Question

Define photophosphorylation and describe cyclic phosphorylation.

Answer 1

Phosphorylation is a process that involves the addition of phosphate to an organic compound. Examples include the addition of phosphate to glucose to produce glucose monophosphate and the addition of phosphate to adenosine diphosphate (ADP) to form adenosine triphosphate (ATP).

It is a process of photophosphorylation in which an electron expelled by the excited photo-centre is returned to it after passing through a series of electron carriers. It occurs under conditions of low light intensity, wave­length longer than 680 nm and when carbon fixation is inhibited. The absence of CO2 fixation results in non-requirement of electrons for for­mation of NADPH. Cyclic photophosphoryla­tion is performed by photosystem I only. Its photo-centre P700 extrudes an electron with a gain of 23 kcal/mole of energy after absorb­ing a photon of light (hv). After losing the electron the photo-centre becomes oxidized.