IGCSE · Practice questions · Anaerobic respiration

Less energy, and a debt to pay.

Six original Cambridge-style questions comparing aerobic and anaerobic respiration, writing the yeast and muscle equations, and explaining the oxygen debt from exercise data.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two equations, one debt.

Anaerobic: breaks down nutrients to release energy without oxygen; much less energy per glucose.
Yeast: glucose → alcohol + carbon dioxide (Extended: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂).
Muscles: glucose → lactic acid. No carbon dioxide.
Oxygen debt (Extended): fast heart rate carries lactic acid to the liver; deep fast breathing supplies oxygen; the liver respires lactic acid aerobically.

Recall

[3 marks]

Describe anaerobic respiration and state the word equation for anaerobic respiration in muscles during vigorous exercise.

The chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. ✓
glucose → ✓
lactic acid ✓

Analysis

[3 marks]

State three differences between aerobic respiration and anaerobic respiration in yeast.

Aerobic uses oxygen; anaerobic does not. ✓
Aerobic produces carbon dioxide and water; anaerobic in yeast produces alcohol (ethanol) and carbon dioxide. ✓
Aerobic releases much more energy per glucose molecule; anaerobic releases much less (glucose is only partly broken down). ✓

Recall

[2 marks]

Extended. Write the balanced chemical equation for anaerobic respiration in yeast.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ ✓
One mark for correct formulae, one mark for correct balancing. ✓

Explain

[3 marks]

Explain how yeast is used in bread-making and in brewing. Name the useful product in each case.

Yeast respires sugar anaerobically, producing alcohol (ethanol) and carbon dioxide. ✓
Bread: the carbon dioxide is the useful product; its bubbles make the dough rise (the alcohol evaporates on baking). ✓
Brewing: the alcohol is the useful product (the carbon dioxide gives fizz or escapes). ✓

Data

[4 marks]

Extended. The table shows the concentration of lactic acid in an athlete's blood before, during and after a 400 m race.

Time / min	0 (rest)	2 (end of race)	10	30	60
Lactic acid / mmol per dm³	1.0	12.0	9.0	4.0	1.2

(a) Explain the change during the race. (b) Explain how the lactic acid is removed after the race.

(a) The muscles need more oxygen than the blood can supply, so they also respire anaerobically, producing lactic acid; it rises from 1.0 to 12.0 mmol per dm³ in the blood (an oxygen debt builds up). ✓
(b) The heart rate stays fast to transport lactic acid in the blood from the muscles to the liver. ✓
Breathing stays deeper and faster to supply extra oxygen. ✓
In the liver lactic acid is respired aerobically, so its concentration falls back towards the resting value by 60 minutes. ✓

Explain

[2 marks]

Extended. Suggest why a sprinter keeps panting for several minutes after finishing a 100 m race even though they are standing still.

During the race lactic acid built up in the muscles and blood from anaerobic respiration, causing an oxygen debt. ✓
Deep, fast breathing continues to supply the extra oxygen needed to respire the lactic acid aerobically (in the liver) and repay the debt. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.