What is aerobic respiration?
Respiration is a series of reactions in which energy is released from glucose.Aerobic respiration is the form of respiration which uses oxygen. It can be summarised by this equation:
glucose + oxygen → carbon dioxide + water (+ energy)
Energy is shown in brackets because it is not a substance. Notice that:
- Glucose and oxygen are used up
- Carbon dioxide and water are produced as waste products
Aerobic respiration happens all the time in the cells of animals and plants. Most of the reactions involved happen inside mitochondria, tiny objects inside the cytoplasm of the cell. The reactions are controlled by enzymes.
Energy released during respiration is used by the organism in several ways. It may be used to build up larger molecules from smaller ones. For example:
- Plants make amino acids from sugars, nitrates and other nutrients
- These amino acids are then built up into larger molecules - proteins
Energy is used by animals to enable the muscles to contract so that the animals can move. Mammals and birds keep their body temperature steady. Energy from respiration is used to do this when their surroundings are colder than they are.
During exercise, the muscle cells respire more than they do at rest. This means that:
- Oxygen and glucose must be delivered to them more quickly
- Waste carbon dioxide must be removed more quickly
This is achieved by increasing the heart rate, rate of breathing and the depth of breathing.
The increased heart rate increases the rate of blood flow around the body. The increased rate and depth of breathing increases the rate of gaseous exchange in the lungs.
The muscles store glucose as glycogen. This can then be converted back to glucose for use during exercise.
Take care not to get confused: plants store glucose as starch and animals store it as glycogen. In addition, respiration and breathing are not the same thing: respiration releases energy, while breathing lets air into and out of our lungs.
Not enough oxygen may reach the muscles during exercise. When this happens, they use anaerobic respiration to obtain energy. Anaerobic respiration involves the incomplete breakdown of glucose. It releases around 5% of the energy released by aerobic respiration, per molecule ofglucose. The waste product is lactic acid rather than carbon dioxide and water:
glucose → lactic acid (+ little energy)
- Muscle fatigue - Muscles become fatigued (tired) during long periods of vigorous activity. This means that they stop contracting efficiently. One cause of this is the build-up of lactic acid in the muscles from anaerobic respiration. The lactic acid is removed from the muscles by blood flowing through them.
- Fitness versus health - Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate is likely to return to normal more quickly after exercise. But being fit is not the same as being healthy. Healthy people are free from disease and infection - they may or may not be fit as well. It is possible to be fit but unhealthy, or healthy but unfit.