Lactic acid is formed in the muscles during exercise or running. It is formed:

Correct — C, to give extra energy from glucose. Running hard raises a muscle's demand for energy faster than the lungs and circulation can deliver oxygen to it. Glucose is broken down in the cytoplasm to pyruvate by a pathway that needs no oxygen at all. When oxygen is plentiful, that pyruvate goes into the mitochondria and is oxidised completely to carbon dioxide and water, releasing a large amount of energy. When oxygen runs short, the mitochondrial route stalls, and the muscle converts pyruvate into lactic acid instead. The yield is small — around two molecules of ATP per glucose against the far larger return from complete oxidation — but it keeps arriving without oxygen, and that is exactly what a sprinting muscle needs. Lactic acid is therefore a by-product of a shortcut taken to keep energy coming from glucose, and its build-up is what is associated with cramp.

• (a)to give extra energy from fat. — Fat cannot be broken down without oxygen. Its oxidation runs entirely through the mitochondria, which is the route that has stalled when lactic acid starts to appear.
• (b)to give extra oxygen from lungs. — Reverses the situation. Lactic acid forms because oxygen is in short supply, and no chemistry in the muscle can manufacture oxygen to make up the shortfall.
• (d)to give extra energy from vitamins. — Vitamins are not fuels. They act as coenzymes and regulators in milligram quantities and release no useful energy on breakdown.

Every cell begins respiration the same way, by splitting glucose into pyruvate in the cytoplasm without using oxygen. What happens next depends on the oxygen supply. With oxygen, pyruvate enters the mitochondria and is broken down to carbon dioxide and water with a large release of energy. Without it, the cell must dispose of pyruvate some other way — muscle cells reduce it to lactic acid, while yeast converts it to ethanol and carbon dioxide, which is the basis of fermentation.

The wording invites a wrong reading. Lactic acid is not made in order to produce energy; it is what is left when the oxygen-free route is used to keep energy coming. The question rewards the student who can name the fuel, and the fuel is glucose in every version of this pathway. Fat is the giveaway distractor, because fat is genuinely the body's main fuel at rest and over long distances — but only while oxygen is available, which is precisely the condition that has failed here. Muscles that cramp during a sprint are muscles that have gone anaerobic.

• Glycolysis splits glucose into pyruvate in the cytoplasm and requires no oxygen.
• With enough oxygen, pyruvate is oxidised in the mitochondria to carbon dioxide and water with a large energy yield.
• Without enough oxygen, muscle converts pyruvate into lactic acid, releasing about two molecules of ATP per glucose.
• Accumulated lactic acid in muscle is associated with cramps during vigorous exercise, and it is later cleared once oxygen is restored.
• Yeast takes the other anaerobic branch, turning pyruvate into ethanol and carbon dioxide.
• Fat cannot be respired anaerobically, since its breakdown runs entirely through the oxygen-dependent mitochondrial pathway.

• Reading lactic acid as something the body makes in order to release energy, rather than as the by-product of the route it was forced to take.
• Choosing fat as the fuel, when fat cannot be respired without oxygen.
• Placing glycolysis in the mitochondria; it happens in the cytoplasm.

Asked as a cause-and-purpose item where three options name the wrong fuel or reverse the role of oxygen, so identifying glucose as the substrate settles it.

• practice — not a real PYQ

  In yeast, the anaerobic breakdown of glucose produces

  • (a)lactic acid and water
  • (b)ethanol and carbon dioxide
  • (c)carbon dioxide and water only
  • (d)acetic acid and oxygen

  Answer(b) ethanol and carbon dioxide — the fermentation branch, which is why yeast is used in brewing and why bread dough rises.

• practice — not a real PYQ

  Glycolysis, the breakdown of glucose to pyruvate, takes place in the

  • (a)mitochondria
  • (b)nucleus
  • (c)cytoplasm
  • (d)ribosome

  Answer(c) cytoplasm — only the later oxygen-dependent stages move into the mitochondria.