Digestion and Absorption in the Digestive System

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The Digestive System: Digestion and Absorption

Overview

The digestive system is responsible for the breakdown of food into absorbable units and the subsequent absorption of nutrients, water, and electrolytes. Digestion involves both mechanical and chemical processes, while absorption refers to the movement of these digested molecules into the body's internal environment.

Digestion

Mechanical and Chemical Breakdown

Mechanical breakdown is accomplished through motility, such as chewing and muscular contractions in the gastrointestinal (GI) tract.
Chemical breakdown is achieved by digestive enzymes that hydrolyze macromolecules into their building blocks.

Digestive Enzymes

α-Amylase and Disaccharidases: Act on carbohydrates.
Proteases: Act on proteins.
Lipases: Act on fats.
Lumenal enzymes: Secreted into the GI tract as part of saliva, gastric, and pancreatic juice.
Brushborder enzymes: Embedded in the membrane at the apical side of epithelial cells in the small intestine.

Hydrolysis

Hydrolysis is the chemical process by which digestive enzymes cleave chemical bonds in macromolecules by the addition of water.

Example: The enzyme sucrase catalyzes the hydrolysis of sucrose () into glucose () and fructose ():

Absorption

Mechanisms of Absorption

Absorption is the movement of nutrients, water, and electrolytes from the lumen of the GI tract across the epithelial layer into the interstitial fluid, and then into blood and lymph.

Transcellular absorption: Transport across the luminal membrane, through the epithelial cell, and out across the basolateral membrane.
Paracellular absorption: Transport between adjacent cells along concentration or electrochemical gradients.

Carbohydrates

Dietary Carbohydrates

Starch and Glycogen: 60% of diet; polymers of glucose.
Sucrose: 20% of diet; composed of glucose and fructose.
Lactose: 6% of diet; composed of glucose and galactose.
Glucose, Galactose, Fructose: 12% of diet.

Key Point: Carbohydrates are absorbed as monosaccharides (glucose, galactose, fructose).

Dietary Fiber

Definition: Polysaccharides that cannot be digested by human enzymes (e.g., cellulose, hemicellulose, pectins).
α-Amylase digests starch (α(1→4) linkage), but not cellulose (β(1→4) linkage).
In the colon, fiber induces osmotic water flow (reducing constipation) and is fermented by bacteria to organic anions and gases.

Carbohydrate Digestion Pathway

Starch and glycogen are broken down by amylase into disaccharides (maltose, sucrose, lactose).
Disaccharidases (maltase, sucrase, lactase) further break down disaccharides into monosaccharides.
Monosaccharides are absorbed in the small intestine.

Location of Digestive Enzymes

Lumenal α-Amylase: Found in the oral cavity (salivary glands) and duodenum (pancreas).
Brush border of enterocytes in the small intestine: Contains maltase, sucrase, and lactase.

Protein Digestion and Absorption

Protein Structure and Digestion

Proteins are chains of amino acids linked by peptide bonds.
Endopeptidases (e.g., pepsin, trypsin) cleave peptide bonds within the protein chain.
Exopeptidases remove amino acids from the ends of peptide chains.

Location and Activation of GI Proteases

Pepsinogen (inactive) is converted to pepsin (active) in the acidic environment of the stomach.
Pancreatic proteases (e.g., trypsinogen) are activated in the small intestine.
Brush border peptidases further digest peptides at the epithelial surface.

Protein Absorption

Amino acids and small peptides are absorbed via secondary active transport mechanisms.
Deficiency in pancreatic enzymes or transporter defects can lead to malabsorption (e.g., chronic pancreatitis, cystic fibrosis).

Lipid Digestion and Absorption

Dietary Lipids

Include triglycerides, cholesterol, and phospholipids.
Lipids are hydrophobic and require emulsification for digestion.

Emulsification and Digestion

Lingual and gastric lipase begin digestion in the mouth and stomach (minor role).
Most lipid digestion occurs in the small intestine, aided by bile salts that emulsify fat droplets.
Bile salts are recycled via portal circulation.

Vitamin Absorption

Fat-Soluble Vitamins

Vitamins A, D, E, and K are absorbed with fats in the small intestine.
Consumption of fat substitutes (e.g., olestra) or lipase inhibitors (e.g., orlistat) can lead to deficiencies in these vitamins.

Water-Soluble Vitamins

Vitamins C and most B vitamins are absorbed by mediated transport.
Vitamin B12 requires intrinsic factor for absorption in the terminal ileum.

Iron and Calcium Absorption

Regulation and Mechanisms

Absorption of iron and calcium is regulated according to body needs.
Iron is absorbed as heme (from meat) or non-heme (from plants), via apical cotransporters and exported by ferroportin.
Calcium absorption increases when body stores are low, via passive paracellular transport or active transport in the duodenum.

Fluid and Electrolyte Absorption

Sodium (Na+), potassium (K+), chloride (Cl-), and water are absorbed via epithelial transport mechanisms.
Water absorption follows osmotic gradients created by solute transport.

Digestion and Absorption in the Large Intestine

Na+ and K+ are absorbed along with water.
Undigested polysaccharides (fiber) are metabolized by bacteria to short-chain fatty acids, which are absorbed by diffusion.
Small amounts of vitamin K and gases (flatus) are produced and absorbed.

Absorption Along the GI Tract

Region	Absorbed Substances
Small Intestine	Fat-soluble vitamins (A, D, E, K), water-soluble vitamins (folic acid, B12), sodium, potassium, water
Colon (Large Intestine)	Sodium, potassium, water, acids, gases, vitamin K

Disorders of Digestion and Absorption

Osmotic diarrhea: Caused by ingestion of indigestible carbohydrates (e.g., artificial sweeteners) or lactose intolerance.
Protein malabsorption: Due to enzyme deficiencies or transporter defects.
Vitamin deficiencies: Result from impaired fat absorption.

Additional info: The above notes expand on the original slides and text, providing definitions, mechanisms, and examples for each major process in digestion and absorption.