Topic 12: Respiration

Respiration is one of the most important processes in living things — it is how every cell releases the energy stored in food. Without it, muscles could not move, nerves could not work, and life would stop. In this topic you will learn the difference between aerobic and anaerobic respiration, write their equations, and see why oxygen makes such a big difference to how much energy is released.

What respiration is and why it matters

Respiration is the chemical reactions in cells that break down nutrient molecules to release energy. It happens in all living cells, all the time. The energy released is used for many processes.

• Muscle contraction so the body can move.
• Growth by building large molecules from smaller ones.
• Active transport to move substances across cell membranes.
• Maintaining a constant body temperature in mammals.

Do not confuse respiration with breathing: breathing moves air in and out of the lungs, while respiration is the release of energy inside cells.

Aerobic respiration

Aerobic respiration is the release of a relatively large amount of energy by the breakdown of glucose using oxygen. It is the main way cells release energy.

• Word equation: glucose + oxygen → carbon dioxide + water (energy is released).
• Balanced symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
• It releases much more energy per glucose molecule than anaerobic respiration.
• The waste products, carbon dioxide and water, are easily removed.

Because it gives so much energy, aerobic respiration is used whenever enough oxygen is available.

Anaerobic respiration

Anaerobic respiration is the release of a relatively small amount of energy by the breakdown of glucose without using oxygen. It is useful when oxygen is in short supply, but it releases far less energy.

• In muscles during hard exercise: glucose → lactic acid (energy is released).
• In yeast (used in brewing and bread-making): glucose → alcohol (ethanol) + carbon dioxide.
• The yeast equation in symbols: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
• Much less energy is released because glucose is only partly broken down.

The reactions of anaerobic respiration in yeast are also called fermentation.

Oxygen debt and recovery

During vigorous exercise, muscles may respire anaerobically because oxygen cannot be delivered fast enough. This produces lactic acid, which builds up and can cause muscle fatigue and cramp.

• After exercise you continue to breathe deeply and quickly for a while.
• This extra oxygen is used to break down the lactic acid that built up.
• The amount of oxygen needed to remove the lactic acid is called the oxygen debt.
• The heart rate also stays high to carry lactic acid in the blood to the liver, where it is broken down.

This is why your breathing and heartbeat stay fast even after you stop running — the body is repaying the oxygen debt.