In the 1940s, biophysicist R. Emerson discovered that the rate of plant photosynthesis generated by light of wavelength 700 nm could be greatly enhanced by adding light of shorter wavelengths (higher energy). He found that a combination of light at, say, 600 and 700 nm supports a rate of photosynthesis higher 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 700 nm or less; PSII, only by shorter-wavelength light (<680 nm).
In chloroplasts, the special-pair reaction-center chlorophylls that initiate photoelectron transport in PSI and in PSII differ in their light-absorption maxima because of differences in their protein environments. For this reason, these chlorophylls are often denoted P680 (PSII) and P700 (PSI). Like a bacterial reaction center, each chloroplast reaction center is as sociated with multiple internal antennas and light-harvesting complexes; the LHCs associated with PSII (e.g., LHCII) and with PSI (e.g., LHCI) contain different proteins. Furthermore, the two photosystems are distributed differently in thylakoid membranes: PSII primarily in regions of stacked thylakoids (grana) and PSI primarily in un stacked regions. The stacking of thylakoid membranes may be due to the binding properties of the proteins associated with PSII, especially LHCII.
Finally, and most important, the two chloroplast photo systems differ significantly in their functions (Figure 1): only PSII oxidizes water to form molecular oxygen, whereas only PSI transfers electrons to the final electron acceptor, NADP+. Photosynthesis in chloroplasts can follow a linear or a cyclical pathway through these photosystems. The linear pathway, which we discuss first, can support carbon fixation as well as ATP synthesis. In contrast, the cyclical path way 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. Similar proteins and pigments compose photosystems I and II in plants and in photosynthetic bacteria.
Fig1. 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 special-pair P680 chlorophyll result in a two-step reduction of the primary electron acceptor QB to QH2. On the luminal side of PSII, electrons removed from H2O by the oxygen-evolving complex are transferred to P680 +, restoring the reaction-center chlorophylls to the ground state after each excitation. The oxygen-evolving complex contains a cluster of four manganese ions (Mn, violet), a Ca2+ ion (green), and a Cl− ion (teal). These bound ions function in the split ting of H2O and maintain the environment essential for high rates of O2 evolution. Four sequential light-induced excitations of the P680 (double the number illustrated here) are required to oxidize two water molecules and release four protons and one molecule of molecular oxygen (O2). A tyrosine on the protein helps conducts electrons from the Mn ions to the oxidized reaction-center chlorophyll (P680 +), reducing it to the ground state (P680) after excitation by each photon. (Center) The cytochrome bf complex then accepts electrons from QH2 and transports two protons into the lumen. Operation of a Q cycle in the cytochrome bf complex translocates additional protons across the membrane to the thylakoid lumen, increasing the proton-motive force. (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 ferredoxin-NADP+ reductase (FNR). This enzyme uses the prosthetic group flavin adenine dinucleotide (FAD) and a proton to reduce 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.
