Proteins are chains of amino acids. Of the 22 amino acids found in animal tissue, dogs cannot synthesize 10 in adequate amounts and cats cannot synthesize 11 — these must come from the diet and are called "essential." Examples: lysine, methionine, threonine, tryptophan, and (for cats) taurine. Protein quality is a function of the amino acid profile, not just total grams. Animal proteins supply a more complete amino acid mix than most plant proteins, which is why named animal sources outperform pea-protein concentrates on a per-gram basis even at the same crude protein percentage.