SECTION Review

The small intestine is the site of most chemical digestion and almost all absorption. Chemical digestion breaks large food molecules down into their chemical building blocks, which can then be absorbed through the intestinal wall and into the general circulation. Intestinal brush border enzymes and pancreatic enzymes are responsible for the majority of chemical digestion. The breakdown of fat also requires bile.

Most nutrients are absorbed by transport mechanisms at the apical surface of enterocytes. Exceptions include lipids, fat-soluble vitamins, and most water-soluble vitamins. With the help of bile salts and lecithin, the dietary fats are emulsified to form micelles, which can carry the fat particles to the surface of the enterocytes. There, the micelles release their fats to diffuse across the cell membrane. The fats are then reassembled into triglycerides and mixed with other lipids and proteins into chylomicrons that can pass into lacteals. Other absorbed monomers travel from blood capillaries in the villus to the hepatic portal vein and then to the liver.

Glossary

α-dextrin

breakdown product of starch

α-dextrinase

brush border enzyme that acts on α-dextrins

aminopeptidase

brush border enzyme that acts on proteins

chylomicron

large lipid-transport compound made up of triglycerides, phospholipids, cholesterol, and proteins

deoxyribonuclease

pancreatic enzyme that digests DNA

dipeptidase

brush border enzyme that acts on proteins

lactase

brush border enzyme that breaks down lactose into glucose and galactose

lipoprotein lipase

enzyme that breaks down triglycerides in chylomicrons into fatty acids and monoglycerides

maltase

brush border enzyme that breaks down maltose and maltotriose into two and three molecules of glucose, respectively

micelle

tiny lipid-transport compound composed of bile salts and phospholipids with a fatty acid and monoacylglyceride core

nucleosidase

brush border enzyme that digests nucleotides

pancreatic amylase

enzyme secreted by the pancreas that completes the chemical digestion of carbohydrates in the small intestine

pancreatic lipase

enzyme secreted by the pancreas that participates in lipid digestion

pancreatic nuclease

enzyme secreted by the pancreas that participates in nucleic acid digestion

phosphatase

brush border enzyme that digests nucleotides

ribonuclease

pancreatic enzyme that digests RNA

sucrase

brush border enzyme that breaks down sucrose into glucose and fructose

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