Key Concepts Of Section

Photosynthetic Stages and Light-Absorbing Pigments

■ The principal end products of photosynthesis in plants are 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 inner membrane surrounding chloroplasts do not participate in photosynthesis (see Figure 8-30).

■ In stage 1 of photosynthesis, light is absorbed by chlorophyll a molecules bound to reaction-center proteins in the thylakoid membrane. The energized chlorophylls donate an electron to a quinone on the opposite side of the membrane, creating a charge separation (see Figure 8-33). In green plants, the positively charged chlorophylls then remove electrons from water, forming oxygen.

■ In stage 2, electrons are transported from the reduced quinone via carriers in 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.

■ In stage 3, movement of protons down their electrochemical gradient through F0F1 complexes powers the synthesis of ATP from ADP and Pi.

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

■ Associated with each reaction center are multiple light-harvesting complexes (LHCs), which contain chlorophylls a and b, carotenoids, and other pigments that absorb light at multiple wavelengths. Energy is transferred from the LHC chlorophyll molecules to reaction-center chlorophylls by resonance energy transfer (see Figure 8-34).

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