Internal Antennas and Light-Harvesting Complexes Increase the Efficiency of Photosynthesis
Although the special-pair chlorophyll a molecules within the reaction center that are involved directly in charge separation and electron transfer are capable of directly absorbing light and initiating photosynthesis, they are most commonly energized indirectly by energy transferred to them from other light-absorbing and energy-transferring pigments. These other pigments, which include many other chlorophylls, absorb photons and pass the energy to the special-pair chlorophylls (Figure 12-42). Some of these pigments are bound to protein subunits that are considered to be intrinsic components of the photosystem, which is made up of several distinct protein chains, and thus are called internal antennas. Others are incorporated into protein complexes that bind to, but are distinct from, the photosystem core proteins and are called light-harvesting complexes (LHCs). Even at the maximum light intensity encountered by photosynthetic organisms (tropical noontime sunlight), each reaction-center chlorophyll a molecule absorbs only about one photon per second, which is not enough to support photosynthesis sufficient for the needs of the plant. The involvement of internal antennas and LHCs greatly increases the efficiency of photosynthesis, especially at more typical light intensities, by increasing absorption of 680-nm light and by extending the range of wavelengths of light that can be absorbed by other antenna pigments.
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.]
Photosystem core proteins and LHC proteins maintain the pigment molecules in the precise orientations and positions that are optimal for light absorption and rapid (<10−9 seconds) energy transfer, called resonance transfer, to one of the special-pair chlorophyll a molecules in the associated reaction center. Resonance energy transfer does not involve the transfer of an electron. Studies on one of the two photosystems in cyanobacteria, which are similar to those in multicellular, seed-bearing plants, suggest that energy from absorbed light is funneled first to a “bridging” chlorophyll in each LHC and then to the special-pair chlorophylls in the reaction center (see Figure 12-42a). Surprisingly, however, the molecular structures of LHCs from plants and cyanobacteria are completely different from those from green and purple bacteria, even though both types contain carotenoids and chlorophylls. Figure 12-42b shows the distribution of the chlorophyll pigments in photosystem I from Pisum sativum (garden pea) together with those from peripheral LHC antennas. The large number of internal and LHC antenna chlorophylls that surround the reaction center permit efficient transfer of absorbed light energy to the special-pair chlorophylls in the reaction center.
Although LHC antenna chlorophylls can transfer light energy absorbed from a photon, they cannot release an electron. As we’ve seen already, this function resides in the two reaction-center chlorophylls. To understand their electron-releasing ability, we examine the structure and function of the reaction center in bacterial and plant photosystems in the next section.