Combustion creates fire and a rise in temperature. An example of combustion is the burning of gas in an oven. Cellular respiration takes place within living cells including human bodies and is a process that breaks down complex molecules into simpler organisms. Combustion and cellular respiration have similarities with oxygen use, carbon dioxide production, energy release and the breakdown of organic substances.
Combustion cannot take place without oxygen. It uses oxygen to such an extent that the average amount in the air surrounding a combustion process falls from 21 per cent to 15 per cent. Cellular respiration also uses oxygen but in a slower and more stable manner than combustion. With cellular respiration, oxygen is necessary to enable cells to obtain energy from food. In humans, the act of breathing provides the oxygen required for cellular respiration.
Carbon dioxide production
Fossil fuels such as gas, oil and coal are the product of dead organisms that contain carbon dioxide. When gas, oil and coal burn during combustion, the process releases the carbon dioxide and allows it to enter the atmosphere. Cellular respiration produces carbon dioxide but in a different manner to combustion. The cellular respiration process creates carbon dioxide when the oxygen that it uses reacts with the glucose carbohydrate in a living cell.
The process of combustion releases energy in the form of light and heat. For example, the burning of fossil fuels is a combustion process that creates the light and heat energy for domestic and industrial use. Cellular respiration releases energy when oxygen combines with glucose. A human uses the energy to work muscles and to create the enzymes necessary for digesting food. The amount of energy that a body needs varies according to factors such as activity level and pregnancy.
Breakdown of organic substances
Organic substances are chemical compounds that contain carbon. Examples of organic substances are coal and oil. When organic substances burn, the combustion process breaks down the chemical bonds that exist within them. As a result, the organic substances turn into their basic elements, which are usually water vapour and carbon dioxide. In cellular respiration, the organic substance is glucose. Cellular respiration breaks down the chemical bonds of glucose to create water, carbon dioxide and energy.