Mammals and birds need energy to maintain a constant body temperature. Energy is also needed for the following life processes:
- cell division
- muscle contraction
- protein synthesis
- active transport
- nerve impulses
Respiration involves chemical reactions that break down nutrient molecules in living cells to release energy.
Aerobic respiration needs oxygen. It is the release of a relatively large amount of energy in cells by the breakdown of food substanes in presence of oxygen:
Glucose + oxygen > carbon dioxide + water
Aerobic respiration happens all the time in animals and plants. Most of the reactions in aerobic respiration happen inside mitocondria.
Anaerobic respiration does not need oxygen. It is the release of the relatively small amount of energy in cells by the breakdown of food substances in the absense of oxygen.
Anaerobic respiration happens in muscles during hard exercise:
Glucose > lactic acid
Glucose is not completely broken down, so much less energy is released than during aerobic respiration.
There is a build up of lactic acid in the muscles during vigorous exercise.The lactic acid needs to be oxidised to carbon dioxide and water later.
This causes an oxygen debt- known as excess post- exercise oxygen consumption (EPOC)- that needs to be 'repaid' after the exercise stops. This is why we keep on breathing deeply for a few minutes after we have finished exercising.
Testing for carbon dioxide
Limewater turns milky in the presence of carbon dioxide. It can be used to show that exhaled air conatins more carbon dioxide than inhaled air.
Carbon dioxide dissolves in water to form a weekly acidic solution. Hydrogencarbonate indicator is used to show the presence of carbon dioxide in the solution. It is:
- red at neutral pH (no carbon dioxide)
- yellow at low pH (carbon dioxide present)