Q.42 Given below are two statements : One is labelled as Assertion (A) and the other is
labelled as Reason (R).
Assertion (A) :In preparatory phase of glycolysis phosphorylation of glucose takes
place and converts into glyaraldehyde-3-phosphate. Hexokinase
like many other kinases requires Mg” for activity.
Reasons (R) :In payoff phase of glycolysis, under aerobic condition two
molecules of NADH are formed in cytosol along with two
molecules of pyruvate and two molecules of ATP.
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
Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
Detailed Analysis
Assertion (A) is correct but factually imprecise: In the preparatory phase of glycolysis (steps 1-5), glucose is phosphorylated twice (hexokinase → G6P → PFK → F1,6BP), then cleaved by aldolase into DHAP + GAP (glyceraldehyde-3-phosphate). Hexokinase indeed requires Mg²⁺ as a cofactor to coordinate ATP’s phosphate with its aspartate residue.
Reason (R) is correct: In the payoff phase (steps 6-10), GAP dehydrogenase produces 2 NADH (GAP → 1,3BPG), followed by 2 pyruvate via pyruvate kinase, netting 2 ATP (under aerobic conditions, NADH shuttles to mitochondria).
However, R describes payoff phase products, completely unrelated to A’s preparatory phase phosphorylation.
Option Breakdown
-
(1) Both correct, R explains A: ❌ R discusses payoff products, not preparatory phosphorylation
-
(2) Both correct, R not explanation: ✅ Correct answer—both statements true, different pathway phases
-
(3) A correct, R incorrect: ❌ R accurately lists payoff stoichiometry
-
(4) A incorrect, R correct: ❌ A correctly describes hexokinase/Mg²⁺
Answer: (2)
Glycolysis Phase Separation (A vs R)
Preparatory Phase (Investment, Steps 1-5):
Glucose + 2 ATP → 2 Glyceraldehyde-3-P + 2 ADP
1. Hexokinase (Mg²⁺): Glucose → G6P
2. Phosphoglucose isomerase: G6P → F6P
3. PFK-1: F6P + ATP → F1,6BP
4. Aldolase: F1,6BP → DHAP + GAP
5. Triose phosphate isomerase: DHAP → GAP
Payoff Phase (R: Energy Extraction, Steps 6-10):
2 GAP + 2 NAD⁺ + 4 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 4 ATP
Net: 2 ATP + 2 NADH per glucose
Why No Explanatory Link
| Statement | Phase | Key Process | Products |
|---|---|---|---|
| A | Preparatory | Phosphorylation | 2 GAP (invests 2 ATP) |
| R | Payoff | Substrate phosphorylation | 2 NADH + 2 pyruvate |
GATE AR Strategy
Pattern recognition: A = energy investment (first half), R = energy payoff (second half). Tests pathway phase separation over biochemical detail linkage. Classic trap: assuming sequential steps = explanatory relationship.
