With an understanding of the principles of how photosynthetic organisms generate high-energy electrons, let us examine the biochemical systems that coordinate the electron capture and their use to generate reducing power and ATP, resources that will be used to power the synthesis of glucose from CO₂. Photosynthesis in green plants is mediated by two kinds of membrane-bound, light-sensitive complexes—photosystem I (PS I) and photosystem II (PS II), each with its own characteristic reaction center (Figure 22.10). Photosystem I responds to light with wavelengths shorter than 700 nm and is responsible for providing electrons to reduce NADP⁺ to NADPH, a versatile reagent for driving biosynthetic processes requiring reducing power. Photosystem II responds to wavelengths shorter than 680 nm, sending electrons through a membrane-bound proton pump called cytochrome b₆ f and then on to photosystem I to replace the electrons donated by PS I to NADP⁺. The electrons in the reaction center of photosystem II are replaced when two molecules of H₂O are oxidized to generate a molecule of O₂. As we will soon see, electrons flow from water through photosystem II, the cytochrome b₆ f complex, and photosystem I and are finally accepted by NADP⁺. In the course of this flow, a proton gradient is established across the thylakoid membrane. This proton gradient is the driving force for ATP production.

The first stage of the light reactions is catalyzed by photosystem I (Figure 22.11). Photosystem I typically includes 14 polypeptide chains and multiple associated proteins and cofactors. The core of this system is a pair of similar subunits, PsaA (83 kDa) and PsaB (82 kDa), which bind 80 chlorophyll molecules as well as other redox factors. A special pair of chlorophyll a molecules lies at the center of the structure and absorbs wavelengths of light that are longer than those absorbed by chlorophyll a not associated with a photosystem. The special pair of the reaction center of photosystem I are called P700. When activated by light (designated P700*), the reaction center initiates photoinduced charge separation that generates the high-energy electrons. The electrons flow down an electron-transport chain through a chlorophyll called A₀ and a quinone called A₁ to a set of 4Fe-4S clusters. From there, the electrons are transferred to ferredoxin (Fd), a protein containing a 2Fe-2S cluster coordinated to four cysteine residues (Figure 22.12).

Although reduced ferredoxin is a strong reductant, it is not useful for driving many reactions, in part because ferredoxin carries only one available electron. The currency of readily available biosynthetic reducing power in cells is NADPH, which shuttles two electrons as a hydride ion. Indeed, NADPH functions exactly as NADH does and differs structurally only in that it contains a phosphoryl group on the 2′-hydroxyl group of one of the ribose units (Figure 22.13). This phosphoryl groups acts as a label identifying NADPH as a reductant for biosynthetic (anabolic) reactions, and so it is not oxidized by the respiratory chain as is NADH. This distinction holds true in all biochemical systems, including that of human beings.

How can reduced ferredoxin be used to drive the reduction of NADP⁺ to NADPH? This reaction is catalyzed by ferredoxin-NADP⁺ reductase, a flavoprotein. The bound FAD component in this enzyme collects two electrons, one at a time, from two molecules of reduced ferredoxin as it proceeds from its oxidized form to its fully reduced form (Figure 22.14). The enzyme then transfers a hydride ion to NADP⁺ to form NADPH. Note that this reaction takes place on the stromal side of the thylakoid membrane. The use of a stromal proton in the reduction of NADP⁺ makes the stroma more basic than the thylakoid lumen and thus contributes to the formation of the proton-motive force across the thylakoid membrane that will be used to synthesize ATP. The first of the raw materials required to reduce CO₂—biosynthetic reducing power as NADPH—has been generated. However, photosystem I is now deficient in electrons. How can these electrons be replenished?

Restocking the electrons of photosystem I requires another source of electrons. One of the roles of photosystem II is to provide electrons to photosystem I. Photosystem II catalyzes the light-driven transfer of electrons from water to photosystem I. In the process, protons are pumped into the thylakoid lumen to generate a proton-motive force. Photosystem II is an enormous assembly of more than 20 subunits. It is formed by D1 and D2, a pair of similar 32-kDa proteins that span the thylakoid membrane (Figure 22.15).

The photochemistry of photosystem II begins with the excitation of a special pair of chlorophyll molecules that are often called P680, generating P680* (Figure 22.16). P680* rapidly transfers electrons to a nearby pheophytin (designated Ph), a chlorophyll with two H⁺ ions in place of the central Mg²⁺ ion. From there, the electrons are transferred first to a tightly bound plastoquinone at site Q_A and then to a mobile plastoquinone at site Q_B. Plastoquinone is an electron carrier that closely resembles ubiquinone, a component of the mitochondrial electron-transport chain. With the arrival of a second electron and the uptake of two protons, the mobile plastoquinone is reduced to plastoquinol, QH₂.

The site of quinone reduction in PS II is on the side of the stroma. Thus, the two protons that are taken up with the reduction of each molecule of plastoquinone come from the stroma, increasing the proton gradient. This gradient is further augmented by the link between photosystem II and photosystem I, the cytochrome b₆ f complex.

The QH₂ produced by photosystem II transfers its electrons to plastocyanin, which in turn donates the electrons to photosystem I, thereby replenishing the missing electrons in photosystem I. The chain starts when the electrons are transferred, one at a time from QH₂, to plastocyanin (Pc), a copper-containing protein in the thylakoid lumen (Figure 22.10). This reaction is catalyzed by cytochrome b₆ f (plastoquinol-plastocyanin reductase), a proton pump:

The cytochrome b₆ f reaction is reminiscent of that catalyzed by ubiquinol cytochrome c oxidoreductase in oxidative phosphorylation. Indeed, most components of the cytochrome b₆ f complex are similar to those of Q-cytochrome c oxidoreductase.

The oxidation of plastoquinol by the cytochrome b₆ f complex takes place by a mechanism nearly identical with the Q cycle of the electron-transport chain of cellular respiration. In photosynthesis, the net effect of the Q cycle is, first, to release protons from QH₂ into the lumen and, second, to pump protons from the stroma to the lumen, strengthening the protonmotive force (Figure 22.17). Thus, electrons from photosystem II are used to replace electrons in photosystem I and, in the process, augment the proton gradient. However, the electron-deficient P680 must now be replenished with electrons.

We experience the results of attaining photosynthetic redox balance with every breath that we take. The electron-deficient P680⁺ is a very strong oxidant and is capable of extracting electrons from water molecules, resulting in the release of oxygen. This reaction, the photolysis of water, takes place at the water-oxidizing complex (WOC), also called the manganese center. The core of this complex includes a calcium ion, four manganese ions, and four water molecules (Figure 22.18A). Manganese is especially suitable for photolyzing water because of its ability to exist in multiple oxidation states and to form strong bonds with oxygen-containing species.

In its reduced form, the WOC oxidizes two molecules of water to form a single molecule of oxygen. Each time the absorbance of a photon powers the removal of an electron from P680, the positively charged special pair extracts an electron from a tyrosine residue of subunit D1 of the WOC, forming a tyrosine radical (Figure 22.18B). The tyrosine radical then removes an electron from a manganese ion. This process occurs four times, with the result that H₂O is oxidized to generate O₂ and H⁺:

Four photons must be absorbed to extract four electrons from a water molecule. The four electrons harvested from water are used to reduce two molecules of Q to QH₂. All oxygenic phototrophs, the most common type of photosynthetic organism, use the same inorganic core and protein components for capturing the energy of sunlight. A single solution to the biochemical problem of extracting electrons from water evolved billions of years ago, and has been conserved for use under a wide variety of phylogenetic and ecological circumstances. The photolysis of water by photosynthetic organisms is essentially the sole source of oxygen in the biosphere. This gas, so precious to our lives, is in essence a waste product of photosynthesis that results from the need to achieve redox balance (Figure 22.19).

The cooperation between photosystem I and photosystem II creates electron flow—an electrical current—from H₂O to NADP⁺. The pathway of electron flow is called the Z scheme of photosynthesis because the redox diagram from P680 to P700* looks like the letter Z (Figure 22.20).

Although we have focused on the green plants, which use H₂O as the electron donor in photosynthesis, Table 22.1 shows that photosynthetic bacteria display a wide variety of electron donors.