The principal end products of photosynthesis are polymers of six-carbon sugars: starch and sucrose. The overall reaction of oxygen-generating photosynthesis is 6 CO2 + 6 H2O → 6 O2 + C6H12O6.
Chloroplasts are surrounded by a permeable outer membrane and an inner membrane that forms the permeability barrier; neither of these membranes participates in photosynthesis. In the chloroplast interior, the thylakoid membrane is folded into numerous flattened vesicles; the light-capturing and ATP-generating reactions of photosynthesis occur on this membrane (see Figure 16-34).
Photosynthesis in plants can be described in four stages, which occur in specific parts of the chloroplast.
In stage 1, 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 16-38). In green plants, the positively charged chlorophylls then remove electrons from water, forming oxygen.
In stage 2, electrons move from the quinone through a chain of electron transport molecules in the thylakoid membrane until they reach the ultimate electron acceptor, usually NADP+, reducing it to NADPH. Transport of electrons is coupled to the movement of protons across the membrane from the stroma to the thylakoid lumen, forming a pH gradient 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.
Chlorophyll a is the only light-absorbing pigment in reaction centers. 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 16-39).
