Chlorophyll and photosynthesis

Abstract

Photosynthesis is the prerequisite of all life on earth. Chlorophyll fulfils the requirements for photosynthesis: the absorption of visible light, the photochemical capabilities, a rich supply of redox levels and chemical stability. The biosynthetic pathway of chlorophyll can be read as the evolutionary history of photosynthesis. Our exegesis is that the primary porphyrins served for early photosynthesis. The porphyrins readily photo-oxidize organic compounds under the reducing, aqueous conditions of this early era. The formation of oxidized substances in a reducing atmosphere supplied the thermodynamic gradient necessary for organized life processes. Conversely, the closed-shell metalloporphyrins, notable magnesium porphyrins, are powerful photoreducing agents. When coupled to the ultimate electron source, water, oxygen was produced and the modern era of photosynthesis was born. At the same time, the efficiency and usefulness of the photopigments was increased by incorporating them into the organized cellular system of membranes. The clear gradient of ionic to hydrophophic structures along the biosynthetic pathway from porphyrins to chlorophyll supports this view. Experimental evidence on the photochemistry of porphyrins pigments in solution and in lipid bilayers form the basis for these arguments. In this way we can relate the structure of chlorophyll to its function in photosynthesis.