Respiration

Aerobic and anaerobic respiration

Cellular respiration is an exothermic reaction that takes place continuously in living cells. It releases the energy organisms need for living processes, including building larger molecules, movement and keeping warm.

Aerobic respiration uses oxygen and takes place mainly in the mitochondria:

glucose + oxygen → carbon dioxide + water

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration happens without oxygen. In muscle cells:

glucose → lactic acid

In plant cells and yeast it produces ethanol and carbon dioxide. In yeast this is called fermentation, and it is used to make bread and alcoholic drinks:

glucose → ethanol + carbon dioxide

Anaerobic respiration releases much less energy than aerobic respiration, because the glucose is only partly broken down.

Response to exercise

During exercise the muscles need more energy, so the body responds by increasing:

• heart rate,
• breathing rate,
• breath volume.

Together these deliver more oxygenated blood to the muscles and remove carbon dioxide faster.

If exercise is intense and the oxygen supply cannot keep up, muscles respire anaerobically and lactic acid builds up. Over long periods of activity the muscles become fatigued and stop contracting efficiently.

After exercise, extra oxygen is needed to deal with the lactic acid. This is the oxygen debt. Blood carries the lactic acid to the liver, where it is converted back into glucose — which is why breathing stays fast for a while after you stop.

Metabolism

Metabolism is the sum of all the reactions in a cell or the body. Many of these reactions use energy released by respiration and are controlled by enzymes. They include:

• converting glucose into starch, glycogen and cellulose,
• making lipids from one molecule of glycerol and three fatty acids,
• combining glucose with nitrate ions to make amino acids, which are then built into proteins,
• respiration itself,
• breaking down excess proteins to form urea, which is excreted.