SUMMARY

• 22.1 Photosynthesis Takes Place in Chloroplasts

  The proteins that participate in the light reactions of photosynthesis are located in the thylakoid membranes of chloroplasts. The light reactions result in (1) the formation of reducing power for the production of NADPH, (2) the generation of a transmembrane proton gradient for the formation of ATP, and (3) the production of O₂.

• 22.2 Photosynthesis Transforms Light Energy into Chemical Energy

  Chlorophyll molecules—tetrapyrroles with a central magnesium ion—absorb light quite efficiently because they are polyenes. An electron excited to a high-energy state by the absorption of a photon can move to a nearby electron acceptor. In photosynthesis, an excited electron leaves a pair of associated chlorophyll molecules known as the special pair located at the reaction center. Light-harvesting complexes that surround the reaction centers contain additional molecules of chlorophyll a, as well as carotenoids and chlorophyll b molecules, which absorb light in the center of the visible spectrum. These accessory pigments increase the efficiency of light capture by absorbing light and transferring the energy to reaction centers through resonance energy transfer.

• 22.3 Two Photosystems Generate a Proton Gradient and NADPH

  Photosynthesis in green plants is mediated by two linked photosystems. In photosystem I, the excitation of special-pair P700 releases electrons that flow to ferredoxin, a powerful reductant. Ferredoxin-NADP⁺ reductase catalyzes the formation of NADPH. In photosystem II, the excitation of a special pair of chlorophyll molecules called P680 leads to electron transfer to plastocyanin, which replenishes the electrons removed from photosystem I. A proton gradient is generated as electrons pass through photosystem II, through the cytochrome b₆ f complex and through ferredoxin-NADP⁺ reductase. The electrons extracted from photosystem II are replenished by the extraction of electrons from a water molecule at a center containing four manganese ions. One molecule of O₂ is generated at this center for each four electrons transferred.

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• 22.4 A Proton Gradient Drives ATP Synthesis

  The proton gradient across the thylakoid membrane creates a proton-motive force used by ATP synthase to form ATP. The ATP synthase of chloroplasts (also called CF₁-CF₀) closely resembles the ATP-synthesizing assembly of mitochondria (F₁-F₀). If the NADPH/NADP⁺ ratio is high, electrons transferred to ferredoxin by photosystem I can reenter the cytochrome b₆ f complex. This process, called cyclic photophosphorylation, leads to the generation of a proton gradient by the cytochrome b₆ f complex without the formation of NADPH or O₂.