Dietary proteins are denatured by cooking and stomach acid
The hormone gastrin stimulates the release of pepsinogen from the chief cells in the stomach
Pepsinogen is converted to the active enzyme pepsin by the acid in the stomach
Pepsin hydrolyzes polypeptide chains into shorter amino acid chains (peptides)
Partially digested proteins move to duodenum
The presence of proteins in the stomach/small intestine stimulates the release of cholecystokinin (CCK) by upper intestinal mucosa
CCK stimulates the pancreas to release the protein splitting enzymes: trypsin, chymotrypsin, and carboxypeptidase into the duodenum
These enzymes will break peptides into smaller peptides and amino acids
Peptides and amino acids are absorbed via active absorption mechanisms (energy required for absorption) into the cells of the small intestine
Absorbed small peptides are eventually broken down to individual amino acids inside the intestinal cells
Amino acids then travel to liver via portal vein