Pancreatic Juice

Pancreatic juice contains enzymes that digest carbohydrates, fats, proteins, and nucleic acids. The carbohydrate-digesting enzyme, pancreatic amylase, splits molecules of starch or glycogen into double sugars (disaccharides). The fat-digesting enzyme, pancreatic lipase, breaks triglyceride molecules into fatty acids and monoglyc-erides. (A monoglyceride molecule consists of one fatty acid bound to glycerol.)

The protein-splitting (proteolytic) enzymes are trypsin, chymotrypsin, and carboxypeptidase. Each of these enzymes splits the bonds between particular combinations of amino acids in proteins. Because no single enzyme can split all possible combinations of amino acids, several enzymes are necessary to completely digest protein molecules.

The proteolytic enzymes are stored in inactive forms within tiny cellular structures called zymogen granules. These enzymes, like gastric pepsin, are secreted in inactive forms and must be activated by other enzymes after they reach the small intestine. For example, the pancreatic cells release inactive trypsinogen, which is activated to trypsin when it contacts the enzyme enterokinase, which the mucosa of the small intestine secretes. Chymotrypsin and carboxypeptidase are activated, in turn, by trypsin. This mechanism prevents enzymatic digestion of proteins within the secreting cells and the pancreatic ducts.

A painful condition called acute pancreatitis results from a blockage in the release of pancreatic juice. Trypsinogen, activated as pancreatic juice builds up, digests parts of the pancreas. Alcoholism, gallstones, certain infections, traumatic injuries, or the side effects of some drugs can cause pancreatitis.

Pancreatic juice also contains two nucleases, which are enzymes that break down nucleic acid molecules into nucleotides, as well as a high concentration of bicarbonate ions that makes the juice alkaline. This alkalinity provides a favorable environment for the actions of the digestive enzymes and helps neutralize the acidic chyme as it arrives from the stomach. At the same time, the

Cystic duct -

Duodenum

Hepatopancreatic ampulla

Common hepatic duct Common bile duct

Tail of pancreas

Cystic duct -

Duodenum

Common hepatic duct Common bile duct

Tail of pancreas

Blood Vessels Pancreas

Hepatopancreatic ampulla

Hepatopancreatic sphincter

Figure 17.24

The pancreas is closely associated with the duodenum.

Intestinal lumen

Hepatopancreatic ampulla

Hepatopancreatic sphincter

Hepatopancreatic sphincter

Figure 17.24

The pancreas is closely associated with the duodenum.

Intestinal lumen

Hepatopancreatic ampulla

Hepatopancreatic sphincter alkaline condition in the small intestine blocks the action of pepsin, which might otherwise damage the duodenal wall.

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