A calorie is a unit of energy. We tend to associate calories with food, but they apply to anything containing energy. Specifically, a calorie is the amount of energy, or heat, it takes to raise the temperature of 1 gram of water by 1 degree Celsius. One calorie is equal to 4.184 joules, a common unit of energy used in the physical sciences. Most of us think of calories in relation to things we eat and drink, as in "This can of orange juice has 200 calories".

Human beings need energy to survive -- to breathe, move, pump blood -- and they acquire this energy from food. The number of calories in a food is a measure of how much potential energy that food possesses. A gram of carbohydrates has 4 calories, a gram of protein has 4 calories and a gram of fat has 9 calories. Foods are a compilation of these three building blocks. So if you know how many carbohydrates, fats and proteins are in any given food, you know how many calories, or how much energy, that food contains. Our bodies "burn" the calories through metabolic processes, by which enzymes break the carbohydrates into glucose and other sugars, the fats into glycerol and fatty acids and the proteins into amino acids. These molecules are then transported through the bloodstream to the cells, where they are either absorbed for immediate use or are sent on to the final stage of metabolism, in which they are reacted with oxygen to release their stored energy.

Check that you understand what these words mean in this document
To associate: to connect.
Physical science: a science such as chemistry, physics, biology etc.
To pump blood: to make the blood go round the different parts of the body.
To acquire: to get.
Potential: possible.
Compilation: collection.
Metabolic processe: the function in which the organism changes the foods we take in into forms that can be of use.
To transport: to carry.

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How food works