Photosynthesis is more complicated in green plants than in photosynthetic bacteria. In green plants, photosynthesis depends on the interplay of two kinds of membrane-bound, light-sensitive complexes— photosystem I (PS I) and photosystem II (PS II), as shown in Figure 19.12. There are similarities in photosynthesis between green plants and photosynthetic bacteria. Both require light to energize reaction centers consisting of special pairs, called P700 for photosystem I and P680 for photosystem II, and both transfer electrons by using electron-transport chains. However, in plants, electron flow is not cyclic but progresses from photosystem II to photosystem I under most circumstances.

Photosystem I, which responds to light with wavelengths shorter than 700 nm, uses light-derived high-energy electrons to create biosynthetic reducing power in the form of NADPH, a versatile reagent for driving biosynthetic processes. The electrons for creating one molecule of NADPH are taken from two molecules of water by photosystem II, which responds to wavelengths shorter than 680 nm. A molecule of O₂ is generated as a side product of the actions of photosystem II. The electrons travel from photosystem II to photosystem I through cytochrome bf, a membrane-bound complex homologous to Complex III in oxidative phosphorylation. Cytochrome bf generates a proton gradient across the thylakoid membrane that drives the formation of ATP. Thus, the two photosystems cooperate to produce NADPH and ATP.

Photosystem II, an enormous transmembrane assembly of more than 20 subunits, catalyzes the light-driven transfer of electrons from water to plastoquinone. This electron acceptor closely resembles ubiquinone, a component of the mitochondrial electron-transport chain. Plastoquinone cycles between an oxidized form (Q) and a reduced form (QH₂, plastoquinol). The overall reaction catalyzed by photosystem II is

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The electrons in QH₂ are at a higher redox potential than those in H₂O. Recall that, in oxidative phosphorylation, electrons flow from ubiquinol to an acceptor, O₂, which is at a lower potential. Photosystem II drives the reaction in a thermodynamically uphill direction by using the free energy of light.

This reaction is similar to one catalyzed by the bacterial system in that a quinone is converted from its oxidized into its reduced form. Photosystem II is reasonably similar to the bacterial reaction center (Figure 19.13). The core of the photosystem is formed by D1 and D2, a pair of similar 32-kDa subunits that span the thylakoid membrane. These subunits are homologous to the L and M chains of the bacterial reaction center. Unlike the bacterial system, photosystem II contains a large number of additional subunits that bind more than 30 chlorophyll molecules altogether and increase the efficiency with which light energy is absorbed and transferred to the reaction center (Section 19.5).

The photochemistry of photosystem II begins with excitation of a special pair of chlorophyll molecules that are bound by the D1 and D2 subunits (Figure 19.14). Because the chlorophyll a molecules of the special pair absorb light at 680 nm, the special pair is often called P680. On excitation, P680 rapidly transfers an electron to a nearby pheophytin. From there, the electron is transferred first to a tightly bound plastoquinone at site Q_A and then to a mobile plastoquinone at site Q_B. This electro flow is entirely analogous to that in the bacterial system. With the arrival of a second electron and the uptake of two protons, the mobile plastoquinone is reduced to QH₂. At this point, the energy of two photons has been safely and efficiently stored in the reducing potential of QH₂.

The major difference between the bacterial system and photosystem II is the source of the electrons that are used to neutralize the positive charge formed on the special pair. P680⁺, a very strong oxidant, extracts electrons from water molecules bound 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 19.15A). Manganese was apparently evolutionarily selected for this role 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 (often designated Z) of subunit D1 of the WOC, forming a tyrosine radical (Figure 19.15B). 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 (Figure 19.16). 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.

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Photosystem II spans the thylakoid membrane such that the site of quinone reduction is on the side of the stroma, whereas WOC lies in the thylakoid lumen. Thus, the two protons that are taken up with the reduction of Q to QH₂ come from the stroma, and the four protons that are liberated in the course of water oxidation are released into the lumen. This distribution of protons generates a proton gradient across the thylakoid membrane characterized by an excess of protons in the thylakoid lumen compared with the stroma (Figure 19.17).

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Electrons flow from photosystem II to photosystem I through the cytochrome bf complex. This complex catalyzes the transfer of electrons from plastoquinol (QH₂) to plastocyanin (Pc), a small, soluble copper protein in the thylakoid lumen.

The two protons from plastoquinol are released into the thylakoid lumen. This reaction is reminiscent of that catalyzed by Complex III in oxidative phosphorylation, and most components of the cytochrome bf complex are homologous to those of Complex III. The cytochrome bf complex includes four subunits: a 23-kDa cytochrome with two b-type hemes, a 20-kDa Rieske-type Fe–S protein, a 33-kDa cytochrome f with a c-type cytochrome, and a 17-kDa chain.

This complex catalyzes the reaction by proceeding through the Q cycle (Figure 18.12). In the first half of the Q cycle, plastoquinol (QH₂) is oxidized to plastoquinone (Q), one electron at a time. The electrons from plastoquinol flow through the Fe–S protein to convert oxidized plastocyanin (Pc) into its reduced form.

In the second half of the Q cycle, cytochrome bf reduces a molecule of plastoquinone from the Q pool to plastoquinol, taking up two protons from one side of the membrane, and then reoxidizes plastoquinol to release these protons on the other side. The enzyme is oriented so that protons are released into the thylakoid lumen and taken up from the stroma, contributing further to the proton gradient across the thylakoid membrane (Figure 19.18).

The final stage of the light reactions is catalyzed by photosystem I, a transmembrane complex consisting of about 15 polypeptide chains and multiple associated proteins and cofactors (Figure 19.19). The core of this system is a pair of similar subunits, psaA (83 kDa, red) and psaB (82 kDa, blue), which bind 80 chlorophyll molecules as well as other redox factors. These subunits are quite a bit larger than the core subunits of photosystem II and the bacterial reaction center. Nonetheless, they appear to be homologous; the terminal 40% of each subunit is similar to a corresponding subunit of photosystem II. A special pair of chlorophyll a molecules lie at the center of the structure and absorb light maximally at 700 nm. This center, called P700, initiates photoinduced charge separation (Figure 19.20). The electron travels from P700 down a pathway through chlorophyll at site A₀ and quinone at site A₁ to a set of 4Fe–4S clusters. The next step is the transfer of the electron to ferredoxin (Fd), a soluble protein containing a 2Fe–2S cluster coordinated to four cysteine residues (Figure 19.21). Ferredoxin transfers electrons to NADP⁺. Meanwhile, P700⁺ captures an electron from reduced plastocyanin provided by photosystem II to return to P700 so that P700 can be excited again. Thus, the overall reaction catalyzed by photosystem I is a simple one-electron oxidation–reduction reaction.

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Given that the reduction potentials for plastocyanin and ferredoxin are +0.37 V and −0.45 V, respectively, the standard free energy for this reaction is +79.1 kJ mol⁻¹ (+18.9 kcal mol⁻¹). This uphill reaction is driven by the absorption of a 700-nm photon, which has an energy of 171 kJ mol⁻¹ (40.9 kcal mol⁻¹).

The reduced ferredoxin generated by photosystem I is a strong reductant that is used as an electron source for a variety of reactions, most notably the fixation of N₂ into NH₃ (Section 24.1). However, ferredoxin is not useful for driving many reactions, in part because ferredoxin carries only one available electron. In contrast, NADPH, a two-electron reductant, is a widely used electron donor in biosynthetic processes, including the reactions of the Calvin cycle (Chapter 20). How is reduced ferredoxin used to drive the reduction of NADP⁺ to NADPH? This reaction is catalyzed by ferredoxin–NADP⁺ reductase, a flavoprotein with an FAD prosthetic group (Figure 19.22A). The bound FAD moiety accepts two electrons and two protons from two molecules of reduced ferredoxin to form FADH₂ (Figure 19.22B). The enzyme then transfers a hydride ion (H⁻) to NADP⁺ to form NADPH. This reaction takes place on the stromal side of the membrane. Hence, the uptake of a proton in the reduction of NADP⁺ further contributes to the generation of the proton gradient across the thylakoid membrane.

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The cooperation between photosystem I and photosystem II creates a flow of electrons 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 19.23).

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