Respiration

IGCSE Edexcel Biology
2.34–2.38 Aerobic and anaerobic respiration
Key Concepts: Respiration is the process of releasing energy from food molecules in cells. Aerobic respiration uses oxygen and releases more energy than anaerobic respiration, which occurs when oxygen is limited.

Section A — Aerobic Respiration

1. Define respiration. [2]
2. Write the word equation for aerobic respiration. [2]
3. Write the balanced symbol equation for aerobic respiration. [2]
4. State where in the cell aerobic respiration takes place. [1]
5. State four uses of energy released by respiration in living organisms. [4]

Section B — Anaerobic Respiration

6. State the conditions under which anaerobic respiration occurs. [1]
7. Write the word equation for anaerobic respiration in: [4]

a) Human muscle cells (during intense exercise)

b) Yeast cells

8. Compare aerobic and anaerobic respiration. [4]
Feature Aerobic respiration Anaerobic respiration
Oxygen required?
Energy yield (ATP)
Products
Location in cell
9. Explain why lactic acid builds up in muscle cells during intense exercise. [3]

Section C — Applications and Investigations

10. A student investigates anaerobic respiration in yeast using glucose solution. Describe how they could measure the rate of anaerobic respiration. [3]
11. Explain the role of anaerobic respiration (fermentation) in yeast in the production of bread. [3]
12. After a sprint race, an athlete continues to breathe heavily for several minutes. Explain why. [3]

Total marks: 32

Mark Scheme

1. The process by which cells break down glucose [1] to release energy (ATP) [1] [2]
2. Glucose + oxygen → carbon dioxide + water [2 — 1 if one product missing]
3. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O [2 — 1 each side correct]
4. Mitochondria [1]
5. Any four: muscle contraction / movement; active transport; protein synthesis; maintaining body temperature; cell division; nerve impulses [4]
6. When oxygen supply is limited / insufficient (e.g. during vigorous exercise or in yeast without oxygen) [1]
7. a) Glucose → lactic acid [2]; b) Glucose → ethanol + carbon dioxide [2] [4]
8. Oxygen required: Yes / No; Energy yield: high (many ATP) / low (2 ATP); Products: CO₂ + H₂O / lactic acid (mammals) or ethanol + CO₂ (yeast); Location: mitochondria / cytoplasm [4]
9. During intense exercise, oxygen supply to muscles is insufficient [1]; muscles respire anaerobically [1]; glucose is incompletely broken down, producing lactic acid as a by-product [1] [3]
10. Set up yeast + glucose in a sealed container/flask [1]; collect CO₂ gas produced in a measuring cylinder / count bubbles per minute [1]; keep temperature constant as a control variable [1] [3]
11. Yeast anaerobically respires (ferments) glucose in the dough [1]; produces CO₂ gas [1]; CO₂ bubbles cause the dough to rise / become lighter in texture [1] [3]
12. During the sprint, anaerobic respiration produced lactic acid in the muscles [1]; extra oxygen is needed to break down / oxidise this lactic acid [1]; this is called the oxygen debt / excess post-exercise oxygen consumption (EPOC) [1] [3]