So far, we have considered only how the photosynthetic electron transport chain leads to the formation of NADPH. However, we know that the Calvin cycle also requires ATP. In chloroplasts, as in mitochondria, ATP is synthesized by ATP synthase, a transmembrane protein powered by a proton gradient (Chapter 7). In chloroplasts, the ATP synthase is oriented such that the synthesis of ATP is the result of the movement of protons from the thylakoid lumen to the stroma, as shown in Fig. 8.14c.

How do protons accumulate in the thylakoid lumen? Two features of the photosynthetic electron transport chain are responsible for the buildup of protons in the thylakoid lumen (Fig. 8.14c). First, the oxidation of water releases protons and O₂ into the lumen. Second, the cytochrome–b₆ f complex, the protein complex situated between photosystem II and photosystem I, and plastoquinone together function as a proton pump that is functionally and evolutionarily related to proton pumping in the electron transport chain of cellular respiration (Chapter 7).

In photosynthesis, the proton pump involves: (1) the transport of two electrons and two protons, by the diffusion of plastoquinone, from the stroma side of photosystem II to the lumen side of the cytochrome–b₆ f complex and (2) the transfer of electrons within the cytochrome–b₆ f complex to a different molecule of plastoquinone, which results in additional protons being picked up from the stroma and subsequently released into the lumen.

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Together, these mechanisms are quite powerful. When the photosynthetic electron transport chain is operating at full capacity, the concentration of protons in the lumen can be more than 1000 times greater than their concentration in the stroma (equivalent to a difference of 3 pH units). This accumulation of protons on one side of the thylakoid membrane can then be used to power the synthesis of ATP by oxidative phosphorylation as described in Chapter 7.