Question

Inhibitors of photosynthesis at photosystem I.

• A. Sulfonylureas
• B. Benzoic acids
• C. Diphenyl ethers
• D. Bipyridyls

Hint:

Recall which herbicide group diverts electrons directly at photosystem I.

Answer 1

The Correct Option is D

This question is about identifying which herbicide class stops photosynthesis by acting at photosystem I specifically.

1. Sulfonylureas: These block the acetolactate synthase enzyme needed to build branched chain amino acids, an action far removed from the photosynthetic electron transport chain.
2. Benzoic acids: Herbicides in this class, like dicamba, act as synthetic auxins that overstimulate and disrupt normal growth hormone signaling, not photosynthesis.
3. Diphenyl ethers: These block protoporphyrinogen oxidase, an enzyme in the chlorophyll making pathway, which causes damage only when light triggers a buildup of reactive molecules, a different mechanism from direct photosystem I action.
4. Bipyridyls: Compounds like paraquat grab electrons straight out of the photosystem I electron transport chain and pass them to oxygen, generating reactive oxygen species that destroy cell membranes almost immediately in sunlight.

Since only bipyridyls directly divert electrons at photosystem I, option 4 is the correct answer.

Let's summarize:

• Bipyridyls, such as paraquat and diquat, intercept electrons at photosystem I.
• The other three groups act on amino acid synthesis, hormone balance, or chlorophyll synthesis instead.

So the final answer is bipyridyls.