Question

Given below are two statements :
one is labelled as
Assertion (A) and the other is labelled as
Reason (R).
Assertion (A) :
Some microorganisms can use nitrate as a terminal electron acceptor under anaerobic conditions.
Reason (R) :
Nitrate can be fully reduced to ammonia in the process of denitrification.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Remember: \[ \text{Denitrification} \Rightarrow NO_3^- \rightarrow N_2 \] while: \[ NO_3^- \rightarrow NH_3 \] occurs in: \[ \text{Dissimilatory nitrate reduction to ammonia (DNRA)} \]
• Nitrate can serve as a terminal electron acceptor in anaerobic respiration.

Answer 1

The Correct Option is C

Concept: Under anaerobic conditions, certain microorganisms perform anaerobic respiration using compounds other than oxygen as terminal electron acceptors. One important alternative electron acceptor is: \[ \text{Nitrate } (NO_3^-) \] This process occurs in:
• Denitrification
• Dissimilatory nitrate reduction During denitrification, nitrate is typically reduced stepwise to gaseous nitrogen products such as: \[ NO_3^- \rightarrow NO_2^- \rightarrow NO \rightarrow N_2O \rightarrow N_2 \]

Step 1: Evaluating Assertion (A). The assertion states: \[ \text{Some microorganisms use nitrate as terminal electron acceptor under anaerobic conditions} \] This statement is correct because many anaerobic bacteria perform nitrate respiration. Therefore: \[ \boxed{(A) \text{ is correct}} \]

Step 2: Evaluating Reason (R). The reason states: \[ \text{Nitrate can be fully reduced to ammonia in denitrification} \] This statement is incorrect because:
• Denitrification usually converts nitrate into gaseous nitrogen compounds.
• Reduction of nitrate to ammonia occurs in a different pathway called: \[ \text{Dissimilatory nitrate reduction to ammonia (DNRA)} \] Hence: \[ \boxed{(R) \text{ is incorrect}} \]

Step 3: Selecting the correct option. Since: \[ (A) \text{ is true and } (R) \text{ is false} \] the correct answer is: \[ \boxed{(C)\ (A)\text{ is correct but }(R)\text{ is not correct}} \]