Which one of the following is not a breakdown product of glucose during respiration?

Correct — D, Methanol. Respiration begins with glycolysis in the cytoplasm, which splits the six-carbon glucose into two three-carbon pyruvate molecules. Pyruvate then takes one of three routes. Without oxygen in yeast it becomes ethanol and carbon dioxide, which is fermentation. Without enough oxygen in our muscle it becomes lactic acid, which is what cramps a sprinter. With oxygen it enters the mitochondrion and is oxidised completely to carbon dioxide and water. Methanol appears on none of these paths; it is an industrial alcohol, and in the body it is a poison rather than a product.

• (a)Ethanol — Ethanol is the fermentation product of pyruvate in yeast, along with carbon dioxide — the reaction behind brewing and behind the rising of bread dough.
• (b)Lactic acid — Lactic acid is what pyruvate becomes in muscle cells when oxygen runs short during hard exercise, and its build-up is felt as cramp.
• (c)Water — Water is a product of the complete aerobic oxidation of glucose, alongside carbon dioxide, and that route releases far more energy than either anaerobic path.

Glycolysis is common to all three routes and yields pyruvate; what happens next depends on oxygen and on the organism. Yeast without oxygen makes ethanol and carbon dioxide. Human muscle without enough oxygen makes lactic acid. Any aerobic tissue with oxygen sends pyruvate into the mitochondrion, where it is oxidised to carbon dioxide and water with a much larger yield of ATP.

Three of the four options sit on the standard NCERT diagram of the fates of pyruvate, so the item is a check on whether that diagram is remembered as a whole. The fourth option relies on the near-identical names of methanol and ethanol. They differ by one carbon and by their biology: yeast makes ethanol from sugar, while methanol is made industrially and is metabolised into formic acid, which is why swallowing it damages the optic nerve.

• Glycolysis takes place in the cytoplasm and converts glucose to pyruvate.
• In yeast, pyruvate becomes ethanol and carbon dioxide when oxygen is absent.
• In muscle cells short of oxygen, pyruvate becomes lactic acid.
• In aerobic respiration, pyruvate is oxidised in the mitochondrion to carbon dioxide and water.
• Aerobic respiration releases far more energy per glucose molecule than either anaerobic route.

• Reading methanol as a variant spelling of ethanol.
• Assuming lactic acid is produced by yeast and ethanol by muscle — the two are the other way round.
• Forgetting that water is a genuine product of aerobic respiration.

Set as a negative recall item on the fates of pyruvate; the sibling papers ask the same diagram positively, through the sprinter's cramp or the oxygen-starved yeast cell.

• practice — not a real PYQ

  Pyruvate is converted into ethanol and carbon dioxide in

  • (a)human muscle cells
  • (b)yeast under anaerobic conditions
  • (c)the mitochondria of all cells
  • (d)root nodules of legumes

  Answer(b) yeast under anaerobic conditions — this is alcoholic fermentation.

• practice — not a real PYQ

  The site of glycolysis in a cell is the

  • (a)mitochondrion
  • (b)nucleus
  • (c)cytoplasm
  • (d)chloroplast

  Answer(c) cytoplasm — only the later aerobic stages take place inside the mitochondrion.