Ch 23 Digestion and Absorption

Digestive System Overview

The small intestine is the primary site for absorption in the digestive system, particularly in the jejunum and, to a lesser extent, the ileum.
Key structures in the small intestine enhancing absorption:
- Circular folds: Increase surface area for absorption.
- Villi: Finger-like projections that further increase surface area.
- Microvilli: Cellular extensions on enterocytes (intestinal cells) that create a brush border, the site of absorption.

Microvilli are found on the apical surface of enterocytes, facing the intestinal lumen, crucial for nutrient absorption.
Brush border enzymes: Located on microvilli, they catalyze the final steps of digestion before absorption. Example: enzymes breaking down disaccharides into monosaccharides.
Basolateral membrane: Or basal membrane, interfaces with connective tissue and interstitial fluid, facilitating nutrient transport after absorption.

Tight junctions: Connect enterocytes, preventing substances from passing between them; nutrients must be transported across membranes and through enterocytes to be absorbed.
Absorption involves:
- Transporting nutrients from the intestinal lumen across the apical membrane into the cytosol of enterocytes.
- Moving nutrients across the basolateral membrane into interstitial fluids.
Lumen: Represents the external environment; anything here is technically outside the body until absorbed.

Digestion begins in the buccal cavity (mouth) with salivary amylase, though its impact is minimal.
Major carbohydrate digestion occurs:
- In the small intestine via pancreatic amylase that breaks down starch.
- By brush border enzymes into monosaccharides (e.g., glucose, galactose, fructose).
Transport mechanisms for monosaccharides:
- Glucose and Galactose: Transported via secondary active transport (co-transport mechanism with sodium).
- Fructose: Utilizes facilitated diffusion, a sodium-independent process.

Protein digestion begins in the stomach with pepsin (activated from pepsinogen) and continues in the small intestine.
Proteins are broken down into peptides by pancreatic enzymes; brush border enzymes further digest them into amino acids.
Transport mechanisms:
- Amino acids and small peptides use secondary active transport dependent on the sodium gradient for absorption into the enterocytes.
- Dipeptides and tripeptides can also be absorbed by this mechanism.

Fat digestion differs as fats are lipid-soluble and do not require carriers like water-soluble molecules.
Begins in the buccal cavity with lingual lipase, but this is minimal.
Digestion process:
- Gastric lipase continues fat digestion in the stomach.
- In the small intestine, bile (from gallbladder) emulsifies fats to increase surface area for pancreatic lipases to act on them.
Fat digestion produces fatty acids and monoglycerides, which can easily diffuse across the apical membrane into enterocytes.
Inside enterocytes, fats are reassembled into triglycerides and packaged into chylomicrons.
Chylomicrons are then secreted via exocytosis into interstitial fluid and transported through lymphatic and circulatory systems.

Digestion: A series of catabolic reactions that breakdown food into simpler units for absorption (e.g., macromolecules into monomers).
Absorption: The process of transporting nutrients from the intestine into the body, predominantly occurring in the small intestine (jejunum and ileum).
Goal: To absorb the simplest forms of nutrients, allowing for efficient transport into the cells of the body.