Zoology Notes

HSSLIVE Plus One Zoology Chapter 5: Digestion and Absorption Notes

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Nutrition acquisition through digestion and absorption is essential for all heterotrophic organisms. This chapter explores the human digestive system, tracing the journey of food from ingestion through mechanical and chemical breakdown in the alimentary canal. It details the roles of digestive enzymes, bile, and other secretions in transforming complex food molecules into absorbable nutrients. The chapter also examines the specialized structures for nutrient absorption in the small intestine and how these nutrients are transported to cells throughout the body.

Chapter 5: Digestion and Absorption

Digestive System in Humans

The human digestive system consists of the alimentary canal (gastrointestinal tract) and associated digestive glands.

Alimentary Canal Components:

  1. Mouth: Entry point for food
  2. Pharynx: Common passage for food and air
  3. Esophagus: Tube connecting pharynx to stomach
  4. Stomach: Temporary storage and partial digestion
  5. Small Intestine: Main site of digestion and absorption
    • Duodenum (first part)
    • Jejunum (middle part)
    • Ileum (last part)
  6. Large Intestine: Water absorption and feces formation
    • Cecum with appendix
    • Colon (ascending, transverse, descending, sigmoid)
    • Rectum
    • Anal canal

Digestive Glands:

  1. Salivary Glands: Produce saliva with amylase
  2. Gastric Glands: Produce gastric juice with pepsin, HCl, mucus
  3. Pancreas: Produces pancreatic juice with various enzymes
  4. Liver: Produces bile for fat emulsification
  5. Intestinal Glands: Produce intestinal juice (succus entericus)

Digestive Process

1. Mouth and Buccal Cavity:

  • Mechanical Process: Chewing (mastication) breaks down food
  • Chemical Process: Salivary amylase begins carbohydrate digestion
  • Salivary Secretion: 1-1.5 liters per day
  • Saliva contains water, mucus, lysozyme (antibacterial), salivary amylase

2. Pharynx and Esophagus:

  • Swallowing (deglutition) moves food from mouth to stomach
  • Peristaltic movements push food through esophagus
  • No digestion occurs in these regions

3. Stomach:

  • Structure: J-shaped sac with rugae (folds)
  • Regions: Cardiac, fundic, body, pyloric
  • Gastric Juice Components:
    • HCl (provides acidic medium, kills bacteria)
    • Pepsinogen (converted to pepsin)
    • Mucus (protects stomach lining)
    • Intrinsic factor (vitamin B12 absorption)
  • Digestion:
    • Proteins → Peptones and proteoses
    • No carbohydrate or fat digestion
  • Regulation: Neural (vagus nerve) and hormonal (gastrin)

4. Small Intestine:

  • Main site for digestion and absorption
  • Pancreatic Juice:
    • Trypsin, chymotrypsin, carboxypeptidase (protein digestion)
    • Pancreatic amylase (carbohydrate digestion)
    • Lipase (fat digestion)
    • Nucleases (nucleic acid digestion)
  • Bile (from liver):
    • Emulsifies fats
    • Activates lipase
    • Contains bile salts, bile pigments, cholesterol
  • Intestinal Juice:
    • Completes digestion of all nutrients
    • Contains enzymes like peptidases, sucrase, maltase, lactase

Digestion of Major Nutrients:

  1. Carbohydrates:
    • Starch → Maltose (by amylase)
    • Maltose → Glucose (by maltase)
    • Sucrose → Glucose + Fructose (by sucrase)
    • Lactose → Glucose + Galactose (by lactase)
  2. Proteins:
    • Proteins → Peptones (by pepsin in stomach)
    • Peptones → Peptides (by trypsin, chymotrypsin)
    • Peptides → Amino acids (by carboxypeptidase, aminopeptidase)
  3. Lipids:
    • Emulsification by bile salts
    • Triglycerides → Monoglycerides + Fatty acids (by lipase)

Absorption of Digested Products

Small Intestine Adaptations for Absorption:

  • Long length (5-7 meters)
  • Circular folds (plicae circulares)
  • Villi (finger-like projections)
  • Microvilli (brush border)
  • These features increase surface area by about 600 times

Absorption Mechanisms:

  1. Simple diffusion: Fat-soluble substances
  2. Facilitated transport: Some sugars and amino acids
  3. Active transport: Most amino acids, glucose, minerals
  4. Endocytosis: Large molecules

Absorbed Nutrients:

  • Glucose, amino acids, minerals: Absorbed into blood capillaries in villi
  • Fatty acids and glycerol: Form chylomicrons and enter lacteal (lymphatic vessel)

Large Intestine Functions:

  • Absorption of water, minerals, and certain vitamins
  • Formation of feces
  • Hosts beneficial bacteria that produce vitamins K and B12
  • Elimination of undigested waste as feces

Disorders of Digestive System:

  • Jaundice: Liver disorder causing yellowing of skin
  • Vomiting: Forceful expulsion of stomach contents
  • Diarrhea: Frequent watery stools
  • Constipation: Difficulty in bowel movements
  • Indigestion: Discomfort from incomplete digestion
  • Peptic Ulcer: Erosion of GI tract lining by acid

Complete Chapter-wise Hsslive Plus One Zoology Notes

Our HSSLive Plus One Zoology Notes cover all chapters with key focus areas to help you organize your study effectively:

  1. Chapter 1 The Living World
  2. Chapter 2 Animal Kingdom
  3. Chapter 3 Structural Organisation in Animals
  4. Chapter 4 Biomolecules
  5. Chapter 5 Digestion and Absorption
  6. Chapter 6 Breathing and Exchange of Gases
  7. Chapter 7 Body Fluids and Circulation
  8. Chapter 8 Excretory Products and their Elimination
  9. Chapter 9 Locomotion and Movement
  10. Chapter 10 Neural Control and Coordination
  11. Chapter 11 Chemical Coordination and integration

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