Plants and microorganisms  can synthesize all 20 amino  acids required for protein  synthesis.
Mammals obtained most of  their amino acids by  nutrition and can synthesis  only the commonly occuring amino acids.

Reasons:
• Intermediate toxicity
• In favor of degradation
• Can be obtained readily

Definitions. Absence of a de novo pathway for the biosynthesis of an amino acid does not necessarily mean that the amino acid must be obtained from the diet. Arginine.

Essential amino acids must be obtained by degradation of ingested proteins.

Proteolysis of proteins catalyzed by proteases in digestive tract produces free amino acids and small peptides.

Further digestion of peptides by multiple forms of peptidase in small intestine produces more free amino acids.