Final answer:
There are 20 standard amino acids commonly found in proteins necessary for life. Additionally, two rare amino acids, Selenocysteine and Pyrrolysine, are found in limited quantities.
Step-by-step explanation:
There are 20 standard amino acids that are used by the human body to build proteins, and these amino acids possess a common structural feature where an amino group is attached to a carboxylic acid group.
Humans are capable of synthesizing about half of these amino acids, while the rest must be obtained through the diet and are known as essential amino acids. Interestingly, two additional amino acids, Selenocysteine and Pyrrolysine, have also been discovered in proteins, although they appear in limited quantities and are not considered part of the standard 20 amino acids.
Amino acids can be classified according to the locations of the core structural functional groups, as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, ionization, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form the second-largest component (water being the largest) of human muscles and other tissues.
Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.
It is thought that they played a key role in enabling life on Earth and its emergence.