Chapter 14: Digestion: Turning a Meal into Cellular Biochemicals

Digestion

The breakdown of large biomolecules (proteins, lipids, polysaccharides) into small absorbable molecules by hydrolytic enzymes

Proteins (digestion product)

Broken down into amino acids

Lipids (digestion product)

Broken down into fatty acids and glycerol

Polysaccharides (digestion product)

Broken down into monosaccharides (mainly glucose)

Hydrolytic enzymes

Enzymes that use water to cleave chemical bonds during digestion

Acetyl-CoA

Formed from digested nutrients

Purpose of digestion

Convert large, insoluble food molecules into small, soluble forms for absorption, transport and energy production.

Main digestive organs

Mouth, esophagus, stomach, small intestine, large intestine, anus

Accessory organs

Salivary glands, pancreas, liver/ gallbladder.

Mouth

Where digestion begins and food is mechanically degraded

Slurry

Produced when food is physically broken down by chewing

Salivary amylase

Enzyme that converts polysaccharides → disaccharides - produced by salivary glands and active in mouth

Stomach

Temporary storage tank for food and where chemical breakdown of proteins begin

Zymogen (proenzyme)

Inactive enzyme precursor - activated only at the site of digestion to prevent self-digestion and ensure enzymes only act where needed (compartmentalization)

Pepsinogen

Inactive zymogen form of pepsin

Pepsin

Active enzyme that breaks internal peptide bonds in proteins (endopeptidase) - only enzyme produced and acts in stomach

Stomach enzymes

Pepsin, gastric lipases (initiate emulsification) and gastric amylase 

Enzyme activation cascade

A small activation event (e.g., trypsin) triggers multiple other enzyme activations.

Enteropeptidase

Enzyme produced in the intestine to activate pancreatic enzymes - activates trypsinogen to trypsin

Trypsin

Key enzyme that activates other pancreatic zymogens

Chyme

Semi-fluid mixture of partially digested food leaving the stomach

Exocrine function (pancreas)

Secretes bicarbonate and digestive enzymes via ducts into the small intestine

Endocrine function (pancreas)

Releases hormones (insulin and glucagon) into the bloodstream to regulate blood sugar

Bicarbonate (HCO₃⁻)

Neutralizes acidic chyme

Pancreatic enzymes

Pancreatic amylase, trypsin, chymotrypsin, elastase carboxypeptidase, and lipase 

Secretin

Hormone released by small intestines when acidic chyme enters small intestine; triggers pancreas to secrete bicarbonate.

Cholecystokinin (CCK)

Hormone released when fats/proteins enter the small intestine; stimulates pancreatic enzyme release and gallbladder contraction (bile release).

Brush border enzymes/ Intestinal enzymes

Enzymes on microvilli surface that complete final digestion of small molecules

Villi

Finger-like folds that increase intestinal surface area for absorption

Microvilli

Tiny projections on villi forming the “brush border”

Oligopeptides

Small fragments produced after proteins are digested by pancreatic enzymes in small intestine

Peptidases (brush border)

Enzymes on the surface of and in intestinal cells that convert oligopeptides → amino acids, dipeptides and tripeptides

Antiport

Release amino acids into blood by coupling with sodium movement in cells

Starch

Main carbohydrate in diet

α-Amylase

Enzyme that breaks starch into smaller chains; secreted by salivary glands and pancreas

α-Glucosidase

Breaks small starch fragments into glucose

α-Dextrinase (isomaltase)

Cleaves α-1,6 bonds in branched starches.

Sucrase

Breaks sucrose → glucose + fructose

Lactase

Breaks lactose → glucose + galactose

Glucose transporter (SGLT1)

Co-transports glucose, galactose and Na⁺ into intestinal cells.

GLUT5

Facilitated diffusion of fructose into intestinal cells

GLUT2

Transports all monosaccharides (glucose, galactose, fructose) into the bloodstream.

Triacylglycerol

Main dietary fat; hydrophobic and must be emulsified for digestion.

Emulsification

Breaking large fat droplets into smaller ones to increase enzyme access. Done by body temperature, peristalsis, and gastric lipases.

Bile salts

a modified cholesterol produced in the liver; emulsify fats in the small intestine

Lipases

Enzymes secreted by pancreas into small intestine that convert the triacylglycerols into two fatty acids and monoacylglycerol

Free fatty acids

Released during fat digestion - Carboxy end interacts with water / Hydrocarbon tail interacts with other lipids

Micelles

Carry lipid digestion products to intestinal epithelium cells/ enterocytes

Chylomicrons

Carry packaged lipids, proteins and reformed triacylglycerols in entereocytes to the blood via lymphatic system.

Celiac disease

Autoimmune disorder where gluten triggers immune attack on the small intestine lining

Gluten

Protein in wheat rich in proline and glutamine, hard to digest

Consequence of celiac disease

Damaged villi → decreased surface area → impaired nutrient absorption

Snake venom enzymes

Contain digestive enzymes like phospholipases and collagenases that pre-digest prey tissues + neurotoxic and hemotoxic peptides

Medical use of snake venom enzymes

Studied for treatments like antihypertensives and anticoagulants