Digestion and absorption of proteins: Nutrition

 Digestion of Proteins

Mouth:

Mechanical digestion: Chewing breaks down food into smaller pieces, increasing the surface area for enzyme action.

Chemical digestion: No significant protein digestion occurs in the mouth.

Stomach:

Mechanical digestion: The churning action of the stomach muscles further breaks down food particles.

Chemical digestion:

Gastric juice: The stomach secretes hydrochloric acid (HCl) and pepsinogen.

Activation of pepsin: HCl converts pepsinogen to pepsin, an active protease.

Protein breakdown: Pepsin hydrolyzes peptide bonds in proteins, breaking them into smaller polypeptides and amino acids.

Small Intestine:

Pancreatic enzymes: The pancreas releases pancreatic juice containing proteases (trypsinogen, chymotrypsinogen, and procarboxypeptidase).

Activation of enzymes: Enterokinase activates trypsinogen to trypsin, which activates chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase.

Protein digestion: These enzymes further break down polypeptides into smaller peptides and amino acids.

Brush border enzymes: Peptidases (aminopeptidase and dipeptidase) on the lining of the small intestine hydrolyze peptides into amino acids, dipeptides, and tripeptides.

Absorption of Proteins

Small Intestine:

Amino acids and small peptides: These are absorbed by the epithelial cells lining the small intestine through active transport and co-transport mechanisms involving sodium ions.

Transport to the liver: Once inside the epithelial cells, amino acids enter the bloodstream and are transported to the liver via the hepatic portal vein.

Metabolism: In the liver, amino acids can be used to synthesize new proteins, converted into other amino acids, or used for energy production if needed.

Large Intestine:

Any undigested proteins or amino acids that reach the large intestine are typically minimal, as most protein digestion and absorption occur in the stomach and small intestine.

Bacterial fermentation: Some amino acids may be partially broken down by intestinal bacteria, producing gases and other byproducts.

The remaining undigested material is eventually excreted as part of the feces.