Plant Biochemistry

Comparative energy yields of biomolecules:
Assertion (A) is false: Triacylglycerols (fats) are far more efficient energy storage molecules ($38\text{ kJ/g} \approx 9\text{ kcal/g}$) than carbohydrates ($17\text{ kJ/g} \approx 4\text{ kcal/g}$), because fats are stored in an anhydrous form.
Reason (R) is false: Carbohydrates are more oxidized (possessing many hydroxyl groups) than hydrocarbon-rich fatty acids; therefore, carbohydrates yield LESS energy upon oxidation, not more.
Because both Assertion (A) and Reason (R) are false, Option 4 is the correct answer.

Amphibolic nature of the TCA cycle:
Assertion (A) is true: The Krebs citric acid cycle functions in both catabolism (oxidation of acetyl-CoA to $\text{CO}_2$ and energy) and anabolism.
Reason (R) is true: Cycle intermediates serve as biosynthetic precursors (e.g., alpha-ketoglutarate for glutamate, oxaloacetate for aspartate and glucose, succinyl-CoA for porphyrins and heme, citrate for fatty acids).
Because utilizing intermediates for anabolic biosynthesis while catabolizing acetyl-CoA is the very definition of an amphibolic pathway, (R) is the correct explanation of (A).

Anaerobic respiration / alcoholic fermentation in higher plant tissues (e.g., flooded roots):

• Under anoxic conditions, glycolysis generates pyruvate and NADH.
• Pyruvate is decarboxylated by pyruvate decarboxylase to acetaldehyde, releasing $\text{CO}_2$.
• Acetaldehyde is then reduced by alcohol dehydrogenase (ADH) into Ethanol, regenerating $\text{NAD}^+$ to sustain glycolytic ATP generation.