In many ways, water is an ideal source of electrons for photo-synthesis. Water is so abundant within cells that it is always available to serve as an electron donor in photosynthesis. In addition, O₂, the by-product of pulling electrons from water, diffuses readily away rather than accumulates. However, from an energy perspective, water is a challenging electron donor: It takes a great deal of energy to pull electrons from water. The amount of energy that a single photosystem can capture from sunlight is not enough both to pull an electron from water and produce an electron donor capable of reducing NADP+. The solution is to use two photosystems arranged in series. The energy supplied by the first photosystem allows electrons to be pulled from water, and the energy supplied by the second photosystem step allows electrons to be transferred to NADP+.

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If you follow the flow of electrons from water through both photosystems and on to NADP+, as shown in Fig. 8.13, you can see a large increase in energy as the electrons pass through each of the two photosystems. You can also see that at every other step along the photosynthetic electron transport chain there is a small decrease in energy. This decrease in energy indicates that these are exergonic reactions (Chapter 6) and thus explains why electrons move in one “direction” through the series of redox reactions that make up the photosynthetic electron transport chain. To run these reactions in the opposite direction would require an input of energy. Because the overall energy trajectory has an up-down-up configuration resembling a “Z,” the photosynthetic electron transport chain is sometimes referred to as the Z scheme.

For the two photosystems to work together to move electrons from water to NADPH, they must have distinct chemical properties. Photosystem II supplies electrons to the beginning of the electron transport chain. When photosystem II loses an electron (that is, when it is itself oxidized), it is able to pull electrons from water. In contrast, photosystem I energizes electrons with a second input of light energy so they can be used to reduce NADP+. The key point here is that photosystem I when oxidized is not a sufficiently strong oxidant to split water, whereas photosystem II is not a strong enough reductant to form NADPH.

The major protein complexes of the photosynthetic electron transport chain include the two photosystems as well as the cytochrome-b₆ f complex (cyt), through which electrons pass between photosystem II and photosystem I (Fig. 8.14). Small, relatively mobile compounds convey electrons between these protein complexes. Plastoquinone (Pq), a lipid-soluble compound similar in structure to coenzyme Q (Chapter 7), carries electrons from photosystem II to the cytochrome–b₆ f complex by diffusing through the membrane, while plastocyanin (Pc), a water-soluble protein, carries electrons from the cytochrome–b₆ f complex to photosystem I by diffusing through the thylakoid lumen.

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Water donates electrons to one end of the photosynthetic electron transport chain, whereas NADP+ accepts electrons at the other end. The enzyme that pulls electrons from water, releasing both H+ and O₂, is located on the lumen side of photosystem II. The mechanism by which water splitting occurs is not known, despite the considerable industrial value of developing a way to use sunlight to generate hydrogen gas (H₂). NADPH is formed when electrons are passed from photosystem I to a membrane-associated protein called ferredoxin (Fd) (Fig. 8.14b). The enzyme ferredoxin–NADP+ reductase then catalyzes the formation of NADPH by transferring two electrons from two molecules of reduced ferredoxin to NADP+ as well as a proton from the surrounding solution:

NADP+ + 2e⁻ + H+ → NADPH

Quick Check 4 Why are two photosystems needed if H₂O is used as an electron donor?