Chapter 10: Food Digestion, Absorption, and Distribution – Biochemistry Study Notes

Food Digestion, Absorption, and Distribution

Introduction

This chapter explores the biochemical processes involved in the digestion, absorption, and distribution of food in the human body. It covers the breakdown of macronutrients, the roles of enzymes and hormones, and the integration of metabolic pathways in different tissues.

Digestion

Overview of Digestion

• Digestion is the process by which complex food substances are broken down into simpler molecules that can be absorbed and utilized by the body.

• It involves both mechanical and chemical processes, beginning in the mouth and continuing through the gastrointestinal tract.

Metabolism

• Metabolism refers to the sum of all chemical reactions that occur within living organisms to maintain life.

• It includes catabolism (breakdown of molecules to release energy) and anabolism (synthesis of complex molecules from simpler ones).

Roles of Metabolism

• Provides energy for cellular processes.

• Supplies building blocks for biosynthesis.

• Regulates storage and mobilization of nutrients.

Chemistry of Foodstuffs

Major Food Components

• Carbohydrates: Main source of energy; include sugars, starches, and fibers.

• Proteins: Provide amino acids for protein synthesis and energy.

• Lipids: Concentrated energy source; essential for cell membranes and signaling.

Digestion and Absorption

General Principles

• Enzymatic hydrolysis breaks down macromolecules into absorbable units.

• Absorption occurs primarily in the small intestine via specialized transport mechanisms.

Protection of Gastrointestinal Tract

• Mucus protects epithelial cells from digestive enzymes and acidic conditions.

• Zymogens (inactive enzyme precursors) prevent autodigestion; activated only in the digestive tract lumen.

Digestion of Proteins

Stomach

• Pepsin is the principal protease in the stomach, activated from pepsinogen by gastric acid.

• HCl production by parietal cells creates an acidic environment (pH ~2) optimal for pepsin activity.

Small Intestine

• Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) further digest proteins into oligopeptides and amino acids.

• Activation of pancreatic zymogens occurs in the small intestine (e.g., trypsinogen to trypsin by enteropeptidase).

Absorption of Amino Acids

• Amino acids are absorbed into enterocytes via active transport mechanisms, often coupled with sodium ions.

• They enter the bloodstream for distribution to tissues.

Digestion of Carbohydrates

Structure of Carbohydrates

• Carbohydrates are composed of monosaccharides linked by glycosidic bonds.

• Common dietary carbohydrates include starch (amylose and amylopectin), glycogen, lactose, and sucrose.

The Glycosidic Bond

• Formed by condensation between two monosaccharides, releasing water.

• Example: Maltose is formed by an α(1→4) glycosidic bond between two glucose units.

Digestion of Starch and Disaccharides

• Amylase (salivary and pancreatic) hydrolyzes starch into maltose and dextrins.

• Disaccharidases (maltase, sucrase, lactase) in the intestinal brush border complete digestion to monosaccharides.

Absorption of Monosaccharides

• Glucose and galactose are absorbed via sodium-dependent active transport (SGLT1).

• Fructose is absorbed by facilitated diffusion (GLUT5 transporter).

Digestion and Absorption of Lipids

Overview

• Lipids are emulsified by bile salts and hydrolyzed by pancreatic lipase into fatty acids and monoglycerides.

• Products are incorporated into micelles for absorption by enterocytes.

Chylomicrons

• Re-esterified triglycerides, cholesterol, and fat-soluble vitamins are packaged into chylomicrons for transport via the lymphatic system.

Energy Considerations in Digestion and Absorption

• Digestion is generally not limited by thermodynamic constraints; enzymes lower activation energy for hydrolysis reactions.

• Absorption often requires energy input, especially for active transport processes.

Integration of Metabolism and Fuel Distribution

Characteristics of Different Tissues

• Liver: Central role in metabolism; regulates glucose, amino acid, and lipid metabolism.

• Brain: Relies primarily on glucose for energy; can adapt to ketone bodies during starvation.

• Skeletal Muscle: Utilizes glucose, fatty acids, and ketone bodies depending on activity and nutritional state.

Overall Control by Hormones

• Hormones such as insulin and glucagon coordinate the distribution and storage of nutrients.

• Insulin promotes glucose uptake and storage; glucagon stimulates glycogenolysis and gluconeogenesis.

Summary Table: Major Digestive Enzymes and Their Functions

Enzyme	Source	Substrate	Product
Salivary amylase	Salivary glands	Starch	Maltose, dextrins
Pepsin	Stomach	Proteins	Peptides
Pancreatic lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides
Trypsin	Pancreas (activated in intestine)	Proteins, peptides	Smaller peptides, amino acids
Lactase	Small intestine	Lactose	Glucose, galactose

Key Equations

• General hydrolysis of a glycosidic bond: $\text{R-O-R'} + H_2O \rightarrow \text{R-OH} + \text{R'-OH}$

• Activation of trypsinogen: $\text{Trypsinogen} \xrightarrow{\text{Enteropeptidase}} \text{Trypsin}$

Additional Info

• Some slides referenced diagrams and metabolic pathways; standard textbook diagrams of digestion and absorption can supplement these notes.

• For more detail on hormonal regulation, see chapters on biosignaling and metabolic integration.