Question

atrazine is an herbicide that prevents plant growth by inhibiting photosynthesis. atrazine works by binding to proteins in the electron transport chain of photosystem ii. once atrazine binds to the proteins, electrons can no longer travel down the chain to reach photosystem i. based on this information, how does atrazine stop plant growth?
a. it prevents chlorophyll from creating electrons, which are needed by the light - dependent reactions to make sugars.
b. it prevents the formation of atp and nadph, which are needed by the light - independent reactions to make sugars.
c. it prevents the formation of nadh and fadh₂, which are needed by the light - independent reactions to fix carbon.
d. it prevents water from releasing oxygen, which is needed by the light - dependent reactions to fix carbon.

Explanation:

1. Recall the process of photosynthesis: The light - dependent reactions of photosynthesis involve the electron transport chain. Photosystem II sends electrons down the chain, and these electrons eventually help in the production of ATP and NADPH.
2. Analyze the effect of atrazine: Atrazine binds to proteins in the electron transport chain of photosystem II, stopping electron flow. Without electron flow, the light - dependent reactions cannot produce ATP and NADPH.
3. Analyze the role of ATP and NADPH: ATP and NADPH are produced in the light - dependent reactions and are then used in the light - independent reactions (Calvin cycle) to make sugars (glucose) which are essential for plant growth.
4. Evaluate each option:

• Option A: Chlorophyll's role is to absorb light, not directly create electrons for sugar production in the way described. The electrons come from water splitting. So A is incorrect.
• Option B: Since atrazine stops electron flow in the light - dependent reactions, ATP and NADPH (which are needed for the light - independent reactions to make sugars) will not be produced. This matches the mechanism of how atrazine stops plant growth.
• Option C: NADH and FADH₂ are involved in cellular respiration, not photosynthesis. So C is incorrect.
• Option D: The release of oxygen from water is part of the light - dependent reactions, but preventing oxygen release is not how atrazine stops plant growth. Plant growth is stopped because sugar production (which needs ATP and NADPH) is halted. So D is incorrect.

Answer:

B. It prevents the formation of ATP and NADPH, which are needed by the light - independent reactions to make sugars.