ICAR AIEEA PG Plant Biotechnology 14 July 2023 Shift-1 Paper

Ionization of polyprotic phosphoric acid ($\text{H}_3\text{PO}_4$):

• Stepwise dissociation: $\text{H}_3\text{PO}_4 \rightleftharpoons \text{H}^+ + \text{H}_2\text{PO}_4^-$, $K_{a1} = 7.1 \times 10^{-3}$, $\text{p}K_{a1} \approx 2.15$.
• Second dissociation: $\text{H}_2\text{PO}_4^- \rightleftharpoons \text{H}^+ + \text{HPO}_4^{2-}$, $K_{a2} = 6.3 \times 10^{-8}$, $\text{p}K_{a2} \approx 7.20$.
• Third dissociation: $\text{HPO}_4^{2-} \rightleftharpoons \text{H}^+ + \text{PO}_4^{3-}$, $K_{a3} = 4.2 \times 10^{-13}$, $\text{p}K_{a3} \approx 12.38$.
• Because each successive proton is removed from an increasingly negative species, $K_{a1} \gt K_{a2} \gt K_{a3}$, meaning $\text{p}K_{a1} \lt \text{p}K_{a2} \lt \text{p}K_{a3}$.

Catalytic perfection and diffusion-controlled enzymes:

• An enzyme is considered 'catalytically perfect' when its catalytic rate is limited only by the rate at which substrate can diffuse into its active site ($k_{\text{cat}}/K_m \approx 10^8 - 10^9 \text{ M}^{-1}\text{s}^{-1}$).
• In glycolysis, Triose Phosphate Isomerase (TIM or TPI) interconverts dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) at the diffusion limit ($k_{\text{cat}}/K_m = 2.4 \times 10^8 \text{ M}^{-1}\text{s}^{-1}$).
• Any further increase in its catalytic efficiency would have no effect on the reaction velocity because substrate diffusion is the sole bottleneck.

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.