Question

Discuss the flow of electrons in the non-cyclic pathway.

Answer 1

During the flow of electrons various cofactors of the electron transfer chain to be oxidized or reduced. Molecules that in their oxidized form are difficult to reduce have a high relative potential once they have accepted electrons. Plastoquinone receive electrons from pheophytin and pass them to the intermediate electron carriers which includes the plastoquinone pool and the cytochromes b and f associated in a complex with an iron-sulphur protein. When NADP+ and a suitable enzyme are present, two ferrodoxin molecules, carrying one electron each , transfer two electrons to NADP+ , which picks up a proton and becomes NADPH. In noncyclic electron flow pathway photophosphorylation of ADP occurs. In contrast to electron flow in light reactions I and II which can occur in memebrane fragments , intacts thylakoid are required for efficient photophosphorylation.

When photosystem II absorbs light excited P680 is captured by the primary electron acceptor. The oxidized chlorophyll acts as a oxidizing agent and splits water molecule into 2 hydrogen ions and an oxygen atom which later on forms O2. Each excited electron transfers into photosystem II to photosystem I via an electron transport chain. By chemiosmosis ATP produced which is then called non-cyclic photophosphorylation. Under certain conditions electrons take an alternative path called cyclic electron flow which uses p700 but not p680. The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle , NADPH will accumulate and the plant may shift from non-cyclic to cyclic electron flow pathway.