Question

9) The herbicide 3-(3,4-chlorophenyl)-1,1,-dimethyl urea(DCMU) blocks the transport of electrons from photosystem II to the cytochrome b6-f complex.

  a) How will this affect a chloroplast’s ability to produce ATP? To produce NADPH?

  b) why the above-mentioned herbicide D CMU an effective herbicide?

Answer 1

a) DCMU also known as diuron is a potent photosynthetic inhibitor which blocks electron flow at the quinone acceptor of photosystem II (PSII). Plastoquinone B is a hydrophobic electron carrier that transfers electron from plastoquinone A to cytochrome b6f complex. DCMU competes for the binding site of plastoquinone that is normally occupied by plastoquinone as a result water oxidation is blocked, proton gradient is not established and NADP+ is not converted in NADPH, which indicates NADPH and ATP are not produced.

b) DCMU is an effective herbicide because it acts as a competitive inhibitor. Competitive inhibitor binds with enzyme's active site hence inhibitor competes with substrate for binding site. In case of DCMU it competes for the binding site of plastoquinone as a result electron flow gets blocked and no synthesis of high energy compounds, ATP and NADPH occurs.