FIGURE 12-42 Light-harvesting complexes and photosystems in cyanobacteria and plants. (a) Diagram of the membrane of a cyanobacterium, in which each multiprotein light-harvesting complex (LHC) contains 90 chlorophyll molecules and 31 other small molecules, all held in a specific geometric arrangement for optimal light absorption and energy transfer. Of the six chlorophyll molecules in the reaction center, two constitute the special-pair chlorophylls that can initiate photoelectron transport (blue arrow) when excited. Resonance transfer of energy (red arrows) rapidly funnels energy from absorbed light to one of two “bridging” chlorophylls and thence to the special-pair chlorophylls in the reaction center. (b) Three-dimensional organization of photosystem I (PSI) and its associated LHCs from Pisum sativum (garden pea), as determined by x-ray crystallography, seen from the plane of the membrane. Only the chlorophylls and the reaction-center electron carriers are shown. (c) Expanded view of the reaction center from (b), rotated 90° about a vertical axis. See W. Kühlbrandt, 2001, Nature 411:896, and P. Jordan et al., 2001, Nature 411:909.

[Parts (b) and (c) data from A. Ben-Sham et al., 2003, Nature 426:630, PDB ID 1qvz; and Y. Mazor, A. Borovikova, and N. Nelson, 2015, Elife 4:e07433, PDB ID 4y28.]