Free Mock Test 2026
Free NEET Respiration: Important MCQs Practice Test — Practice Questions
1. The respiratory pathway is called amphibolic because it:
- (A) Works both with and without oxygen
- (B) Produces both ATP and NADH
- (C) Occurs in both the cytoplasm and the mitochondria
- (D) Is involved in both the breakdown and the synthesis of biomolecules
Answer: (D) Is involved in both the breakdown and the synthesis of biomolecules
Respiratory intermediates are used to build other molecules: acetyl CoA can be used for fatty acid synthesis and TCA intermediates for amino acids. So the pathway is both catabolic and anabolic.
2. The conversion of pyruvic acid to ethanol or lactic acid in fermentation is important mainly because it:
- (A) Produces more ATP than glycolysis
- (B) Fixes atmospheric CO₂
- (C) Supplies acetyl CoA to the TCA cycle
- (D) Regenerates NAD⁺ so that glycolysis can continue
Answer: (D) Regenerates NAD⁺ so that glycolysis can continue
Glycolysis needs NAD⁺. In fermentation NADH is oxidised back to NAD⁺ while pyruvic acid is reduced, so glycolysis keeps running without oxygen.
3. A germinating seed shows a respiratory quotient (RQ) of about 0.7. The respiratory substrate is most likely:
- (A) Fat
- (B) Carbohydrate
- (C) Organic acid
- (D) Protein
Answer: (A) Fat
RQ is 1 for carbohydrates, about 0.9 for proteins, and about 0.7 for fats, which need more oxygen per CO₂ released. Organic acids have an RQ above 1.
4. During electron transport in mitochondria, protons accumulate in the:
- (A) Matrix
- (B) Cytosol
- (C) Intermembrane (perimitochondrial) space
- (D) Inner membrane lipid layer
Answer: (C) Intermembrane (perimitochondrial) space
Protons are pumped across the inner membrane into the intermembrane space. They return to the matrix through the F₀ channel of ATP synthase, which drives ATP synthesis in the F₁ headpiece.
5. Assertion (A): Aerobic respiration releases far more energy from glucose than fermentation. Reason (R): In aerobic respiration, pyruvic acid is completely oxidised to CO₂ and H₂O, and electrons pass through the ETS to oxygen.
- (A) Both A and R are true, but R is not the correct explanation of A
- (B) Both A and R are true, and R is the correct explanation of A
- (C) A is true, but R is false
- (D) A is false, but R is true
Answer: (B) Both A and R are true, and R is the correct explanation of A
Fermentation releases less than 7 per cent of the energy in glucose and leaves much of it in ethanol or lactic acid. Complete oxidation and oxidative phosphorylation give a much larger ATP yield.
6. In the TCA cycle, the 2-carbon acetyl group of acetyl CoA condenses with oxaloacetic acid to form:
- (A) Succinic acid
- (B) Malic acid
- (C) Citric acid
- (D) α-Ketoglutaric acid
Answer: (C) Citric acid
Acetyl CoA (2C) joins oxaloacetic acid (4C) to form citric acid (6C), catalysed by citrate synthase. This is the first step of the cycle.
7. In glycolysis, ATP is formed by substrate-level phosphorylation during the conversion of:
- (A) Glucose to glucose-6-phosphate
- (B) Fructose-6-phosphate to fructose-1,6-bisphosphate
- (C) 1,3-Bisphosphoglyceric acid to 3-PGA and PEP to pyruvic acid
- (D) Dihydroxyacetone phosphate to PGAL
Answer: (C) 1,3-Bisphosphoglyceric acid to 3-PGA and PEP to pyruvic acid
ATP is produced when 1,3-BPGA becomes 3-phosphoglyceric acid and when PEP becomes pyruvic acid. The first two steps listed use ATP rather than make it.
8. When fat is used as the respiratory substrate, glycerol enters the glycolytic pathway as:
- (A) Pyruvic acid
- (B) Acetyl CoA
- (C) PGAL (3-phosphoglyceraldehyde)
- (D) Oxaloacetic acid
Answer: (C) PGAL (3-phosphoglyceraldehyde)
Fats are broken down into glycerol and fatty acids. Glycerol is converted to PGAL and enters glycolysis, while fatty acids are converted to acetyl CoA for the TCA cycle.
9. The link reaction (oxidative decarboxylation of pyruvic acid) produces, per molecule of glucose:
- (A) 1 acetyl CoA, 1 CO₂ and 1 NADH
- (B) 2 acetyl CoA, 2 CO₂ and 2 NADH
- (C) 2 acetyl CoA, 4 CO₂ and 4 NADH
- (D) 2 acetyl CoA, 2 CO₂ and 2 FADH₂
Answer: (B) 2 acetyl CoA, 2 CO₂ and 2 NADH
One glucose gives 2 pyruvic acid. Each pyruvic acid yields 1 acetyl CoA, 1 CO₂ and 1 NADH in the mitochondrial matrix, so the totals are doubled.
10. What is the net gain of ATP and NADH from glycolysis per molecule of glucose?
- (A) 4 ATP and 2 NADH
- (B) 2 ATP and 2 NADH
- (C) 2 ATP and 4 NADH
- (D) 4 ATP and 4 NADH
Answer: (B) 2 ATP and 2 NADH
Glycolysis uses 2 ATP and produces 4 ATP, a net gain of 2 ATP. Two NADH are formed and 2 molecules of pyruvic acid are produced.
11. One turn of the TCA cycle produces:
- (A) 3 NADH, 1 FADH₂ and 1 GTP (ATP)
- (B) 2 NADH, 2 FADH₂ and 1 ATP
- (C) 4 NADH, 1 FADH₂ and 2 ATP
- (D) 3 NADH, 2 FADH₂ and no ATP
Answer: (A) 3 NADH, 1 FADH₂ and 1 GTP (ATP)
Each turn gives 3 NADH, 1 FADH₂ and 1 GTP (converted to ATP). Per glucose, the cycle runs twice, giving 6 NADH, 2 FADH₂ and 2 ATP.
12. Which pair of enzymes converts pyruvic acid to ethanol and CO₂ during alcoholic fermentation in yeast?
- (A) Pyruvic acid decarboxylase and alcohol dehydrogenase
- (B) Lactate dehydrogenase and pyruvate kinase
- (C) Pyruvate dehydrogenase and citrate synthase
- (D) Hexokinase and aldolase
Answer: (A) Pyruvic acid decarboxylase and alcohol dehydrogenase
Pyruvic acid decarboxylase removes CO₂ to give acetaldehyde, and alcohol dehydrogenase reduces it to ethanol using NADH.
13. A plant tissue is treated with cyanide, which blocks cytochrome c oxidase. What will happen first?
- (A) Electron flow through Complex I increases
- (B) Electron flow stops, NADH is not reoxidised and ATP synthesis by ATP synthase falls
- (C) The proton gradient across the inner membrane becomes steeper
- (D) The TCA cycle speeds up because NADH accumulates
Answer: (B) Electron flow stops, NADH is not reoxidised and ATP synthesis by ATP synthase falls
Without the final step to oxygen, electron transport stops, and proton pumping and ATP synthase activity drop. NADH builds up and the TCA cycle slows because NAD⁺ is not regenerated.
14. As per NCERT, the oxidation of one NADH and one FADH₂ through the ETS yields how many ATP molecules, respectively?
- (A) 3 and 2
- (B) 2 and 3
- (C) 4 and 2
- (D) 2 and 2
Answer: (A) 3 and 2
NCERT uses 3 ATP for each NADH and 2 ATP for each FADH₂ in the balance sheet of aerobic respiration.
15. Which component carries electrons from Complex III to Complex IV in the mitochondrial electron transport system?
- (A) Ubiquinone
- (B) NADH dehydrogenase
- (C) Succinate dehydrogenase
- (D) Cytochrome c
Answer: (D) Cytochrome c
Ubiquinone carries electrons from Complexes I and II to Complex III. Cytochrome c, a small protein on the outer face of the inner membrane, passes them from Complex III to cytochrome c oxidase (Complex IV).