9.1 Digestive System
The human digestive system consists of the alimentary canal (mouth, esophagus, stomach, small intestine, large intestine, rectum, anus) and associated organs (salivary glands, liver, gallbladder, pancreas).
Types of Digestion
- Physical Digestion: Mechanical breakdown of large food pieces into smaller particles (e.g., chewing in the mouth, churning in the stomach, emulsification of lipids by bile) to increase surface area for enzyme action.
- Chemical Digestion: Complex non-diffusible food molecules are broken down into simple diffusible molecules by digestive enzymes via hydrolysis.
9.2 Digestion
1. Digestion in the Mouth
- Chewing physically breaks down food. Salivary glands secrete saliva containing salivary amylase (pH 6.5–7.5).
- Reaction: $\text{Starch} + \text{Water} \xrightarrow{\text{Salivary Amylase}} \text{Maltose}$
- Food is rolled into a bolus and swallowed down the esophagus via peristalsis (rhythmic contraction and relaxation of circular and longitudinal muscles).
2. Digestion in the Stomach
- Gastric glands secrete gastric juice containing hydrochloric acid (HCl) and pepsin (pH 1.5–2.0).
- Functions of HCl: Provides an acidic medium for pepsin, kills bacteria, and stops the action of salivary amylase.
- Reaction: $\text{Protein} + \text{Water} \xrightarrow{\text{Pepsin}} \text{Polypeptides}$
- Food is churned into a semi-fluid mixture called chyme.
3. Digestion in the Small Intestine (Duodenum & Ileum)
- Duodenum receives:
- Bile (produced by liver, stored in gallbladder): Neutralizes acidic chyme and emulsifies lipids into tiny droplets to increase surface area for lipase.
- Pancreatic Juice (secreted by pancreas, pH 7.5–8.0):
- Pancreatic Amylase: $\text{Starch} + \text{Water} \rightarrow \text{Maltose}$
- Trypsin: $\text{Polypeptides} + \text{Water} \rightarrow \text{Peptides}$
- Pancreatic Lipase: $\text{Lipids} + \text{Water} \rightarrow \text{Fatty acids} + \text{Glycerol}$
- Ileum secretes Intestinal Juice (pH 7.0–8.0):
- Maltase: $\text{Maltose} + \text{Water} \rightarrow \text{Glucose}$
- Sucrase: $\text{Sucrose} + \text{Water} \rightarrow \text{Glucose} + \text{Fructose}$
- Lactase: $\text{Lactose} + \text{Water} \rightarrow \text{Glucose} + \text{Galactose}$
- Erepsin (Peptidase): $\text{Peptides} + \text{Water} \rightarrow \text{Amino acids}$
9.3 Absorption
Absorption of digested nutrients takes place primarily in the ileum of the small intestine.
Adaptations of the Ileum for Absorption
- Extremely Long: Increases surface area and time for absorption.
- Folded Internal Walls: Covered with millions of finger-like projections called villi (singular: villus).
- Microvilli: Epithelial cells of villi have microscopic projections providing a vast surface area.
- Thin Epithelium: One cell thick for rapid diffusion.
- Dense Capillary Network & Lacteals: Efficient transport of absorbed nutrients.
Nutrient Transport Routes
- Into Blood Capillaries (Water-Soluble): Glucose, amino acids, fructose, galactose, water-soluble vitamins (B and C), and mineral salts are absorbed via active transport or facilitated diffusion into capillaries $\rightarrow$ transported to the liver via the hepatic portal vein.
- Into Lacteals (Fat-Soluble): Fatty acids and glycerol recombine inside epithelial cells to form droplets, which enter lacteals along with fat-soluble vitamins (A, D, E, K) $\rightarrow$ transported via the lymphatic system into the bloodstream at the left subclavian vein.
9.4 Assimilation
Assimilation is the process where absorbed nutrients are taken up by body cells and converted into complex cellular components or used for energy.
Role of the Liver in Assimilation
- Metabolism of Digested Food: Converts excess glucose into glycogen for storage (regulated by insulin). Excess amino acids undergo deamination to form urea.
- Detoxification: Neutralizes toxic substances (drugs, alcohol, toxins).
- Bile Production: Produces bile salts for fat emulsification.
9.5 Defecation
- Undigested material (cellulose/fiber, dead cells, mucus) enters the large intestine (colon).
- Colon Function: Reabsorbs water and mineral salts, converting liquid waste into solid feces.
- Role of Microorganisms: Symbiotic bacteria (e.g., E. coli) synthesize Vitamin K and Vitamin B12.
- Feces are stored temporarily in the rectum and expelled through the anus via defecation.
9.6 Balanced Diet and Health Issues
- Balanced Diet: Contains all seven nutrient classes (carbohydrates, proteins, lipids, vitamins, minerals, water, dietary fiber) in correct proportions.
- Energy Value of Food: Measured in kilojoules (kJ) or calories (cal) using a bomb calorimeter.
$$\text{Energy Value} = \frac{\text{Mass of water (g)} \times 4.2\text{ J g}^{-1}{^\circ\text{C}}^{-1} \times \text{Temperature rise } ({^\circ\text{C}})}{\text{Mass of food sample (g)} \times 1000}$$
- Health Issues:
- Obesity: Excessive fat accumulation leading to cardiovascular disease, diabetes mellitus, hypertension.
- Gastritis: Inflammation of the stomach lining caused by irregular meals, excess HCl secretion, or bacterial infection (Helicobacter pylori).
- Anorexia Nervosa / Bulimia Nervosa: Eating disorders leading to severe malnutrition and metabolic failure.