Final answer:
Animals are unable to synthesize about half of the 20 essential amino acids that are required to make proteins.
Step-by-step explanation:
The proteins in all living species, from bacteria to humans, are constructed from the same set of 20 amino acids. Humans can synthesize only about half of the needed amino acids; the remainder must be obtained from the diet and are known as essential amino acids. An essential amino acid is an amino acid that cannot be synthesized by the body in sufficient quantities and must be obtained from the diet.