IGCSE · Practice questions · Aerobic respiration

Glucose plus oxygen.

Six original Cambridge-style questions on aerobic respiration: its definition, the word and (Extended) balanced equations, where it happens and why exercise increases breathing.

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.

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Reactants left, products right.

Definition: chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
Word equation: glucose + oxygen → carbon dioxide + water.
Extended: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; count atoms to check.
Energy is released, not made, and is not written in the balanced equation.

Recall

[3 marks]

Describe what is meant by aerobic respiration and state its word equation.

The chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. ✓
glucose + oxygen → ✓
carbon dioxide + water ✓

Recall

[2 marks]

Extended. Write the balanced chemical equation for aerobic respiration.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O ✓
One mark for correct formulae, one mark for correct balancing. ✓

Calculation

[3 marks]

Extended. Use the balanced equation to show that the numbers of carbon, hydrogen and oxygen atoms are the same on both sides. Then state how many molecules of carbon dioxide are released when 5 molecules of glucose are respired aerobically.

Carbon: 6 on the left, 6 × 1 = 6 on the right. Hydrogen: 12 on the left, 6 × 2 = 12 on the right. ✓
Oxygen: 6 + (6 × 2) = 18 on the left; (6 × 2) + (6 × 1) = 18 on the right. ✓
5 × 6 = 30 molecules of carbon dioxide. ✓

Explain

[3 marks]

Sperm cells and muscle cells contain many mitochondria. Explain why.

Mitochondria are where most of the reactions of aerobic respiration take place. ✓
Aerobic respiration releases a large amount of energy from glucose. ✓
Muscle cells need the energy for contraction, and sperm cells need it to swim (move the tail); more mitochondria release more energy. ✓

Data

[4 marks]

A student's breathing was measured at rest and during exercise.

	Breaths per minute	Volume per breath / dm³
At rest	12	0.5
Jogging	28	1.8

(a) Calculate the volume of air breathed per minute in each case. (b) Explain why the volume increases during exercise.

(a) At rest: 12 × 0.5 = 6.0 dm³ per minute. Jogging: 28 × 1.8 = 50.4 dm³ per minute. ✓
(b) During exercise the muscles contract more, so they need more energy. ✓
Their rate of aerobic respiration increases, so they use more oxygen and produce more carbon dioxide. ✓
Faster, deeper breathing supplies the extra oxygen and removes the extra carbon dioxide. ✓

Analysis

[3 marks]

Compare the word equations for photosynthesis and aerobic respiration. State two similarities and one difference.

Similarity: both involve the same four substances, glucose, oxygen, carbon dioxide and water. ✓
Similarity: both are chemical reactions controlled by enzymes in cells (both happen in plant cells in the light). ✓
Difference: respiration uses glucose and oxygen and releases energy; photosynthesis uses carbon dioxide and water and needs light energy absorbed by chlorophyll (the reactants and products are reversed). ✓

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.