Cyclic Photophosphorylation

Abstract

Cyclic photophosphorylation can be defined as the synthesis of ATP coupled to electron transport activated by Photosystem I solely, and can therefore proceed in long-wave-length light (03BB 2265 700 nm). This proces is unaffected by the specific inhibitors of Photosystem II, such as CMU, DCMU and orthophenanthroline. The current interpretation of cyclic phosphorylation is that the light-activated electrons of chlorophyll aI are used to reduce an acceptor, the nature of which is still uncertain, and then a sequence of electron carriers transport the electron down the electrochemical gradient, back to the oxidizing side of photosystem I. ATP formation is coupled to this electron transport. Experimentally, cyclic phosphorylation can therefore be distinguished from any other kind of chloroplast photophosphorylation by two criteria: 1) the resistence to CMU, DCMU and orthophenanthroline; 2) the fact that light absorbed by photosystem I at the far-red end of chlorophyll a absorption spectrum is sufficient to activate the reaction.