Feeding and breathing are two of the most important activities carried out by living organisms. This chapter studies the connections between these two activities.

An organism needs food for three main reasons:

Energy in Food
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The foods that provide living things with the most energy are ones which contain a lot of sugar, fats, oils or proteins. To release this energy oxygen is needed. The release of energy from food in a living organism is called respiration and it takes place all over our body.

Respiration is like combustion. When we burn wood, oil or coal we need oxygen. Burning wood in a fire produces carbon dioxide gas and water vapour as well as heat and light energy. When we 'burn' sugar in our bodies by respiration, the energy that is released is used so that we can move or keep warm.

The carbon dioxide that we produce by respiration must be breathed out through our lungs. We can reuse some of the water which is also made during respiration (our bodies are mostly made of water).

Food for Growth
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Not all the food that we eat is consumed in respiration. A large amount of the protein in the meat and vegetables that we eat is used for growth. Also the sugars, oils and fats are stored in our bodies for use at some other time.

The food that animals eat can be used in three different ways:

Not all animals use their food in the same way. Some animals have diets which are difficult to digest, so a lot of food is wasted. In general herbivores have to eat more food and to digest it for longer than carnivores.

Some small animals may need to keep eating all the time to stay warm. Animals that are very active, such as birds which fly, also need food that has a high energy content.

Green plants
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Green plants make their own food by photosynthesis.
The herbivores feed upon the leaves, roots and other parts that the plant grows.
The carnivores feed upon the muscles and blood that the herbivore grows.

Decomposers

When animals and plants respire they release carbon dioxide into the atmosphere. When they produce wastes or when they die their dead bodies return the minerals to the soil.

The decomposers, such as fungi and bacteria, play an important role in returning these minerals to the soil.

This means that the chemicals in our bodies were once part of a plant. When we die these chemicals will be taken up by the roots of a plant once again. Therefore the chemicals that living things are made of are constantly being recycled.

Summary

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© Paul Billiet and Shirley Burchill 2008