Which one of the following statements is correct?

Correct — C, Glucose breaks down into CO₂, H₂O and energy in all aerobic tissues. Complete oxidation to carbon dioxide and water is what respiration achieves when oxygen is available, and any tissue respiring aerobically ends there. The qualifier 'aerobic' is what makes this statement safe, and the absence of that qualifier is what sinks option (a).

• (a)Glucose breaks down to CO₂, H₂O and energy in all organisms — Not all organisms. Obligate anaerobes are poisoned by oxygen and never reach carbon dioxide and water, and yeast in the absence of air stops at ethanol. The claim is right for aerobic organisms and overreaches by covering every organism.
• (b)Glucose breaks down into ethanol, CO₂ and energy in our muscle cells — Muscle cells short of oxygen produce lactate, not ethanol. Ethanol is the yeast pathway, and the option has swapped the two organisms' end products.
• (d)Glucose breaks down into lactate and energy in yeast — Yeast fermenting anaerobically gives ethanol and carbon dioxide. Lactate is the animal muscle product, so this option is the mirror image of the same swap.

Every route starts the same way, with glycolysis splitting glucose into pyruvate in the cytoplasm. The fate of that pyruvate is what differs. With oxygen it enters the mitochondrion and is oxidised fully to carbon dioxide and water, releasing a great deal of energy. Without oxygen it is reduced instead — to lactate in animal muscle, and to ethanol with carbon dioxide in yeast.

Two of the four options are built from a single swap, which makes them useful to read as a pair: whichever way round they are stated, ethanol belongs to yeast and lactate to muscle. The anaerobic routes exist because glycolysis needs a supply of oxidised NAD to continue, and reducing pyruvate is how a cell regenerates it when the mitochondrial route is closed. The energy return is poor, which is why lactate builds up quickly in a sprinting muscle and why fermentation supports only slow growth.

• Aerobic respiration oxidises glucose completely to carbon dioxide and water, releasing about 30 to 32 ATP per glucose.
• Anaerobic respiration in animal muscle produces lactate; in yeast it produces ethanol and carbon dioxide.
• Glycolysis in the cytoplasm is common to all these routes and yields a net two ATP.
• Obligate anaerobes cannot survive in oxygen, so complete oxidation is not a universal fact about living things.
• The anaerobic pathways exist to regenerate the oxidised NAD that glycolysis needs to keep running.

• Swapping ethanol and lactate between yeast and muscle.
• Accepting an unqualified 'all organisms' when anaerobes exist.
• Assuming anaerobic respiration releases as much energy as aerobic; it releases only a small fraction.

A correct-statement item whose wrong options are built by over-generalising or by exchanging two end products. Check the quantifier in every option before the chemistry.

• practice — not a real PYQ

  During vigorous exercise, the compound that accumulates in human muscle when the oxygen supply falls short is

  • (a)ethanol
  • (b)lactic acid
  • (c)acetic acid
  • (d)pyruvic acid only

  Answer(b) lactic acid — pyruvate is reduced to lactate so that glycolysis can keep running without oxygen.

• practice — not a real PYQ

  Anaerobic respiration in yeast is the basis of

  • (a)brewing and baking
  • (b)curdling of milk
  • (c)photosynthesis
  • (d)nitrogen fixation

  Answer(a) brewing and baking — the ethanol matters to the brewer and the carbon dioxide to the baker.