Light-harvesting complexes of pigments and proteins enable photosynthetic organisms to use a broad range of the solar spectrum and to grow even at low light intensities. Following absorption of a photon by an antenna pigment, the exciton moves from pigment to pigment via so-called resonance energy transfer, until becoming trapped in a photosynthetic reaction center.
A variety of light-harvesting complexes exist, differing in different taxonomic groups. Plants have an inner antenna of chlorophyll-a-binding proteins and an outer antenna of chlorophyll-a/b (CAB) binding proteins. The inner antenna, typically containing 80-100 chlorophyll-a molecules, can be an integral part of the reaction center, as in the case of photosystem I, or be composed of two separate chlorophyll-a binding proteins associated with photosystem II. The CAB proteins bind half of the thylakoid chlorophyll-a and all of the chlorophyll-b, and they comprise an outer antenna of several hundred chlorophyll molecules. The various CAB proteins associated with photosystem II or photosystem I are integral membrane proteins, in the 25-kDa molecular weight range, with three transmembrane-spanning regions. Each protein has been estimated to bind up to 14 chlorophyll and one carotenoid molecules.
Cyanobacteria and red algae use water-soluble protein structures, known as phycobilisomes, as outer light-harvesting antennae. Purple bacteria have two types of light-harvesting proteins (LHI and LHII). They are composed of small proteins that span the membrane once and bind bacteriochlorophylls and carotenoids, and they form ring-like structures around the reaction center. Green sulfur bacteria have pigment assemblies known as chlorosomes. For further details, see Photosynthesis.
