Overview
This lecture covers the chemical digestion and absorption of nutrients from food—using a cheeseburger as an example—and details the roles of key digestive organs, hormones, and processes.
Digestive Anatomy & Function
- After the stomach, digestion focuses on chemical breakdown and nutrient absorption in the small intestine.
- The liver produces bile for fat digestion; the pancreas secretes bicarbonate and digestive enzymes.
- The small intestine is the main site for nutrient absorption; the large intestine absorbs water and houses vitamin-producing microbes.
- Sphincters separate GI tract regions, controlling movement between them.
Small Intestine Structure & Processes
- The small intestine's mucosa has villi (projections) and crypts (invaginations) for increased surface area.
- Absorptive cells with microvilli (brush border) perform digestion and nutrient uptake.
- Endocrine cells release hormones in response to the acidic chyme.
Hormonal Regulation & Neutralization
- Acidic chyme triggers secretin release, which inhibits stomach acid and stimulates the pancreas to secrete bicarbonate, raising pH.
- CCK (cholecystokinin) is released in response to fats, slows intestinal movement, and signals gallbladder bile release and pancreatic enzyme secretion.
Carbohydrate Digestion & Absorption
- Carbohydrates are broken down by amylase into disaccharides, then by brush border enzymes into monosaccharides (e.g., glucose).
- Monosaccharides are absorbed via secondary active transport using a sodium gradient.
Protein Digestion & Absorption
- Proteins are broken down by endopeptidases (e.g., pepsin) and exopeptidases into amino acids, dipeptides, and tripeptides.
- Amino acids use sodium-dependent secondary active transport; di/tripeptides use proton gradients.
- Small peptides can be transported via transcytosis (vesicle-mediated transport).
Fat Digestion & Absorption
- Bile salts emulsify fats into micelles, allowing enzymes to produce monoglycerides and free fatty acids.
- Fatty acids and monoglycerides diffuse into cells, are processed, and then transported into lymph via lacteals.
- Cholesterol is absorbed using specific transporters.
Large Intestine Processes & Defecation
- The large intestine absorbs water, stores feces, and houses a microbiome generating essential vitamins.
- Defecation is controlled by internal (smooth) and external (skeletal, voluntary) anal sphincters.
Key Terms & Definitions
- Chyme — acidic, semi-liquid food mixture exiting the stomach.
- Villi/Microvilli — finger-like projections increasing intestinal surface area for absorption.
- Secretin — hormone that neutralizes stomach acid via bicarbonate secretion.
- CCK (Cholecystokinin) — hormone reducing gut motility and stimulating bile/pancreatic enzyme release.
- Amylase — enzyme breaking down starch into sugars.
- Endopeptidase/Exopeptidase — enzymes for breaking protein bonds internally/externally.
- Micelle — tiny fat droplet coated with bile salts for absorption.
- Lacteal — lymphatic vessel for fat absorption in the intestine.
Action Items / Next Steps
- Review key digestive hormones and their functions.
- Study mechanisms of nutrient absorption for carbohydrates, proteins, and fats.
- Prepare for questions on digestive anatomy and processes for the next class.