Alpha-amino acids are the building blocks of proteins. A protein forms via the condensation of amino acids to form a chain of amino acid "residues" linked by peptide bonds. Each different protein has a unique sequence of amino acid residues; this sequence is the primary structure of the protein. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a huge variety of proteins.
There are twenty standard amino acids used by cells in protein biosynthesis, and these are specified by the general genetic code. These twenty amino acids are biosynthesized from other molecules, but organisms differ in which ones they can synthesize and which ones must be provided in their diet. The ones that cannot be synthesized by an organism are called essential amino acids.
