The excitation energy collected by the antenna complex is delivered to the special pair in the photochemical reaction center. The reaction center is a transmembrane chlorophyll–protein complex that lies at the heart of photosynthesis. It harbors the special pair of chlorophyll molecules, which acts as an irreversible trap for excitation energy (see Figure 14–45).
Chloroplasts contain two functionally different although structurally related photosystems, each of which feeds electrons generated by the action of sunlight into an electron-transfer chain. In the chloroplast thylakoid membrane, photo-system I is confined to the unstacked stroma thylakoids, while the stacked grana thylakoids contain photosystem II. The two photosystems were named in order of their discovery, not of their actions in the photosynthetic pathway, and electrons are first activated in photosystem II before being transferred to photosystem I (Figure 14–46). The path of the electron through the two photosystems can be described as a Z-like trajectory and is known as the Z scheme. In the Z scheme, the reaction center of photosystem II first withdraws an electron from water. The electron passes via an electron-transport chain (composed of the electron carrier plastoquinone, the cytochrome b6-f complex, and the protein plastocyanin) to photosystem I, which propels the electron across the membrane in a second light-driven charge-separation reaction that leads to NADPH production.
Figure 14–45 A photosystem. Each photosystem consists of a reaction center plus a number of light-harvesting antenna complexes. the solar energy for photosynthesis is collected by the antenna complexes, which account for most of the chlorophyll in a plant cell. the energy hops randomly by resonance energy transfer (red arrows) from one chlorophyll molecule to another, until it reaches the reaction center complex, where it ionizes a chlorophyll in the special pair. the chlorophyll special pair holds its electrons at a lower energy than the chlorophyll in the antenna complexes, causing the energy transferred to it from the antenna complex to become trapped there. Note that it is only energy that  moves from one chlorophyll molecule to another in the antenna complex, not electrons (Movie 14.10).
Figure 14–46 The Z scheme for photosynthesis. the thylakoids of plants and cyanobacteria contain two different photosystems, known as photosystem I and photosystem II, which work in series. Each of the photosystem I and II reaction centers receives excitation energy from its own set of tightly associated antenna complexes, known as LhC-I and LhC-II, by resonance energy transfer. Note that, for historical reasons, the two photosystems were named opposite to the order in which they act, with photosystem II passing its electrons to photosystem I.
The Z scheme is necessary to bridge the very large energy gap between water and NADPH (Figure 14–47). A single quantum of visible light does not contain enough energy both to withdraw electrons from water, which holds on to its electrons very tightly (redox potential +820 mV) and therefore is a very poor electron donor, and to force them on to NADP+, which is a very poor electron acceptor (redox potential –320 mV). The Z scheme first evolved in cyanobacteria to enable them to use water as a universally available electron source. Other, simpler photosynthetic bacteria have only one photosystem. As we shall see, they cannot use water as an electron source and must rely on other, more energy-rich substrates instead, from which electrons are more readily withdrawn. The ability to extract electrons from water (and thereby to produce molecular oxygen) was acquired by plants when their ancestors took up the endosymbiotic cyanobacteria that later evolved into chloroplasts (see Figure 1–31).
photosystem II Uses a Manganese Cluster to Withdraw Electrons From Water