the same way. (pDB code: 3arC.) one at a time to the mobile electron carrier plastocyanin (a small copper-containing protein that takes the place of the cytochrome c in mitochondria), which will transfer them to photosystem I (Figure 14–50). As we discuss next, photosystem I then harnesses a second photon of light to further energize the electrons that it receives.
photosystem I Carries Out the Second Charge-Separation Step  in the Z Scheme
Photosystem I receives electrons from plastocyanin in the thylakoid space and transfers them, via a second charge-separation reaction, to the small protein ferredoxin on the opposite membrane surface (Figure 14–51). Then, in a final step, ferredoxin feeds its electrons to a membrane-associated enzyme complex, the ferredoxin-NADP+ reductase, which uses the electrons to produce NADPH from NADP+ (see Figure 14–50).