Describe the process of digestion in humans. Explain the role of different digestive enzymes.

Process of Digestion in Humans

Digestion in humans is a complex process involving mechanical and chemical breakdown of food into absorbable nutrients. It occurs in the alimentary canal (mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus) with the help of digestive enzymes and accessory organs (teeth, tongue, liver, pancreas, and gallbladder).

1. Mouth (Ingestion & Carbohydrate Digestion)

• Mechanical digestion: Teeth cut and grind food into smaller pieces, while the tongue mixes it with saliva.
• Chemical digestion: Salivary glands secrete salivary amylase, which breaks down starch (polysaccharides) into maltose and dextrin.
• Role of enzyme: Salivary amylase works best at pH 6.8–7.0 (neutral pH).

2. Esophagus (Transport)

• Food is swallowed and pushed down via peristalsis (wave-like muscle contractions) into the stomach.

3. Stomach (Protein Digestion & Storage)

• Mechanical digestion: Muscular walls churn food into a semi-liquid called chyme.
• Chemical digestion:
  • Gastric glands secrete hydrochloric acid (HCl), which:
    • Kills bacteria.
    • Converts pepsinogen into pepsin (active enzyme).
  • Pepsin breaks down proteins into peptides (pH 1.5–3.5, acidic).
  • Mucus protects the stomach lining from HCl.
  • Lipids are emulsified by bile salts (from liver, stored in gallbladder) but not chemically digested here.

4. Small Intestine (Major Digestion & Absorption)

• Duodenum (First part):
  • Pancreatic juice (from pancreas) contains:
    • Trypsin (converts proteins → peptides; activated by enterokinase).
    • Pancreatic amylase (further breaks carbs → maltose).
    • Pancreatic lipase (fats → fatty acids + glycerol).
  • Bile (from liver/gallbladder) emulsifies fats into micelles for lipase action.
• Ileum (Later part):
  • Brush border enzymes (on intestinal lining) complete digestion:
    • Maltase (maltose → glucose).
    • Peptidase (peptides → amino acids).
    • Sucrase & lactase (sucrose/lactose → glucose/fructose/galactose).
  • Absorption: Nutrients pass into blood capillaries (carbs/proteins) or lacteals (fats).

Glucose (C₆H₁₂O₆)Glucose (C₆H₁₂O₆)Glycerol (C₃H₈O₃)Glycerol (C₃H₈O₃)

5. Large Intestine (Water & Electrolyte Absorption)

• Mechanical action: Peristalsis moves undigested food.
• Chemical action: Bacteria ferment some undigested material, producing vitamin K and B-complex vitamins.
• Water & salt absorption: ~90% of water is reabsorbed here.

6. Rectum & Anus (Egestion)

• Undigested waste (feces) is expelled via the anus.

Role of Key Digestive Enzymes

Key Points

• Carbohydrates are digested mainly in the mouth and small intestine.
• Proteins begin digestion in the stomach and complete in the small intestine.
• Fats require bile salts (emulsification) and pancreatic lipase for breakdown.
• Enzymes work optimally at specific pH levels (acidic in stomach, alkaline in small intestine).
• Absorption occurs primarily in the small intestine, while the large intestine absorbs water and electrolytes.