Chapter 14

Digestion Overview

• Digestion is the process of converting food into molecules that can be used for energy or building blocks.
• Different enzymes are responsible for digesting different biomolecules.
• Many digestive enzymes are hydrolases.
• Most digestive enzymes are secreted as inactive zymogens or proenzymes.
• Zymogens are activated by proteolytic cleavage.

Gastric and Pancreatic Zymogens

• Pepsinogen is synthesized in the stomach and becomes pepsin.
• Chymotrypsinogen, trypsinogen, procarboxypeptidase, and proelastase are synthesized in the pancreas.
• Chymotrypsinogen becomes chymotrypsin.
• Trypsinogen becomes trypsin.
• Procarboxypeptidase becomes carboxypeptidase.
• Proelastase becomes elastase.

Protein Digestion

• Protein digestion begins in the stomach, which has an acidic environment with a pH of 1-2.
• The acidic environment denatures proteins by breaking hydrogen and ionic bonds.
• Pepsin begins degrading proteins into fragments in the stomach.
• Pancreatic proteases hydrolyze proteins into small fragments called oligopeptides in the intestine, which continues protein digestion.
• Peptidases in the plasma membrane of intestinal cells further break down oligopeptides into tripeptides, dipeptides, or amino acids.
• Peptidases inside intestinal cells break down tripeptides and dipeptides into amino acids.

Carbohydrate Digestion

• α-amylase in saliva cleaves α-1,4 glycosidic bonds of polysaccharides, beginning carbohydrate digestion.
• Minimal digestion occurs in the mouth because α-amylase is denatured in the stomach.
• Pancreatic α-amylase in the intestine cleaves α-1,4 glycosidic bonds of starch, but not α-1,6 bonds.
• This produces maltose (disaccharide), maltotriose (trisaccharide), and limit dextrin.
• Enzymes on the surface of intestinal cells further break down carbohydrates for absorption.
• α-glucosidase digests maltose, maltotriose, and other oligosaccharides.
• α-dextrinase digests limit dextrin into simple sugars.
• Sucrase breaks down sucrose into glucose and fructose.
• Lactase breaks down lactose into glucose and galactose.

Lipid Digestion

• Most lipids are ingested as triacylglycerols and must be broken down into fatty acids to be absorbed in the intestine.
• Lipids and fatty acids are hydrophobic, which presents a challenge for digestion.
• The stomach converts lipids into an emulsion, which is a mixture of lipid droplets and water.
• Bile salts in the small intestine further emulsify lipids.
• Bile salts are amphipathic molecules made in the liver from cholesterol and secreted by the gallbladder.
• Bile salts insert into lipid droplets to make triacylglycerol more available for enzymes to digest.
• Pancreatic lipases attach to the surface of lipid droplets and digest triacylglycerol.
• Fatty acids and monoacylglycerol are produced from lipid breakdown.