Hubi 2003 (MUN) - Chapter 2: Digestion (Protein Digestion)

Where does protein digestion start?

Protein digestion begins in the stomach.

Step 1 of Protein Digestion

The acidic nature of the stomach denatures proteins, which makes them better substrates for degradation by exposing more peptide linkages that can be bound by proteases.

Step 2 of Protein Digestion

The protease pepsin begins to degrade proteins that are found in the stomach.

Step 3 of Protein Digestion

The action of pepsin produces protein fragments, which are then further degraded by the proteases of the intestine.

Step 1 of Digestion in the Intestine

Partly digested proteins, carbohydrates, and lipids move from the acidic environment of the stomach to the beginning of the small intestine.

Step 2 of Digestion in the Intestine

The low pH of the food stimulates the cells of the small intestine to release the hormone secretin.

Secretin

A hormone that promotes the release of sodium bicarbonate (NaHCO3) from pancreatic cells to help neutralize the pH of food as it exits the stomach.

Step 3 of Digestion in the Intestine

The oligopeptide products of pepsin then stimulate the release of cholecystokinin (CCK) by intestinal cells.

Cholecystokinin (CCK)

A peptide hormone secreted by the endocrine cells of the upper intestine, which stimulates the release of digestive enzymes by the pancreas and bile salts.

Step 4 of Digestion in the Intestine

The pancreas responds to CCK by releasing a host of digestive enzymes and bile to the intestine.

Step 5 of Digestion in the Intestine

In the intestine, the digestion of proteins and carbohydrates continues, and the digestion of lipids starts. In this stage, the pancreas also secretes lipases to break down lipids.

Step 6 of Digestion in the Intestine

Pancreatic proteases hydrolyze proteins into small oligopeptides.

Step 7 of Digestion in the Intestine

Digestion is then completed by enzymes called peptidases that are attached to the external surfaces of intestinal cells. These enzymes also cleave oligopeptides into amino acids.