Chloroplasts Contain Two Functionally and Spatially Distinct Photosystems

In the 1940s, biophysicist R. Emerson discovered that the rate of plant photosynthesis generated by light of wavelength 700 nm can be greatly enhanced by adding light of shorter wavelength. He found that a combination of light at, say, 600 and 700 nm supports a greater rate of photosynthesis than the sum of the rates for the two separate wavelengths. This so-called Emerson effect led researchers to conclude that photosynthesis in plants involves the interaction of two separate photosystems, referred to as PSI and PSII. PSI is driven by light of wavelength 700 nm or less; PSII, only by light of shorter wavelength <680 nm.

As in the reaction center of green and purple bacteria, each chloroplast photosystem contains a pair of specialized reaction-center chlorophyll a molecules, which are capable of initiating photoelectron transport. The reaction-center chlorophylls in PSI and PSII differ in their light-absorption maxima because of differences in their protein environment. For this reason, these chlorophylls are often denoted P680

▲ FIGURE 8-37 Linear electron flow in plants, which requires both chloroplast photosystems, PSI and PSII. Blue arrows Indicate flow of electrons; red arrows Indicate proton movement. LHCs are not shown. (Left) In the PSII reaction center, two sequential light-Induced excitations of the same P680 chlorophylls result in reduction of the primary electron acceptor Qb to QH2. On the luminal side of PSII, electrons removed from H2O in the thylakoid lumen are transferred to P680+, restoring the reaction-center chlorophylls to the ground state and generating O2. (Center) The cytochrome bf complex then accepts electrons from QH2, coupled to the release of two protons into the lumen. Operation of a Q cycle in the cytochrome bf complex

(PSII) and P700 (PSI). Like a bacterial reaction center, each chloroplast reaction center is associated with multiple light-harvesting complexes (LHCs); the LHCs associated with PSII and PSI contain different proteins.

The two photosystems also are distributed differently in thylakoid membranes: PSII primarily in stacked regions (grana) and PSI primarily in unstacked regions. The stacking of the thylakoid membranes may be due to the binding properties of the proteins in PSII. Evidence for this distribution came from studies in which thylakoid membranes were gently fragmented into vesicles by ultrasound. Stacked and unstacked thylakoid vesicles were then fractionated by density-gradient centrifugation. The stacked fractions contained primarily PSII protein and the un-stacked fraction, PSI.

Finally, and most importantly, the two chloroplast photosystems differ significantly in their functions: only PSII splits water to form oxygen, whereas only PSI transfers electrons to the final electron acceptor, NADP+. Photosynthesis in chloroplasts can follow a linear or cyclic pathway, again like green and purple bacteria. The linear pathway, which we discuss first, can support carbon fixation as well as ATP synthesis. In contrast, the cyclic pathway supports only ATP synthesis and generates no reduced NADPH for use in carbon fixation. Photosynthetic algae and cyanobacteria contain two photosystems analogous to those in chloroplasts.

translocates additional protons across the membrane to the thylakoid lumen, increasing the proton-motive force generated. (Right) In the PSI reaction center, each electron released from light-excited P700 chlorophylls moves via a series of carriers in the reaction center to the stromal surface, where soluble ferredoxin (an Fe-S protein) transfers the electron to FAD and finally to NADP+, forming NADPH. P700+ is restored to its ground state by addition of an electron carried from PSII via the cytochrome bf complex and plastocyanin, a soluble electron carrier. The protonmotive force generated by linear electron flow from PSII to NADP-FAD reductase powers ATP synthesis by the F0Fn complex.

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