Animals need to gather nutrients so that they are able to grow and survive. They need a range of different compounds for energy and for growth and repair. Plants are able to create their own food using energy from the sun. Animals, however, can only get their food energy from plants (herbivores) or from other animals (carnivores).
When an animal takes in food the complex mixture of molecules that make up that food need to be broken down by the digestive system. They are then turned into useful molecules that can be transported in the bloodstream to the parts of the body that need them.
If the animal has more of these useful molecules than it needs at that moment then it will store the extra molecules as fat. Fat is a useful, high-energy molecule that can be stored in the body. When the animal needs the extra energy, at a later time, it can convert the fat back into useful molecules so that they can be transported in the bloodstream and used in the body.
The problem for us, however, is that we often take in far too much food for our body's needs. Our body stores the extra food as fat and our health can sometimes begin to be affected by it.
Carbohydrates and Proteins
Foods can be classified into different groups. There are carbohydrates which are molecules made up of carbon, hydrogen and oxygen. Sugars and starches are types of carbohydrate and they can be found in many different foods, such as bread, pasta, rice and potatoes. Carbohydrates are a good source of energy.
Proteins are another food group: they are used by animals for growth and repair - although they do also provide some energy. Proteins are complex molecules that are made up of smaller molecules called amino acids. Our bodies are able to make their own proteins but we can't make all the amino acids that are required to build all the proteins. The amino acids that we can't make are called essential amino acids and we need these in our diets to remain healthy. If you are a vegetarian then you have to be careful that you get all your amino acids (because you aren't eating animal protein).
|Corn Flakes||Muesli||Rice Snaps||Wheat Biscuits|
|Energy||1580 kJ||1520 kJ||1635 kJ||1435 kJ|
(from Kellog's Corn Flakes; Tesco Swiss Style Muesli; Tesco Rice Snaps; and Tesco Wheat Biscuits. Feb 2009)
Generally, though the foods that we eat contain many different types of nutrient molecule. For example, the table shows the nutrition information from the packets of four breakfast cereals. Notice that, in Science, all energy is measured in Joules or KiloJoules. Energy used to be measured in calories and many food and diet books still use these units.
All of the cereals contain a foodstuff called fibre (sometimes also called roughage) and it is made out of things like the plant cell walls which are hard for us to digest. In fact, this food doesn't give humans any energy or growth benefits at all - but some animals (such as rabbits) are able to digest it. Fibre is important in our diet, however, as it helps the food to pass along our digestive system.
A calorimeter is a special device that collects heat energy in a chemical reaction. You can make a simple calorimeter to find out the amount of energy in a nut. If you hold a test-tube with water in it over a burning nut then the heat given out by the chemical energy in the nut will warm the water up. If you use a thermometer to measure the change in temperature of the water you can work out how much heat energy was needed to make the temperature change. You can then assume that the same amount of energy was in the nut.
The calorimeters used to calculate the energy in the cereals shown above would be quite a bit more sophisticated than that. However, they work on the same principles.