Respiration involves the breakdown of carbohydrates in living organisms to release energy. The respiration we are going to talk about here is a chemical process that occurs in cells and involves the use of enzymes, not the process of breathing, which is sometimes also called respiration.

A lot of the processes that takes place in cells of living organisms require energy to proceed. This energy is released from food by a process of respiration. In a living organism, energy is essential for:

  • the contraction of muscle cells to create any type of movement of the organism.
  • synthesising proteins from amino acids during assimilation.
  • cell division to create more cells.
  • active transport of molecules across a cell membrane against the concentration gradient.
  • growth of an organism.

Aerobic respiration

Aerobic respiration is the breakdown of glucose molecules in the presence of oxygen to produce carbon dioxide, water and energy. An important word here is aerobic which means that oxygen is needed for this type of respiration.

Aerobic respiration is an example of oxidation because the carbon and hydrogen in the food molecules are oxidised to carbon dioxide (CO2) and water (H2O) respectively, accompanied by the release of energy.

The equation for aerobic respiration is:

glucose + oxygen → carbon dioxide + water

Anaerobic respiration

Anaerobic respiration is the break down of food molecules to release energy in the absence of oxygen. An important word here is anaerobic which means in the absence of oxygen.

A common example of anaerobic respiration is the fermentation of sugar solution by yeast to produce alcohol. The sugar is not completely broken down to carbon dioxide and water but converted to carbon dioxide and alcohol.

The equation for fermentation is:

glucose → alcohol + carbon dioxide + energy

Less energy is produced by anaerobic respiration than by aerobic respiration because the alcohol still contains a great deal of
energy due to the fact that glucose is not completely oxidised.

Anaerobic respiration also occurs in human muscles, especially during vigorous exercise, because of oxygen insufficiently supplied to meet the large oxygen demand by respiring muscle cells. In humans, the products are different to those produced by the fermentation mentioned above.

The equation for anaerobic respiration in humans is:

glucose → lactic acid

The build up of lactic acid in the muscles cause muscle cramps. Due to the fact that food is not completely oxidised, anaerobic respiration is very much less efficient than aerobic respiration.

During anaerobic respiration, lactic acid builds up in the muscles. The blood then needs to circulate quickly enough to remove this lactic acid. So the heart rate increases. The lactic acid is taken to the liver where some of it is oxidised to carbon dioxide and water, by oxygen. This is the reason why the breathing rate of a person increases during and after exercise.

Practise questions

  1. What are the main differences between aerobic and anaerobic respiration?
  2. Why do you think your breathing rate and heart rate stay high for some time after completing a spell of vigorous exercise?