Q.Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal.

Protein digestion is a stepwise process carried out at successive regions of the alimentary canal:

1. Mouth

Saliva contains no proteolytic enzyme, so no protein digestion occurs here — food is only chewed and mixed with saliva for swallowing.

2. Stomach

The gastric glands secrete gastric juice containing:

• HCl, which activates inactive pepsinogen into active pepsin (and further activates more pepsinogen autocatalytically).
• Pepsin, which hydrolyses dietary proteins into shorter proteoses and peptones.
• Rennin (mainly in infants), which coagulates the milk protein casein into paracasein, subsequently acted on by pepsin.

3. Small intestine (duodenum) — pancreatic juice

• Trypsinogen → trypsin (activated by enterokinase from the intestinal mucosa) acts on proteins, proteoses and peptones, breaking them into smaller peptides.
• Chymotrypsinogen → chymotrypsin (activated by trypsin) similarly breaks proteins/peptones into peptides, and also clots milk casein.
• Procarboxypeptidase → carboxypeptidase (activated by trypsin), an exopeptidase, removes amino acids one at a time from the carboxyl end of peptide chains.

4. Small intestine — intestinal juice (succus entericus)

The brush border of intestinal mucosal cells carries dipeptidases and aminopeptidases that break the remaining short peptides and dipeptides down into free amino acids. …

Walk through the alimentary canal region by region (mouth → stomach → small intestine's pancreatic secretions → small intestine's own intestinal juice), naming the enzyme(s) active at each …

• Skipping the intermediate proteose/peptone/peptide stages and jumping straight from "protein" to "amino acids" without naming the enzymes responsible at each step. …