The redox potential of the NADP+/NADPH pair (–320 mV) is already very low, and reduction of NADP+ therefore requires a compound with an even lower redox potential. This turns out to be a chlorophyll molecule near the stromal membrane surface of photosystem I that has a redox potential of –1000 mV (chlorophyll A0), making it the strongest known electron donor in biology. The reduced NADPH is released into the chloroplast stroma, where it is used for biosynthesis of glyceraldehyde 3-phosphate, amino acid precursors, and fatty acids, much of it to be exported to the cytoplasm.
Figure 14–50 Electron flow through the cytochrome b6-f complex to NADPH.
the cytochrome b6-f complex is the functional equivalent of cytochrome c reductase (the cytochrome b-c1 complex) in mitochondria (see Figure 14–22). Like its mitochondrial homolog, the b6-f complex receives its electrons from a quinone and engages in a complicated Q cycle that pumps two protons across the membrane (details not shown). It hands its electrons, one at a time, to plastocyanin (pC). plastocyanin diffuses along the membrane surface to photosystem I and transfers the electrons via ferredoxin (Fd) to the ferredoxin-NaDp+ reductase (FNr), where they are utilized to produce NaDph. p700 is a special pair of chlorophylls that absorbs light of wavelength 700 nm.
Figure 14–51 Structure and function of photosystem I. at the heart of the photosystem I complex assembly is the electron-transfer chain shown. at one end is a special pair of chlorophylls called p700 (because it absorbs light of 700 nm wavelength), receiving electrons from plastocyanin (pC). at the other end are the a0 chlorophylls, which hand the electrons on to ferredoxin via two plastoquinones (pQ; purple) and three iron–sulfur clusters. Even though the roles of photosystems I and II in photosynthesis are very different, their central electron-transfer chains are structurally similar, indicating a common evolutionary origin (see Figure 14–53).  Note that in photosystem I both branches of the electron-transfer chain are active, unlike in photosystem II (see Figure 14–48). (pDB code: 3LW5.)
Figure 14–52 Summary of electron and proton movements during photosynthesis in the thylakoid membrane. Electrons are withdrawn, through the action of light energy, from a water molecule that is held by the manganese cluster in photosystem II. the electrons pass on to plastoquinone, which delivers them to the cytochrome b6-f complex that resembles the cytochrome c reductase of mitochondria and the b-c complex of bacteria. they are then carried to photosystem I by the soluble electron carrier plastocyanin, the functional equivalent of cytochrome c in mitochondria. From photosystem I they are transferred to ferredoxin-NaDp+ reductase (FNr) by the soluble carrier ferredoxin (Fd; a small protein containing an iron–sulfur center). protons are pumped into the thylakoid space by the cytochrome b6-f complex, in the same way that protons are pumped into mitochondrial cristae by cytochrome c reductase (see Figure 14–21). In addition, the h+ released into the thylakoid space by water oxidation, and the h+ consumed during NaDph formation in the stroma, contribute to the generation of the electrochemical h+ gradient across the thylakoid membrane. as illustrated, this gradient drives atp synthesis by an atp synthase that sits in the same membrane (see Figure 14–47).
the Chloroplast atp Synthase Uses the proton Gradient Generated by the photosynthetic Light reactions to produce atp