Final answer:
There are 20 naturally occurring amino acids that are common to all living organisms and are used to construct proteins. Humans can only synthesize half and must obtain the remaining, known as essential amino acids, from the diet. Additionally, selenocysteine and pyrrolysine have been found in limited quantities in proteins.
Step-by-step explanation:
There are 20 naturally occurring amino acids that are used by living organisms to construct proteins. These amino acids each contain an a-amine and an α-carboxylic acid group. However, humans can synthesize only about half of the needed amino acids; the rest, known as essential amino acids, must be obtained from the diet.
Proteins are crucial to all life on Earth, from single-celled organisms to humans, as they rely on these amino acids for various functions. Each of the 20 amino acids has a distinctive side chain, known as the R group, which defines its chemical and physical properties. The composition of amino acids in proteins is uniform across different species of life, indicating the universal nature of these biomolecules.
In addition to the common 20 amino acids, two more have been identified in limited amounts in proteins: Selenocysteine and pyrrolysine. Natural non-proteinogenic amino acids also exist and are much more numerous than the proteinogenic amino acids. These non-standard amino acids are not incorporated into proteins during synthesis but may play roles in other biological processes.