The Human Digestive System & Enzymes

• Digestion Definition: Digestion is the biochemical breakdown of large, complex, insoluble food molecules into small, simple, soluble and diffusible molecules that can be absorbed across the intestinal wall into the bloodstream. • Mechanical (Physical) Digestion: - The physical breakdown of large chunks of food into smaller pieces to increase the surface area for enzyme action. - Takes place in: * The Mouth: Teeth chew, tear, grind, and crush food (mastication); tongue mixes food with saliva into a slippery ball called a bolus. * The Stomach: Muscular walls churn and contract to grind food into an acidic semi-liquid paste called chyme. * The Duodenum: Bile salts emulsify large lipid droplets into tiny droplets (physical emulsification, not enzymatic). • Chemical Digestion: - The breakdown of complex covalent bonds in food using biological catalysts called ENZYMES. - Characteristics of Digestive Enzymes: * Protein in nature; destroyed (denatured) by high temperatures above 45°C. * Highly specific: Each enzyme works on only one type of substrate (e.g. amylase digests only starch, not protein). * Work best at an optimal temperature (~37°C in humans) and specific pH (acidic in stomach, alkaline in intestine).

Food travels along a continuous 9-meter muscular tube called the alimentary canal: 1. The Mouth (Oral Cavity): - Teeth perform mechanical mastication. - Salivary glands secrete Saliva containing the enzyme Salivary Amylase (Ptyalin). - Reaction: Starch + Water --[Amylase]--> Maltose (a sweet disaccharide sugar). 2. Oesophagus (Gullet): - A muscular tube connecting the pharynx to the stomach. - Food moves down by PERISTALSIS—wave-like rhythmic contractions and relaxations of longitudinal and circular muscles. No enzymes secreted here. 3. The Stomach: - Muscular sac that churns food with Gastric Juice secreted by gastric glands: * Hydrochloric Acid (HCl): Kills ingested pathogenic bacteria, provides an acidic pH (pH 1.5 - 2.0) needed to activate pepsin. * Pepsin: A protease enzyme that breaks down complex proteins into shorter peptide chains called Peptones. * Renin: Clots liquid milk protein in infants for digestion. * Mucus: Coats stomach lining to prevent self-digestion by acid and pepsin. 4. The Duodenum (First part of Small Intestine): - Receives secretions from two major accessory organs: * Liver: Produces BILE (stored in gall bladder). Bile contains bile salts that neutralize acidic chyme and emulsify fats (breaks big oil drops into tiny droplets). Bile contains NO enzymes. * Pancreas: Secretes Pancreatic Juice containing Pancreatic Amylase (starch → maltose), Trypsin (proteins → peptides), and Pancreatic Lipase (fats → fatty acids + glycerol). 5. The Ileum (Main Small Intestine): - Secretes intestinal juice (succus entericus) to complete digestion: * Maltase: Maltose → Glucose. * Peptidases (Erepsin): Peptides → Amino acids. * Lipase: Remaining fats → Fatty acids + Glycerol. - All digested food is now in its simplest, soluble forms: Glucose, Amino Acids, Fatty Acids, and Glycerol.

• Absorption in the Ileum: Absorption is the process by which simple, soluble end-products of digestion pass through the intestinal lining into blood and lymph vessels. • Structural Adaptations of the Ileum for Absorption: 1. Enormous Surface Area: Very long (~6 meters in adults) with inner walls folded into millions of microscopic finger-like projections called VILLI and microvilli. 2. Extremely Thin Walls: Epithelial layer of each villus is only one cell thick, providing a very short diffusion pathway. 3. Dense Capillary Network: Each villus contains rich blood capillaries that rapidly absorb and transport glucose, amino acids, minerals, and water-soluble vitamins directly to the liver via the Hepatic Portal Vein. 4. Central Lacteal: A lymph vessel in each villus that absorbs fatty acids and glycerol into the lymphatic system. • The Large Intestine (Colon & Rectum): - Colon: Reabsorbs water, mineral salts, and vitamins from undigested food residue. Failure of water reabsorption causes watery diarrhea. - Rectum: Temporarily stores semi-solid waste (faeces). - Anus: Sphincter muscle controlling the expulsion of faeces (Egestion / Defaecation).