Digestion Terms


Amylase: A digestive enzyme that breaks down starch (mostly amylose) and glycogen into oligosaccharides and glucose; found in saliva (salivary amylase) and pancreatic juice (pancreatic amylase).


Bile: Greenish fluid secreted by the liver, stored in the gallbladder, and released into the small intestine, where it emulsifies lipids prior to their digestion; it consists of cholesterol, phospholipids, and salts of oxidized steroids and heme.


Brush border: The microvilli-covered surface of the intestinal mucosa, which has digestive enzymes bound to the apical plasma membrane; the disaccharide digesting enzymes include sucrase that breaks down sucrose into glucose and fructose, maltase that breaks down maltose into glucose, and lactase that breaks down lactose into glucose and galactose; the protein digesting enzymes carboxypeptidase, aminopeptidase , and dipeptidase break down peptides into amino acids; the nucleic acid digesting enzymes include nucleotidase that breaks down nucleotides into nucleosides and phosphate, and the nucleosidase that breaks down nucleosides into nitrogenous bases and either ribose (from RNA) or deoxyribose (from DNA).


Chyle: A milky fluid of chylomicrons absorbed by the lacteals in the small intestine after digestion of lipids.


Intrinsic factor: A glycoprotein that is secreted by the parietal cells of the gastric glands, which facilitates intestinal absorption of vitamin B12 , which is required for hematopoiesis; deficiency causes pernicious anemia.


Ketoacidosis: Acidosis caused by the enhanced production of ketone bodies, such as in diabetes or starvation.


Ketone bodies: Fatty acid metabolites that are organic acids.


Lacteal: Special lymphatic vessels in the villi of the small intestine that absorb lipids.


Lipase: A fat digesting enzyme that breaks down emulsified triglycerides into fatty acids and monoglycerides.


Lipoprotein: One of several types of transport proteins that carry lipids in the blood; dietary lipids are combined with chylomicrons, enter lacteals in the villi, and transported via lymphatics to the bloodstream; high levels of low-density lipoproteins (LDL) are associated with increased risk of atherosclerosis, but high levels of high-density lipoproteins (HDL) are associated with decreased risk of atherosclerosis.


Nucleases: Digestive enzymes that break down nucleic acids; the enzymes that cleave the phosphodiester bonds between the nucleotide subunits of DNA or RNA include pancreatic deoxyribonuclease (DNAase) and pancreatic ribonuclease (RNAase); the nucleotide digesting enzymes include nucleotidase that breaks down nucleotides into nucleosides and phosphate, and nucleosidase that breaks down nucleosides into nitrogenous bases and either ribose (from RNA) or deoxyribose (from DNA).


Proenzyme: The inactive precursor of an enzyme, which can be converted into the active enzyme. Also called zymogen.


Proteases: Protein digesting enzymes that break down proteins into peptides and amino acids; the soluble protease in the stomach is pepsin that is formed from pepsinogen; the soluble protease in the small intestine include pancreatic trypsin that is formed from trypsinogen, pancreatic chymotrypsin that is formed from chymotrypsinogen, and pancreatic carboxypeptidase that is formed from procarboxypeptidase.


Rennin: A digestive enzyme produced in infants that breaks down milk proteins. (Do not confuse rennin with renin, which regulates blood pressure).