Photosynthetic Functions of Chlorophylls

Abstract

Chlorophylls (Chls) are the signature pigments of photosynthetic organisms and have several distinct functions, including photochemical activity and antenna function. Chls carry out reversible photochemical oxidations and reductions, which determine the basic mechanism of functioning of the photosynthetic reaction center (RC). The light-harvesting function of chlorophylls is based on their ability to absorb light over a wide spectral region. The variety of Chls (and bacteriochlorophylls) that are found in photosynthetic systems is formed by peripheral substitutions and reductions of the molecule’s macrocycle. Chls undergo specifi c adjustments of their absorption properties due to pigment-pigment and pigment-protein interactions. Complexes of RC supple- mented with light-harvesting antennas and additional electron transfer proteins are known as photosynthetic units (PSU). Despite the structural variety of the chlorophyll-based photosynthetic antennas known to date, the principles of the antenna design conform to fulfi lling its biological function, capturing the energy of sun and conveying it via excitation energy transfer to the reaction center. A number of very different strategies for energy collection, delivery to the RC and regulation can be found in different photosynthetic systems. Chls are also involved in photoprotective processes of excess excitation deactivation in carotenoid-Chl complexes and Chl clusters, accumulation of stress-related Chl binding polypeptides and specifi c photoprotective electron transfer pathways.