Proteins
Proteins are very large molecules with molecular weights that range from 6000 to millions of grams per mole. While proteins can be used as an energy source by the body, this is not their primary role. Proteins are essential for the catalysis of most of the body's chemical reactions, for structural support and for the transport/storage of vital nutrients. As proteins and amino acids (the components of proteins) are not stored in the body, some protein intake is required each day.
Amino acids are the building blocks of proteins. In humans, the amino acids used are 
-amino acids, which means the carboxylic acid group and the amino group are located on the same carbon.
When amino acids are linked together, amide or peptide bonds are formed. The formation of an amide group (in red) is shown in the reaction below, in which two amino acids react to form a dipeptide.
Reactions of this type can continue to link many amino acids together to form polypeptides. When the number of amino acids in the molecule reaches about 50, it is considered a protein.
Proteins can be classified as simple or complex. A simple protein is composed only of amino acids. Complex proteins, which are far more common, incorporate other non-amino acid groups in their structure.
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