Out of the 20 naturally occurring amino acids, at least 10 are classified as essential because they must be obtained from the diet. These include lysine, tryptophan, histidine, phenylalanine, leucine, isoleucine, threonine, methionine, valine, and arginine. The remaining 10 are nonessential, as they can be synthesized endogenously from other amino acids or metabolic precursors. Additionally, selenocysteine, which is derived from serine, is recognized as the 21st amino acid due to its role in protein synthesis.