Explain why two different proteases, pepsin and trypsin, are needed for the complete chemical digestion of proteins in the alimentary canal.

Pepsin is secreted by the stomach and functions best at an acidic pH of about 1.5, where it cleaves peptide bonds in proteins. Trypsin is released in the small intestine and works optimally at a neutral to slightly alkaline pH (≈7.5). Each enzyme would be denatured or inactive if it encountered the opposite pH; pepsin would lose activity in the intestine and trypsin would be inactivated in the stomach. Therefore, both enzymes are required to ensure proteins are progressively broken down into peptides and amino acids throughout the alimentary canal, achieving complete digestion.