Key Concepts Of Section

Molecular Analysis of Photosystems

■ In the single photosystem of purple bacteria, cyclic electron flow from light-excited chlorophyll a molecules in the reaction center generates a proton-motive force, which is used mainly to power ATP synthesis by the FqFj complex in the plasma membrane (Figure 8-36).

M FIGURE 8-40 Distribution of multiprotein complexes in the thylakoid membrane and the regulation of linear versus cyclic electron flow. (Top) In sunlight, PSI and PSII are equally activated, and the photosystems are organized in state I. In this arrangement, light-harvesting complex II (LHCII) is not phosphorylated and is tightly associated with the PSII reaction center in the grana. As a result, PSII and PSI can function in parallel in linear electron flow. (Bottom) When light excitation of the two photosystems is unbalanced, LHCII becomes phosphorylated, dissociates from PSII, and diffuses into the unstacked membranes, where it associates with PSI and its permanently associated LHCI. In this alternative supramolecular organization (state II), most of the absorbed light energy is transferred to PSI, supporting cyclic electron flow and ATP production but no formation of NADPH and thus no CO2 fixation. PC = plastocyanin. [Adapted from F. A. Wollman, 2001, EMBO J. 20:3623.]

■ Plants contain two photosystems, PSI and PSII, which have different functions and are physically separated in the thylakoid membrane. PSII splits H2O into O2. PSI reduces NADP+ to NADPH. Cyanobacteria have two analogous photosystems.

■ In chloroplasts, light energy absorbed by light-harvesting complexes (LHCs) is transferred to chlorophyll a molecules in the reaction centers (P680 in PSII and P700 in PSI).

■ Electrons flow through PSII via the same carriers that are present in the bacterial photosystem. In contrast to the bacterial system, photochemically oxidized P680+ in PSII is regenerated to P680 by electrons derived from the splitting of H2O with evolution of O2 (see Figure 8-37, left).

■ In linear electron flow, photochemically oxidized P700+ in PSI is reduced, regenerating P700, by electrons transferred from PSII via the cytochrome bf complex and soluble plastocyanin. Electrons lost from P700 following excitation of PSI are transported via several carriers ultimately to NADP+, generating NADPH (see Figure 8-37, right).

■ In contrast to linear electron flow, which requires both PSII and PSI, cyclic electron flow in plants involves only PSI. In this pathway, neither NADPH nor O2 is formed, although a proton-motive force is generated.

■ The proton-motive force generated by photoelectron transport in plant and bacterial photosystems is augmented by operation of the Q cycle in cytochrome bf complexes associated with each of the photosystems.

■ Reversible phosphorylation and dephosphorylation of the PSII light-harvesting complex control the functional organization of the photosynthetic apparatus in thylakoid membranes. State I favors linear electron flow, whereas state II favors cyclic electron flow (see Figure 8-40).

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