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Cambridge IGCSE Biology · 0610

Chapter 12: Respiration

Respiration and uses of energy

Definition
Respiration is a metabolic reaction (chemical reaction) which occurs in almost all living cells to produce energy from nutrient molecules.
Uses of energy
The energy released is used for vital processes, including:
  1. Muscle contraction.
  2. Protein synthesis.
  3. Cell division.
  4. Active transport.
  5. Growth.
  6. Passage of nerve impulses.
  7. Maintenance of a constant body temperature.
Enzyme control
Respiration is catalysed by enzymes, meaning the rate is influenced by factors like temperature and pH.

Aerobic respiration

Definition
The chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
Location
Occurs in the mitochondria. Cells with high energy requirements, such as muscle cells, contain large numbers of mitochondria.
Word equation
Glucose + oxygen → carbon dioxide + water.
Balanced chemical equation
C6H12O6 + 6O2 → 6CO2 + 6H2O

Exam Traps

  • Do not confuse respiration with breathing — breathing ventilates lungs; respiration releases energy in cells.

Anaerobic respiration

Definition
The chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
Efficiency
It is less efficient than aerobic respiration and releases much less energy per glucose molecule.
Location
Occurs in the cell cytoplasm and does not require mitochondria.
In animals (muscles)
Occurs during vigorous exercise when insufficient oxygen is delivered to muscles.
  • Word equation: glucose → lactic acid.
In microorganisms (yeast)
Used in bread-making and the production of ethanol for biofuels.
  • Word equation: glucose → alcohol + carbon dioxide.
  • Balanced chemical equation:
    C6H12O6 → 2C2H5OH + 2CO2

Exam Traps

  • Do not give the yeast alcohol equation for human muscle — muscles produce lactic acid, not ethanol.
  • Avoid saying anaerobic respiration occurs in mitochondria — it takes place in the cytoplasm.

The oxygen debt

Lactic acid build-up
During vigorous exercise, lactic acid builds up in the muscles and blood, causing muscle fatigue.
Oxygen debt
This build-up creates an ‘oxygen debt’ that must be repaid after exercise.
Removal of oxygen debt
  1. Fast heart rate: Continues after exercise to transport lactic acid in the blood from the muscles to the liver.
  2. Deeper and faster breathing: Continues to supply extra oxygen for the aerobic respiration of lactic acid.
  3. Liver function: Lactic acid is broken down into carbon dioxide and water through aerobic respiration in the liver.

Exam Traps

  • Do not say lactic acid is excreted in urine — it is broken down in the liver by aerobic respiration.
  • Avoid claiming oxygen debt means oxygen was stored in muscles during exercise.

Respiration in yeast

Effect of temperature
As temperature increases up to the optimum (approx. 35°C), the rate of respiration increases as enzymes and substrates gain kinetic energy.
Denaturation
Above the optimum temperature, the rate slows down because the high heat denatures the enzymes.
Investigating respiration in yeast with glucose solution, oil layer, delivery tube, and limewater to detect carbon dioxide
Diagram 1: Investigating respiration in yeast. A test tube contains yeast in a glucose solution, topped with a layer of oil to ensure anaerobic conditions. A delivery tube leads into a second test tube containing limewater to detect the production of carbon dioxide.

Exam Traps

  • Do not say rate keeps increasing above 35°C — high temperature denatures enzymes.

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