19.1 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 creation 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.

19.2 Light Absorption by Chlorophyll Induces Electron Transfer

Chlorophyll molecules 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 nearby electron acceptors. In photosynthesis, an excited electron leaves a pair of associated chlorophyll molecules known as the special pair. The functional core of photosynthesis, a reaction center from a photosynthetic bacterium, has been studied in great detail. In this system, the electron moves from the special pair (containing bacteriochlorophyll) to a bacteriopheophytin (a bacteriochlorophyll lacking the central magnesium ion) to quinones. The reduction of quinones leads to the generation of a proton gradient, which drives ATP synthesis in a manner analogous to that of oxidative phosphorylation.

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19.3 Two Photosystems Generate a Proton Gradient and NADPH in Oxygenic Photosynthesis

Photosynthesis in green plants is mediated by two linked photosystems. In photosystem II, the excitation of a special pair of chlorophyll molecules called P680 leads to electron transfer to plastoquinone in a manner analogous to that of the bacterial reaction center. The electrons are replenished by the extraction of electrons from a water molecule at the water-oxidizing complex, which contains four manganese ions. One molecule of O₂ is generated at this center for each four electrons transferred. The plastoquinol produced at photosystem II is reoxidized by the cytochrome bf complex, which transfers the electrons to plastocyanin, a soluble copper protein. From plastocyanin, the electrons enter photosystem I. In photosystem I, the excitation of special pair P700 releases electrons that flow to ferredoxin, a powerful reductant. Ferredoxin–NADP⁺ reductase, a flavoprotein, then catalyzes the formation of NADPH. A proton gradient is generated as electrons pass through photosystem II, through the cytochrome bf complex, and through ferredoxin–NADP⁺ reductase.

19.4 A Proton Gradient across the Thylakoid Membrane 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 the CF₀–CF₁ complex) closely resembles the ATP-synthesizing assemblies of bacteria and mitochondria. If the NADPH:NADP⁺ ratio is high, electrons transferred to ferredoxin by photosystem I can reenter the cytochrome bf complex. This process, called cyclic photophosphorylation, leads to the generation of a proton gradient by the cytochrome bf complex without the formation of NADPH or O₂.

19.5 Accessory Pigments Funnel Energy into Reaction Centers

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.

19.6 The Ability to Convert Light into Chemical Energy Is Ancient

The photosystems have structural features in common that suggest a common evolutionary origin. Similarities in organization and molecular structure to those of oxidative phosphorylation suggest that the photosynthetic apparatus evolved from an early energy-transduction system.