Aerobic and anaerobic respiration

Your body runs on an exothermic reaction

We breathe in oxygen and breathe out carbon dioxide. Inside the cells, digested food is oxidised to release energy. This is respiration, and it is exothermic.

Aerobic respiration uses oxygen. Step 1, glycolysis, happens in the cytoplasm: one glucose molecule splits into two pyruvate, through enzyme-catalysed reactions, with a net gain of 2 ATP. Step 2 happens in the mitochondria: pyruvate becomes 6CO₂ and 6H₂O, with more energy released.

Cells store the energy as ATP and break ATP down whenever they need energy for their work.

Anaerobic respiration happens without oxygen, for example in yeast. It is also exothermic: glucose becomes pyruvate in the cytoplasm, and then ethanol and carbon dioxide, with energy released. For storage, the body keeps lipids (fats, waxes, sterols) in adipose cells, about half of its fuel, and glycogen, the main store of glucose, in the liver and muscles.

Both kinds start with glycolysis in the cytoplasm. Oxygen decides whether pyruvate goes on to CO₂ and water, or to ethanol and CO₂.

Key terms

Glycolysis

The first stage of both kinds of respiration.

ATP

Energy cells use for their work.

Anaerobic respiration

Yeast making ethanol and CO₂.

Lipids

The body's energy reserve.

Short questions with model answers

1. Q1. Follow one glucose molecule through aerobic respiration. Where does each stage happen and what is made? (p. 77)

   • C₆H₁₂O₆ → 2 pyruvate + 2 ATP (net)
   • 2 pyruvate → 6CO₂ + 6H₂O + energy
   • Energy out: exothermic

   C₆H₁₂O₆ → 6CO₂ + 6H₂O + energy

2. Q2. What is the role of glycogen in our body? (Short Q v, p. 81)

   • Glycogen = stored glucose
   • Kept in liver and muscles

   It stores glucose in the liver and muscles, to be released when energy is needed.

Common mistakes

• ✗ Anaerobic respiration absorbs energy because there is no oxygen.

  ✓ It is also exothermic; it releases energy (p. 78).

• ✗ Glycolysis happens in the mitochondria.

  ✓ Glycolysis is in the cytoplasm; the mitochondria finish aerobic respiration.

• ✗ Glycogen is stored in adipose cells.

  ✓ Lipids are in adipose cells; glycogen is in the liver and muscles.

MCQs

1. 1. Glycolysis splits one glucose molecule into:

   1. (a) Two pyruvate
   2. (b) Six CO₂
   3. (c) Ethanol only
   4. (d) Glycogen
   Show answer

   (a) p. 77.

2. 2. The net ATP made in glycolysis is:

   1. (a) 1
   2. (b) 2
   3. (c) 6
   4. (d) 36
   Show answer

   (b) p. 77: a net production of 2 ATP.

3. 3. Anaerobic respiration in yeast gives:

   1. (a) CO₂ and H₂O
   2. (b) Ethanol and CO₂
   3. (c) Glycogen
   4. (d) Oxygen
   Show answer

   (b) p. 78: glucose is converted into carbon dioxide and ethanol.

4. 4. Excess food is stored as lipids in:

   1. (a) Adipose cells
   2. (b) Red blood cells
   3. (c) Mitochondria only
   4. (d) Lungs
   Show answer

   (a) p. 78.

5. 5. Aerobic and anaerobic respiration are both:

   1. (a) Endothermic
   2. (b) Exothermic
   3. (c) Free of enzymes
   4. (d) Only in mitochondria
   Show answer

   (b) pp. 77–78: both release energy.

Quick revision

• Aerobic: glucose → 2 pyruvate (cytoplasm, net 2 ATP) → 6CO₂ + 6H₂O + energy (mitochondria).
• Anaerobic: no oxygen; glucose → pyruvate → ethanol + CO₂ + energy, as in yeast.
• Lipids (adipose cells) and glycogen (liver, muscles) are the body's energy stores.