Cyclic Electron Transport System

Cylic reaction transport occurs only occasionally when synthesis of carbohydrate is curtailed due to limited supply of CO₂ and NADPH₂ starts accumulating. At this time, there would be no need for additional NADPH₂. Moreover, the cyclic electron transport serves the purpose of more production of ATP where needed by the chloroplast.

The cyclic electron transport pathway begins after the PS-I pigment complex absorbs solar energy. They transfer their energy to PS-I reaction centre - P₇₀₀. The outer valence electron of excited P₇₀₀ is raised to higher energy level which is captured by the primary acceptor of PS-I. The primary acceptor then transfers electron to ferredoxin.

Reduced ferredoxin, unable to reduce NADP⁺ returns the electron to PS-I via cyt b₆, PQ, cytf and PC.

The electron transport is called cyclic because the electron emitted from PS-I returns back to PS-I passing through several intermediate carriers.