Air is a mixture of gases. About one-fifth of air is oxygen. The remaining four-fifths are mostly made up of nitrogen and a small amount of carbon dioxide, water vapour, and some other gases.
Oxygen
Most living things need oxygen to stay alive. Living things take in oxygen in order to break down digested food to produce energy. This process is called respiration.
Living things on land take in oxygen from the air around them. In addition, oxygen dissolves in water. Animals and plants that live in water use this dissolved oxygen. Oxygen is also found in the soil. Animals living in the ground and roots of plants are able to use oxygen in the soil.
Oxygen is also needed for burning. When we light a candle, burn a piece of paper, or set fire to wood, oxygen is used up. A continuous supply of fresgh air rich in oxygen is needed to keep things burning.
Nitrogen
Although nitrogen makes up four-fifths of the air, this gas is not used by most living things. Only some bacteria living in the soil are able to use nitrogen from the air around them. Plants must have nitrogen to live, but they are not able to use nitrogen from the air. Bacteria that live in the roots of plants change nitrogen from the air that is in the soil, into a form plants can use.
Carbon dioxide, water vapour, and other gases
When living things respire or when things burn, oxygen is used up and carbon dioxide is produced. Carbon dioxide is important to plants. During the day, plants use carbon dioxide and water to make food in a process called photosynthesis. Oxygen is produced at the same time.
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