The chemical reactions inside cells are controlled by enzymes
During aerobic respiration (respiration that uses oxygen) chemical reactions occur that:
■ use glucose (a sugar) and oxygen
■ release energy
Aerobic respiration takes place continuously in both plants and animals.
Most of the reactions in aerobic respiration take place inside mitochondria
Aerobic respiration is summarised by the equation:
glucose + oxygen ➞ carbon dioxide + water (+ energy)
Energy that is released during respiration is used by the organism. The energy may be used:
■ to build larger molecules from smaller ones
■ in animals, to enable muscles to contract
■ in mammals and birds, to maintain a steady body temperature in colder surroundings
■ in plants, to build up sugars, nitrates and other nutrients into amino acids which are then
built up into proteins.
During exercise a number of changes take place:
■ the heart rate increases
■ the rate and depth of breathing increases.
These changes increase the blood flow to the muscles and so increase the supply of sugar and oxygen and increase the rate of removal of carbon dioxide.
Muscles store glucose as glycogen, which can then be converted back to glucose for use during exercise.
During exercise, if insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain energy.
Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.
As the breakdown of glucose is incomplete, much less energy is released than during aerobic respiration. Anaerobic respiration results in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water.
If muscles are subjected to long periods of vigorous activity they become fatigued, ie they stop contracting efficiently. One cause of muscle fatigue is the build-up of lactic acid in the muscles. Blood flowing through the muscles removes the lactic acid.