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Stage-by-stage electron flow comparison between the respiratory chain (black descending path) and photosynthesis (gray pathway), with energy changes indicated. Ph, pheophytin; Qa and Qb, quinone carriers; cyt bf, cytochrome bf complex; Pc, plastocyanin; A₀ and A₁, electron acceptors; Fe-S, iron-sulfur center; Fd, ferredoxin; Fp, NADP reductase

Light energy absorbed at photosystem I (PS I) is used for ATP synthesis rather than NADPH synthesis. High-energy electrons generated by photon absorption are transferred to the cytochrome bf complex rather than to NADP⁺. At the cytochrome bf complex, electrons are transferred to a lower energy state and protons are pumped into the thylakoid lumen. The electrons are then returned to photosystem I by plastocyanin (PC).

The mobile electron carriers in the chain are plastoquinone (which closely resembles the ubiquinone of mitochondria), plastocyanin (a small copper-containing protein), and ferredoxin (a small protein containing an iron-sulfur center). The b₆-f complex closely resembles the b-c₁ complex of mitochondria and the b-c complex of bacteria (see Figure 14-61): all three complexes accept electrons from quinones and pump H⁺ across the membrane. Note that the H⁺ released by water oxidation and the H⁺ taken up during NADPH formation also contribute to the generation of the electrochemical H⁺ gradient, which drives ATP synthesis by an ATP synthase present in this same membrane (not shown).

The mobile electron carriers in the chain are plastoquinone (which closely resembles the ubiquinone of mitochondria), plastocyanin (a small copper-containing protein), and ferredoxin (a small protein containing an iron-sulfur center). The cytochrome b_6-f complex resembles the b-c₁ complex of mitochondria and the b-c complex of bacteria (see Figure 14-71): all three complexes accept electrons from quinones and pump H⁺ across the membrane. The H⁺ released by water oxidation into the thylakoid space, and the H⁺ consumed during NADPH formation in the stroma, also contribute to the generation of the electrochemical H⁺ gradient. This gradient drives ATP synthesis by an ATP synthase present in this same membrane (not shown here).