This document discusses amino acids, which are the building blocks of proteins. It notes that there are 20 common amino acids that make up proteins. All amino acids have an amino group, a carboxyl group, and a side chain except for glycine. The side chains give each amino acid unique properties. Amino acids can be classified based on the properties of their side chains, such as polarity. Some amino acids are essential for animals to obtain through their diets as animals cannot synthesize them. The essential amino acids vary slightly between different animal species. The document provides details on various amino acid properties and classifications.