Harvesting light

This chapter describes what photosynthetic light harvesting is, why it is needed, and how it works. Light absorption by two types of photosystems drives oxygenic photosynthesis. The light-harvesting capacity and efficiency of these photosystems depend on their structure, pigment composition, and number of pigments per photosystem. The structures of the photosystems are highly conserved throughout evolution. On the contrary, the antenna complexes, which are pigment-protein complexes that increase the light absorption, can be very different for different species. Light harvested by antenna pigments needs to be transferred from one pigment to the other to finally reach the reaction centre of the photosystem where the energy is used to drive charge separation. The rate of energy transfer depends on the distance, energy levels, and relative orientations of the pigments involved. The transfer of energy to the reaction centre competes with other excited-state decay processes in the pigment. As such, the quantum efficiency of excitation energy trapping is high when the transfer rate to the reaction centre is high, while the intrinsic decay rate of the pigment is low.