Chapter 14

zymogen

A catalytically inactive precursor of an enzyme. Also called a proenzyme.

micelle

A globular structure formed by amphipathic molecules in which the hydrophilic part is exposed to water and the hydrophobic part is sequestered inside, away from the water.

secretin

A hormone, released by the cells of the small intestine, that stimulates the release of sodium bicarbonate, which neutralizes the low pH of the partly digested proteins, carbohydrates, and lipids coming from the stomach.

chylomicron

A lipoprotein particle that transports dietary triacylglycerol from the intestine to other tissues; apolipoprotein B-48 is a protein component of chylomicrons.

emulsion

A mixture of two immiscible substances.

trypsin

A pancreatic enzyme that digests dietary proteins and activates most of the pancreatic zymogens.

α-amylase

A pancreatic enzyme that digests starch and glycogen by cleaving the α-1,4 bonds of starch and glycogen but not the α-1,6 bonds. The products are the di- and trisaccharides maltose and maltotriose as well as the material not digestible by the enzyme.

trypsinogen

A pancreatic zymogen that is the precursor of trypsin.

cholecystokinin (CCK)

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

bile salt

A polar derivative of cholesterol that is made in the liver, stored in the gall bladder, and released into the small intestine, where it acts as a detergent to solubilize dietary lipids, facilitating their digestion and absorption.

pepsin

A proteolytic enzyme that begins the process of protein degradation in the stomach. Pepsin's action yields protein fragments that will be further degraded by the proteases of the intestine.

enteropeptidase

A proteolytic enzyme, secreted by the epithelial cells of the small intestine, that activates the pancreatic zymogen trypsinogen to form trypsin. Also called enterokinase.

lipase

An enzyme that degrades a triacylglycerol into free fatty acids and monoacylglycerol.

proteolytic enzyme

Belongs to a class of enzymes that hydrolyze the peptide bonds between amino acids, thus digesting proteins. Also called protease.

digestion

The process of breaking large molecules in food down into smaller units by hydrolytic enzymes. Proteins are hydrolyzed to the 20 different amino acids, polysaccharides are hydrolyzed to simple sugars such as glucose, and fats are hydrolyzed to fatty acids.