Unit 1: Digestion

1. Definition of Digestion

Digestion is the complex biochemical process of breaking down large, insoluble food macromolecules (like carbohydrates, proteins, and fats) into smaller, water-soluble molecules that can be absorbed into the bloodstream and used by the body's cells.

This involves two types of processes:

• Mechanical Digestion: The physical breakdown of food into smaller pieces (e.g., chewing, churning in the stomach).
• Chemical Digestion: The chemical breakdown of food by enzymes.

2. Human Digestive System (Alimentary Canal)

The human digestive system consists of the alimentary canal (a long, continuous tube) and accessory organs.

[Image of the human digestive system]

Alimentary Canal

1. Mouth (Buccal Cavity): Site of ingestion, mechanical digestion (chewing), and initial chemical digestion of carbohydrates.
2. Pharynx: Passageway for food (and air).
3. Esophagus: A muscular tube that transports food from the pharynx to the stomach via peristalsis (wave-like muscle contractions).
4. Stomach: A J-shaped muscular organ that stores food, churns it (mechanical digestion), and mixes it with gastric juices to begin protein digestion. The semi-fluid mass is called chyme.
5. Small Intestine: A long, coiled tube and the primary site for chemical digestion and absorption of nutrients. It has three parts:
   • Duodenum: Receives chyme from the stomach, plus bile and pancreatic juice.
   • Jejunum: Main site of absorption.
   • Ileum: Final section, leads to the large intestine.
6. Large Intestine: Absorbs water and electrolytes from undigested food, forming feces. Houses gut bacteria that synthesize vitamins.
7. Rectum & Anus: Stores and eliminates feces from the body.

Accessory Organs

These organs produce or store secretions that aid in digestion.

• Salivary Glands: Produce saliva.
• Liver: Produces bile.
• Gall Bladder: Stores and concentrates bile.
• Pancreas: Produces pancreatic juice containing enzymes and bicarbonate.

3. Process of Digestion

3a. Digestion in Buccal Cavity (Mouth)

• Mechanical: Mastication (chewing) breaks food into smaller pieces, increasing surface area.
• Chemical: Salivary amylase (an enzyme in saliva) begins the hydrolysis of starch (a carbohydrate) into smaller sugars like maltose.

3b. Digestion in Stomach

• Mechanical: Churning actions of the stomach muscles mix the food.
• Chemical: The stomach's gastric juice contains:
  • Hydrochloric Acid (HCl): Kills microbes and provides a highly acidic pH (~1.5-3.5).
  • Pepsinogen: An inactive enzyme (zymogen). HCl converts it into Pepsin.
  • Pepsin: The active enzyme that begins the breakdown of proteins into smaller polypeptides.

3c. Digestion in Intestine (Small Intestine)

This is where most chemical digestion and all nutrient absorption occur.

1. Neutralization: Acidic chyme from the stomach is neutralized by bicarbonate from the pancreatic juice.
2. Fat Digestion: Bile (from the liver) emulsifies large fat globules into smaller droplets. This increases the surface area for lipase (from the pancreas) to break down fats (triglycerides) into fatty acids and monoglycerides.
3. Carbohydrate Digestion: Pancreatic amylase continues breaking down starch. Other enzymes in the intestinal wall (e.g., lactase, sucrase, maltase) break disaccharides into monosaccharides (glucose, fructose, galactose).
4. Protein Digestion: Trypsin and Chymotrypsin (from the pancreas) break polypeptides into smaller peptides. Then, peptidases (from the pancreas and intestinal wall) break these down into amino acids.

4. Basic Composition of Digestive Juices

5. Role of Liver, Gall Bladder, and Pancreas

a) Liver

• Primary Digestive Role: Produces bile. Bile is essential for fat digestion as it emulsifies fats, breaking large globules into smaller droplets.
• Other Metabolic Roles: The liver is a major metabolic hub. After absorption, blood from the intestine goes directly to the liver, which processes nutrients, detoxifies substances, and stores glycogen.

b) Gall Bladder

• Role: A small organ tucked under the liver.
• Function: It does *not* produce anything. Its sole function is to store and concentrate the bile produced by the liver. It releases this concentrated bile into the small intestine when food (especially fat) is present.

c) Pancreas

• Exocrine Role (Digestive): Produces pancreatic juice, which is a powerful cocktail of digestive enzymes (amylase, lipase, trypsin, chymotrypsin) and bicarbonate. The bicarbonate is critical for neutralizing stomach acid, allowing the intestinal enzymes to function at their optimal pH.
• Endocrine Role (Hormonal): (See Unit 5) Produces the hormones insulin and glucagon, which regulate blood sugar levels.