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An essential part of organisms, proteins are made up of amino acids. Some amino acids can be synthesized by the human body while some must be obtained through food. Proteins are major contributors to cells' functions.
Sources of protein in the diet include nuts, legumes, grains, eggs, meats and dairy products. Proteins have a distinct behaviour in cooking compared to fats and and carbohydrates. Proteins in general stiffen when heated, for example a egg yolk becomes more solid when heated, because it's rich in protein.
Some proteins are toxic for humans but become edible via cooking, for that reasons some legumes must be boiled before they can be eaten.
Amino acids 
The building blocks of protein molecules are called amino acids. There are only 20 amino acids, but these can be combined in a huge number of combinations. The proteins are digested in the intestine, and split up in amino acids. These amino acids are later recombined to new proteins in the cells of the body.