NDA (I) 2025 General Ability Test · Question 106 of 150

Correct — B, Pyruvate. During hard exercise the muscles run short of oxygen, so glucose is first broken down by glycolysis into pyruvate, and that pyruvate is then converted into lactic acid (by the enzyme lactate dehydrogenase). It is the build-up of this lactic acid that produces the ache and cramps. The direct, immediate precursor of the lactic acid is therefore pyruvate, not glucose itself.

• (a)ATP — ATP is the cell's energy currency produced during respiration; it is not the raw material that is turned into lactic acid. Lactic-acid formation actually helps regenerate the coenzyme needed to keep making a little ATP, but ATP is not its source.
• (c)Ethanol — Ethanol is the end product of anaerobic respiration in yeast and some plants, not in human muscle. Human muscle converts pyruvate to lactic acid, not to ethanol.
• (d)Glucose — Glucose is the ultimate starting fuel, but it is not converted straight into lactic acid. Glucose is first broken down to pyruvate in glycolysis; the pyruvate is the immediate precursor of lactic acid, which is what the question asks for.

Cells release energy from glucose by respiration. Glycolysis, the first stage, splits one glucose into two molecules of pyruvate in the cytoplasm. If oxygen is plentiful the pyruvate is fully oxidised (aerobic respiration); if oxygen is short, as in a hard-working muscle, the pyruvate is instead converted to lactic acid (anaerobic respiration).

The question hinges on the word 'from' — it wants the direct precursor, not the original fuel. Trace the pathway: glucose → pyruvate → lactic acid. Glucose is one step too far back; the molecule that is actually turned into lactic acid is pyruvate. Recognising that human muscle (not yeast) makes lactic acid also rules out ethanol.

• Glycolysis breaks one glucose into two pyruvate molecules in the cytoplasm.
• In oxygen-short muscle, pyruvate is converted to lactic acid (anaerobic respiration).
• Yeast, by contrast, converts pyruvate to ethanol and carbon dioxide (fermentation).
• Accumulated lactic acid lowers muscle pH and causes fatigue and cramps.

• Choosing glucose — it is the fuel, but the direct precursor of lactic acid is pyruvate.
• Choosing ethanol — that is the yeast pathway, not human muscle.

Respiration is asked by pathway step or by end product — learn the glucose to pyruvate to lactic-acid/ethanol chain.

• practice — not a real PYQ

  In yeast, anaerobic respiration converts pyruvate into

  • (a)Lactic acid and oxygen
  • (b)Ethanol and carbon dioxide
  • (c)Glucose and water
  • (d)ATP and oxygen

  Answer(b) Ethanol and carbon dioxide — the fermentation pathway used by yeast.

• practice — not a real PYQ

  Glycolysis, the first stage of respiration, takes place in the

  • (a)Nucleus
  • (b)Mitochondrial matrix
  • (c)Cytoplasm
  • (d)Ribosome

  Answer(c) Cytoplasm — glucose is split into pyruvate in the cytoplasm.