Protein digestion only occurs in what organs of the gastrointestinal tract?

After protein is ingested, its digestion begins in the stomach with HCl (hydrochloric acid) and pepsin acting on it to break the central peptide bond in proteins. This requires an acidic environment which is provided by gastric acid.

Once the chyme (fluid partly-digested food) passes from the stomach to the small intestine, it enters the duodenum. Here the acid is neutralised by the addition of pancreatic bicarbonate which creates a slightly alkaline environment.

This alkaline environment is required for the pancreatic digestive enzymes to work effectively.

In the duodenum (the first part of the small intestine), trypsin breaks down protein into single amino acids (the building blocks of proteins) by a hydrolytic process. This process involves insertion of a water molecule between the amino acid links to break them.

Trypsin also activates chymotrypsin, carboxypeptidase, and elastase, enzymes which further break down the single amino acid chains for better absoprtion.

Aborption of these amino acid segments takes place in the jejunum and the ileum (the second and third part of the small intestine).