AP BIOLOGY

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PHOTOSYNTHESIS

C 4 PHOTOSYNTHESIS

Question [CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
A researcher wanted to study light reaction during photosynthesis by blocking photosynthetic electron flow using the herbicide, dichlorophenyldimethylurea (DCMU) and Paraquat. The researcher listed the following observations A) Both DCMU and Paraquat block the electron flow in Photosystem IIB) Both DCMU and Paraquat block the electron flow in Photosystem IC) DCMU blocks electron flow in Photosystem I while paraquat blocks in Photosystem IID) DCMU blocks electron flow in Photosystem II while Paraquat blocks in Photosystem IWhich of the following combinations of the above statements is INCORRECT?
A
A, B and C
B
A, B and D
C
A, C and D
D
B, C and D
Explanation: 

Detailed explanation-1: -DCMU blocks photosynthetic electron flow from photosystem II to the PQ pool. DBMIB blocks photosynthetic and respiratory electron flow [which are intermingled in the thylakoid membrane of cyanobacteria (Schmetterer 1994)] from the PQ pool to cytochrome b6f (Fig.

Detailed explanation-2: -DCMU is a very specific and sensitive inhibitor of photosynthesis. It blocks the QB plastoquinone binding site of photosystem II, disallowing the electron flow from photosystem II to plastoquinone.

Detailed explanation-3: -DCMU kills the weed by inhibition of carbon dioxide fixation, as it is a strong inhibitor of photosystem-II. DCMU acts by blocking electron flow at the guanine acceptors of PS-II (water-plastoquinone oxidoreductase) pigment or pigment system, by attaching itself with the binding site called plastoquinone.

Detailed explanation-4: -DCMU is a very specific and sensitive inhibitor of photosynthesis. It blocks the plastoquinone binding site of photosystem II, disallowing the electron flow from where it is generated, in photosystem II, to plastoquinone.

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