Final answer:
Each of the 20 amino acids in proteins is distinguished by its unique R group, which can be acidic, basic, polar, or nonpolar, influencing how the amino acid behaves chemically and interacts with other molecules.
Step-by-step explanation:
What makes each of the 20 amino acids found in proteins unique is primarily their R group (side chain), which determines the chemical nature of the amino acid. These side chains can vary greatly, dictating whether an amino acid is acidic, basic, polar, or nonpolar. For instance, valine, methionine, and alanine have nonpolar or hydrophobic side chains, making them averse to water. In contrast, serine, threonine, and cysteine are polar and attract water molecules due to their hydrophilic side chains. Amino acids like lysine and arginine possess positively charged side chains.
All living organisms, from bacteria to humans, employ these 20 amino acids to construct their proteins. While humans can synthesize about half of these amino acids, the remaineder are essential amino acids and must be ingested through the diet to fulfill biological needs. The amino group in these amino acids is tethered to the alpha-carbon of the carboxylic acid group, thus they are known as a-amino acids. This common structure underpins all amino acids, but the distinct R groups lead to various amino acid properties and allow proteins to perform their specialized functions within the body.