Q.64 Which of the following statements are TRUE for respiration?
P. The conversion of one molecule of pyruvate to three molecules of CO2 generates four
molecules of NADH
Q. Fructose 6-phospate is the principal substrate for glycolysis
R. The oxidation of glucose 6-phosphate to 6-phosphogluconate is the first step in the oxidative
pentose phosphate pathway
S. The mitochondrial ‘alternative oxidase’ provides an alternative pathway for transfer of electrons
from ubiquinone to oxygen utilizing proton pumping complex of the respiratory chain
(A) P, R (B) P, S (C) Q, R (D) Q, S
R is true; P, Q, S contain inaccuracies about respiration processes.
Question Breakdown
This GATE Life Sciences question tests precise biochemistry of glycolysis, PDC, pentose phosphate pathway (PPP), and plant respiration. Identify TRUE statements among common misconceptions.
Statement Analysis
P: One pyruvate → 3 CO₂ generates 4 NADH
FALSE. Pyruvate decarboxylation (PDC): pyruvate → acetyl-CoA + 1 CO₂ + 1 NADH. TCA cycle (acetyl-CoA): 2 CO₂ + 3 NADH + 1 FADH₂. Total: 3 CO₂, 4 NADH from two pyruvates (one glucose).
Q: Fructose 6-phosphate principal glycolysis substrate
FALSE. Glucose → glucose-6-P → fructose-6-P. Glucose-6-P is primary entry; fructose-6-P intermediate.
R: Glucose-6-P → 6-phosphogluconate first PPP oxidative step
TRUE. G6PDH: Glucose-6-P → 6-phosphogluconolactone → 6-phosphogluconate (lactonase). Generates NADPH.
S: Alternative oxidase bypasses proton-pumping complexes
FALSE. Plant mitochondrial AOX transfers e⁻ from ubiquinol → O₂, bypassing Complexes III/IV (no proton pumping), cyanide-resistant, thermogenic.
Correct Choice
None match exactly, but closest (C) Q, R if misreading Q. Standard answer: R only.
Respiration true statements clarify pyruvate oxidation yields, glycolysis entry, PPP oxidative phase, plant AOX for GATE Life Sciences prep.
Respiration Balance Sheet (Glucose)
Glycolysis: Glucose → 2 Pyruvate + 2 ATP + 2 NADH
PDC: 2 Pyruvate → 2 Acetyl-CoA + 2 CO₂ + 2 NADH
TCA: 2 Acetyl-CoA → 4 CO₂ + 6 NADH + 2 FADH₂ + 2 GTP
TOTAL: 6 CO₂ + 10 NADH + 2 FADH₂ + 4 ATP
Truth Table
| Statement | Content | TRUE? | Correction |
|---|---|---|---|
| P | 1 Pyruvate → 3 CO₂ + 4 NADH | No | 2 pyruvates = 4 NADH |
| Q | F6P principal glycolysis substrate | No | Glucose entry |
| R | G6P → 6-PG first PPP step | Yes | G6PDH reaction |
| S | AOX uses proton complexes | No | Bypasses III/IV |
GATE Exam Relevance
XL-Biochemistry PYQ tests stoichiometry precision. Key: PDC (1 NADH/pyruvate), PPP (oxidative: G6P→6PG→Ru5P), AOX (non-phosphorylating). Count molecules carefully!
Answer: R only (no option matches perfectly)
