Question

DCMU inhibits the ability of the primary electron acceptor for Photosystem II to reduce plastoquinone. Explain how this affects the Hill reaction and how it affects photosynthesis. Be VERY specific. (2)  

Answer 1

DCMU belongs to arylurea class and used as algicide and herbicide to inhibit photosynthesis.

It blocks the transfer of electrons between primary quinone (Q_A) electron acceptor and secondary quinone (Q_B) electron acceptor on the reducing site of photosystem II and thus interrupt the electron transport from plastaquinone (Q_B)to cyt b6f complex. Because of this Electron transport chain of the photosynthesis mechanism is inhibited.

DCMU directly inhibits the reaction centre and the acceleration of deactivation of certain intermediate states of enzymes that splits water and thus reduces the generation of oxygen from the splitting of water molecules from photosystem II. It doesn't have much effect on Photosystem I but it supress the reduction of NADP⁺ to NADPH.