Q75.Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A:
Animals cannot convert fatty acids to sugars.
Reason R:
Plants possess glyoxylate cycle.
In the light of the above statements, choose the most appropriate answer from the options given below:
(1) Both A and R are correct and R is the correct explanation of A
(2) Both A and R are correct but R is not the correct explanation of A
(3) A is correct but R is not correct
(4) A is not correct but R is correct
The correct answer is option (2): Both A and R are correct but R is not the correct explanation of A.
Statement Analysis
Assertion A is true—animals cannot achieve net synthesis of glucose from fatty acids, as β-oxidation yields acetyl-CoA, which enters the TCA cycle where carbons are lost as CO₂ before reaching gluconeogenic precursors like oxaloacetate (no net gain beyond 2 carbons). Reason R is true—plants possess the glyoxylate cycle (in glyoxysomes), bypassing TCA decarboxylations via isocitrate lyase and malate synthase to convert 2 acetyl-CoA → 1 succinate (4C) for gluconeogenesis (e.g., seed germination). However, R does not explain A: the assertion holds due to animals’ biochemical inability (TCA-only path), while R describes a plant-specific solution—contrast, not causation.
Option Breakdown
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Option (1): Wrong—R true but doesn’t causally justify animal limitation (plants’ presence highlights difference, not “why” animals can’t).
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Option (2): Correct—both facts accurate; animal defect is absence of glyoxylate enzymes, unrelated to plant possession phrasing.
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Option (3): Wrong—R factual (plants have it).
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Option (4): Wrong—A standard biochemistry principle.
Introduction: Animals Cannot Convert Fatty Acids to Sugars Explained
NEET metabolism tests gluconeogenesis limits via assertions like “animals cannot convert fatty acids to sugars” vs. plant glyoxylate cycle. Both true but unrelated causally (option 2), key to fat-carbohydrate interconversion. This breaks down acetyl-CoA fate, plant bypass, and exam traps.
Why Animals Can’t: Net Carbon Loss
Fatty acids → acetyl-CoA (β-oxidation) → TCA cycle:
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Isocitrate → α-KG (CO₂ loss).
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α-KG → succinyl-CoA (CO₂ loss).
No net >2C for PEP → glucose; label experiments show transient ¹⁴C in glucose but no mass increase.
Plants’ Glyoxylate Cycle: Succinate Shortcut
Glyoxysomes host:
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Isocitrate lyase: Isocitrate → glyoxylate + succinate.
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Malate synthase: Glyoxylate + acetyl-CoA → malate.
Net: 2 acetyl-CoA → succinate (gluconeogenic).
NEET Key: Option (2) No Causal Link
R contrasts, doesn’t “explain” animal incapacity (that’s enzyme absence). Trap: Misreading as direct cause.
Fatty Acid Fate Table
NEET Tip: Mnemonic—”Animals TCA-Trapped, Plants Glyoxylate-Glucose.” Link to seed oil mobilization.
