Q.41 Given below are two statements, one is labelled as Assertion (A) and the other is labelled as Reason (R):
Assertion (A): During starvation, when β-oxidation of fatty acids is predominant in liver, acetylCoA formed enters citric acid cycle.
Reason (R): AcetylCoA must combine with oxaloacetate to gain entry to the citric acid cycle.
In the light of the above statements, choose the most appropriate answer from the options given below.
- Both (A) and (R) are correct and (R) is the correct explanation of (A)
- Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- (A) is correct but (R) is not correct
- (A) is not correct but (R) is correct
Assertion (A) is not correct, but Reason (R) is correct. The correct option is: (A) is not correct but (R) is not correct.
Detailed Explanation
During starvation, liver mitochondria ramp up β-oxidation of fatty acids, producing excess acetyl-CoA. However, this acetyl-CoA cannot efficiently enter the citric acid cycle (TCA cycle) due to oxaloacetate depletion from gluconeogenesis. Instead, it diverts to ketone body synthesis.
Reason (R) states a true biochemical fact: Acetyl-CoA entry into the TCA cycle requires condensation with oxaloacetate, catalyzed by citrate synthase, to form citrate.
Option Analysis
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Both (A) and (R) correct, (R) explains (A): Incorrect. (A) is false as acetyl-CoA does not predominantly enter TCA during starvation; (R) does not explain it.
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Both (A) and (R) correct, but (R) not explanation: Incorrect. (A) is false despite true (R).
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(A) correct, (R) not: Incorrect. (A) false; (R) true.
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(A) not correct, (R) correct: Correct. Matches the metabolic reality in liver during starvation.
Introduction: β-Oxidation in Starvation and Citric Acid Cycle Entry
In starvation, liver β-oxidation of fatty acids generates excess acetyl-CoA, but it rarely enters the citric acid cycle due to limited oxaloacetate. This Assertion Reason question tests understanding of this metabolic shift, crucial for exams on metabolism.
Key Metabolic Pathway
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β-oxidation breaks fatty acids into acetyl-CoA units in liver mitochondria during starvation.
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Acetyl-CoA needs oxaloacetate for citrate synthase reaction to join TCA cycle.
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Gluconeogenesis consumes oxaloacetate, blocking TCA entry; acetyl-CoA forms ketones instead.
Assertion Reason Breakdown
| Statement |
Correct? |
Why? |
| A: β-oxidation acetyl-CoA enters TCA in liver starvation |
No |
Diverts to ketogenesis due to oxaloacetate deficit. |
| R: Acetyl-CoA requires oxaloacetate for TCA entry |
Yes |
Forms citrate; first TCA step. |
Exam Options Evaluated
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Option 1: Both correct, R explains A → False; A incorrect.
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Option 2: Both correct, R not explanation → False; A incorrect.
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Option 3: A correct, R not → False; R true.
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Option 4: A incorrect, R correct → Right answer.
This concept links fatty acid metabolism, gluconeogenesis, and TCA regulation—key for molecular biology exams.
