Question

DCMU is an inhibitor of the
a. PS-I
b. PS-II
c. Calvin Cycle
d. Krebs Cycle

Answer 1

Hint: DCMU \[\left( {3 - \left( {3,4 - dichlorophenyl} \right) - 1,1 - dimethylurea} \right)\] is an algaecide and herbicide which restrains photosynthesis in the aryl-urea class. In 1954, it was introduced under the trade name of Diuron by Bayer. DCMU will obstruct the transfer of electrons between the acceptor of the primary quinone electron and the acceptor of the secondary quinone electron on the reducer side.

Complete step by step answer:
DCMU is a photosynthesis inhibitor that is very precise and sensitive. It blocks photosystem II's QB plastoquinone binding site, preventing electron flow from photosystem II to plastoquinone. This inhibits the photosynthetic electron transport chain in photosynthesis and thus decreases the plant's capacity to transform light energy into chemical energy (ATP and reductant potential).

DCMU only blocks photosystem II electron flow; it has no effect on photosystem I or other photosynthesis reactions, such as light absorption or Calvin cycle carbon fixation.
A common herbicide is DCMU or \[\left( {3 - \left( {3,4 - dichlorophenyl} \right) - 1,1 - dimethylurea} \right)\]. DCMU is a photophosphorylation inhibitor. DCMU competes for the binding site in PS-II with plastoquinone (QB).

Hence, the correct answer is option (B).

Note: However, "linear" photosynthesis is effectively shut down because it prevents electrons generated from water oxidation in PSII from entering the plastoquinone reservoir. The explanation is that there are no electrons available to exit the photosynthetic electron flow cycle for NADP+ to NADPH reduction. In fact, it was found that not only does DCMU not inhibit the cyclic photosynthetic pathway, but activates it under certain circumstances. DCMU is also used for studying energy flows in photosynthesis because of these effects.