Key Concepts of Section 12.6

• The principal end products of photosynthesis in plants are molecular oxygen and polymers of six-carbon sugars (starch and sucrose).

• The light-capturing and ATP-generating reactions of photosynthesis occur in the thylakoid membrane located within chloroplasts. The permeable outer membrane and the inner membrane surrounding chloroplasts do not participate directly in photosynthesis (see Figures 12-37 and 12-38).

• There are four stages in photosynthesis: (1) absorption of light, generation of high-energy electrons, and formation of O₂ from H₂O; (2) electron transport leading to reduction of NADP⁺ to NADPH, and to generation of a proton-motive force; (3) synthesis of ATP; and (4) conversion of CO₂ into carbohydrates (carbon fixation).

• In stage 1 of photosynthesis, light energy is absorbed by one of two “special-pair” chlorophyll a molecules bound to reaction-center proteins in the thylakoid membrane. The energized chlorophylls donate, via intermediates, an electron to a quinone on the opposite side of the membrane, creating a charge separation (see Figure 12-41). In green plants, the positively charged chlorophylls then remove electrons from water, forming molecular oxygen (O₂).

• In stage 2, electrons are transported from the reduced quinone via intermediates associated with the thylakoid membrane until they reach the ultimate electron acceptor, usually NADP⁺, reducing it to NADPH. Electron transport is coupled to movement of protons across the membrane from the stroma to the thylakoid lumen, forming a pH gradient (proton-motive force) across the thylakoid membrane.

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• In stage 3, movement of protons down their electrochemical gradient through F₀F₁ complexes (ATP synthase) powers the synthesis of ATP from ADP and P_i.

• In stage 4, the NADPH and ATP generated in stages 2 and 3 provide the energy and the electrons to drive the fixation of CO₂, which results in the synthesis of carbohydrates. These reactions occur in the thylakoid stroma and cytosol.

• Associated with each reaction center are multiple internal antennas and external light-harvesting complexes, which contain chlorophylls a and b, carotenoids, and other pigments that absorb light at multiple wavelengths. Energy, but not an electron, is transferred from the internal antenna and LHC chlorophyll molecules to the special-pair chlorophylls in the reaction center by resonance energy transfer (see Figure 12-42).