Unit 2: Digestion and Absorption

Table of Contents

1. Digestive system of Human

The human digestive system is a long, complex tube (the alimentary canal) and its associated glands. Its function is to break down large food molecules into smaller, absorbable units.

Components of the Digestive System

  1. Alimentary Canal (Gastrointestinal Tract):
    • Mouth & Oral Cavity: Site of ingestion and initial digestion (chewing, saliva).
    • Pharynx: Common passage for food and air.
    • Oesophagus: A muscular tube that transports food to the stomach via peristalsis.
    • Stomach: A J-shaped muscular bag that churns food and mixes it with gastric juices (acid, pepsin).
    • Small Intestine: The main site for digestion and absorption. Divided into the Duodenum, Jejunum, and Ileum.
    • Large Intestine: Absorbs water and electrolytes, forms feces. Includes the Colon, Rectum, and Anus.
  2. Associated Glands:
    • Salivary Glands: Produce saliva (contains salivary amylase).
    • Liver: Produces bile, which is stored in the Gallbladder. Bile emulsifies fats.
    • Pancreas: Produces pancreatic juice (contains enzymes like trypsin, amylase, lipase) and bicarbonate to neutralize stomach acid.

2. Process of digestion

Digestion is the breakdown of large, insoluble food molecules into small, water-soluble molecules.

Digestion in the Mouth

  • Mechanical: Mastication (chewing) breaks food into smaller pieces.
  • Chemical: Salivary glands release salivary amylase (also called ptyalin), an enzyme that begins the digestion of starch (a carbohydrate) into smaller sugars.

Digestion in the Stomach

  • The stomach secretes gastric juice, which contains:
    • Hydrochloric Acid (HCl): Kills bacteria and provides an acidic pH (1.5-3.5) for enzymes to work.
    • Pepsin: An enzyme that begins the digestion of proteins into smaller peptides.
  • The stomach churns the food, turning it into a semi-liquid paste called chyme.

Digestion in the Small Intestine

This is where most chemical digestion and all absorption occurs.

  1. The chyme enters the duodenum (first part).
  2. Bile from the liver acts like a detergent, breaking large fat globules into tiny droplets. This is called emulsification. It does *not* digest fat, but it increases the surface area for enzymes to work.
  3. Pancreatic Juice from the pancreas contains:
    • Pancreatic Amylase: Finishes carbohydrate digestion.
    • Trypsin: Finishes protein digestion.
    • Lipase: Digests the emulsified fats into fatty acids and glycerol.
Summary of Digestion:
  • Carbohydrates → Glucose
  • Proteins → Amino Acids
  • Fats (Lipids) → Fatty Acids and Glycerol

3. Absorption of digested products

Absorption is the process of moving the small, digested food molecules from the intestine into the bloodstream or lymphatic system.

  • Site: Occurs almost entirely in the small intestine.
  • Adaptations: The small intestine lining is folded and covered with millions of tiny, finger-like projections called villi and microvilli. These vastly increase the surface area for efficient absorption.
  • Pathways:
    • Glucose and Amino Acids: Are absorbed directly into the blood capillaries within the villi.
    • Fatty Acids and Glycerol: Are absorbed into the lacteals (lymph vessels) within the villi, and travel through the lymphatic system before rejoining the blood.

4. Food and its types

Food provides the energy and building materials the body needs. The main components are:

  • Macronutrients (Needed in large amounts):
    • Carbohydrates: The primary source of energy (e.g., bread, rice, sugars).
    • Proteins: Used for building and repairing tissues (e.g., meat, eggs, beans).
    • Fats (Lipids): A dense source of stored energy, insulation, and protection for organs (e.g., oil, butter).
  • Micronutrients (Needed in small amounts):
    • Vitamins: Organic compounds that help regulate body processes (e.g., Vitamin C for skin, Vitamin D for bones).
    • Minerals: Inorganic elements needed for various functions (e.g., Calcium for bones, Iron for blood).
  • Water: Essential for all body functions, including transport and chemical reactions.
  • Fibre (Roughage): Aids in digestion and keeps the bowels healthy, although it is not digested.

5. Diseases due to nutritional deficiency

A lack of essential nutrients in the diet leads to deficiency diseases.

Nutrient Deficiency Disease Symptoms
Protein (and Energy) Kwashiorkor / Marasmus Severe muscle wasting, stunted growth, edema (swelling).
Vitamin A Night Blindness Inability to see in dim light.
Vitamin B1 (Thiamine) Beri-beri Nerve damage, heart problems, muscle weakness.
Vitamin C Scurvy Bleeding gums, slow wound healing, joint pain.
Vitamin D Rickets (in children) Soft, weak, and deformed bones (e.g., bowed legs).
Iron (Mineral) Anemia Fatigue, weakness, pale skin, shortness of breath.
Iodine (Mineral) Goitre Swelling of the thyroid gland in the neck.