Class 11 Biology Notes Chapter 16 (Digestion and absorption) – Biology Book

Detailed Notes with MCQs of Chapter 16: Digestion and Absorption. This is a crucial chapter, not just for understanding human physiology but also frequently tested in various government examinations. Pay close attention to the details, especially the enzymes, glands, and processes involved.

Chapter 16: Digestion and Absorption - Detailed Notes

1. Introduction

• Food: Provides energy and organic materials for growth and tissue repair. Major components include carbohydrates, proteins, fats, vitamins, minerals, and water.
• Digestion: The process of converting complex, non-diffusible food substances into simple, absorbable forms through mechanical and biochemical methods.
• Absorption: The process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.

2. Digestive System

Consists of:
* Alimentary Canal
* Associated Digestive Glands

A. Alimentary Canal

A long, muscular tube starting from the mouth and ending at the anus.

• Mouth: Leads to the buccal cavity (oral cavity).

  • Teeth: Hard structures for mastication (chewing).
    • Thecodont: Embedded in jaw sockets.
    • Diphyodont: Two sets during life (milk/deciduous and permanent).
    • Heterodont: Different types (Incisors-I, Canines-C, Premolars-PM, Molars-M).
    • Dental Formula (Human Adult): 2123/2123 (Total 32 teeth). (I-2, C-1, PM-2, M-3 in each half of upper and lower jaw).
    • Dental Formula (Milk Teeth): 2102/2102 (Total 20 teeth). (Premolars are absent).
    • Enamel: Hardest substance, covers the chewing surface.
  • Tongue: Freely movable muscular organ attached to the floor of the oral cavity by the frenulum.
    • Papillae: Small projections on the upper surface, some bear taste buds.

• Pharynx: Common passage for food and air.

  • Opens into the oesophagus (food pipe) and larynx (windpipe).
  • Epiglottis: A cartilaginous flap prevents food entry into the glottis (opening of the windpipe) during swallowing (deglutition).

• Oesophagus: A thin, long tube extending posteriorly, passing through the neck, thorax, and diaphragm.

  • Leads to the stomach.
  • Gastro-oesophageal sphincter: Muscular valve regulating the opening into the stomach.
  • Peristalsis: Wave-like muscular contractions propelling food downwards.

• Stomach: J-shaped muscular bag located in the upper abdominal cavity.

  • Parts: Cardiac (receives oesophagus), Fundic, Body (main central region), Pyloric (opens into the small intestine).
  • Pyloric sphincter: Regulates the opening from the stomach to the duodenum.
  • Rugae: Irregular folds in the inner lining when empty.

• Small Intestine: Longest part of the alimentary canal, coiled. Major site for digestion and absorption.

  • Parts:
    • Duodenum: C-shaped, first part. Receives secretions from the liver and pancreas.
    • Jejunum: Middle part, coiled.
    • Ileum: Highly coiled, terminal part. Opens into the large intestine.
  • Villi: Finger-like projections of the mucosa increasing surface area for absorption. Lined by epithelial cells bearing microvilli (brush border appearance). Villi contain capillaries and a large lymph vessel called the lacteal.
  • Crypts of Lieberkühn: Glands found between the bases of villi.
  • Brunner's Glands: Found only in the sub-mucosa of the duodenum; secrete alkaline mucus.

• Large Intestine: Wider than the small intestine.

  • Parts:
    • Caecum: Small blind sac, hosts symbiotic microorganisms. Vermiform appendix (a narrow finger-like projection, vestigial organ) arises from it.
    • Colon: Divided into ascending, transverse, descending, and sigmoid colon.
    • Rectum: Stores faecal matter temporarily.
  • Functions: Absorption of water, some minerals, and drugs; secretion of mucus for adhesion and lubrication.
  • Ileocaecal valve: Prevents backflow of faecal matter into the ileum.

• Anus: Terminal opening for egestion (defecation). Guarded by the anal sphincter (internal involuntary, external voluntary).

B. Histology of Alimentary Canal (Wall Layers from Outer to Inner)

1. Serosa: Outermost layer, thin mesothelium (epithelium of visceral organs) with some connective tissues.
2. Muscularis: Smooth muscles. Usually arranged as an inner circular and an outer longitudinal layer. (An oblique muscle layer is present in the stomach). Responsible for peristalsis.
3. Sub-mucosa: Loose connective tissue containing nerves, blood vessels, and lymph vessels. Glands (like Brunner's glands in duodenum) are present here.
4. Mucosa: Innermost layer lining the lumen.
   • Forms irregular folds (rugae) in the stomach and villi in the small intestine.
   • Contains goblet cells (secrete mucus).
   • Forms gastric glands in the stomach and Crypts of Lieberkühn in the intestine.
   • Lamina propria: Connective tissue layer below the epithelium.
   • Muscularis mucosae: Thin layer of smooth muscle.

3. Digestive Glands

Associated glands secreting digestive juices.

• Salivary Glands: Three pairs located just outside the buccal cavity.

  • Parotid glands: Cheek.
  • Sub-maxillary / Sub-mandibular glands: Lower jaw.
  • Sub-lingual glands: Below the tongue.
  • Saliva: Secretion contains water, electrolytes (Na+, K+, Cl-, HCO3-), salivary amylase (ptyalin), and lysozyme (antibacterial). pH ≈ 6.8.

• Liver: Largest gland (1.2-1.5 kg in adult human). Located in the upper right side of the abdominal cavity.

  • Lobules: Structural and functional units, containing hepatic cells arranged in cords. Each lobule is covered by Glisson's capsule (thin connective tissue sheath).
  • Bile: Secreted by hepatic cells, stored and concentrated in the gall bladder.
    • Composition: Bile pigments (bilirubin, biliverdin - breakdown products of haemoglobin), bile salts (aid fat emulsification), cholesterol, phospholipids. No enzymes.
  • Ducts: Hepatic ducts from liver lobes form the common hepatic duct. Cystic duct from the gall bladder joins it to form the common bile duct.
  • Hepato-pancreatic duct: Formed by the joining of the common bile duct and the pancreatic duct. Opens into the duodenum, guarded by the Sphincter of Oddi.

• Pancreas: Compound gland (both exocrine and endocrine). Located between the limbs of the C-shaped duodenum.

  • Exocrine part: Secretes alkaline pancreatic juice containing enzymes.
    • Inactive enzymes: Trypsinogen, Chymotrypsinogen, Procarboxypeptidase.
    • Active enzymes: Pancreatic amylase, Lipases, Nucleases.
  • Endocrine part (Islets of Langerhans): Secretes hormones (insulin, glucagon).

• Gastric Glands (in Stomach Mucosa):

  • Mucus neck cells: Secrete mucus.
  • Peptic / Chief cells: Secrete proenzyme pepsinogen.
  • Parietal / Oxyntic cells: Secrete HCl (Hydrochloric acid) and Intrinsic factor (essential for Vitamin B12 absorption).

• Intestinal Glands (Crypts of Lieberkühn):

  • Secrete intestinal juice or succus entericus.
  • Contains enzymes like disaccharidases (maltase, lactase, sucrase), dipeptidases, lipases, nucleosidases.
  • Secretions of goblet cells and Brunner's glands (mucus) along with succus entericus provide an alkaline medium (pH ≈ 7.8) and protect the intestinal mucosa.

4. Digestion of Food

• In the Buccal Cavity:

  • Mastication and mixing with saliva.
  • Initiation of carbohydrate digestion: Starch ---(Salivary Amylase, pH 6.8)---> Maltose (a disaccharide). About 30% of starch is hydrolysed here.
  • Lysozyme acts as an antibacterial agent.
  • Food mixed with saliva forms a bolus.

• In the Stomach: (Stores food for 4-5 hours)

  • Mixing with acidic gastric juice (pH 1.8) forms chyme.
  • HCl: Kills bacteria, provides acidic pH for pepsin, activates pepsinogen.
  • Pepsinogen ---(HCl)---> Pepsin (active proteolytic enzyme).
  • Proteins ---(Pepsin)---> Proteoses and Peptones (peptides).
  • Mucus and bicarbonates protect the stomach lining from HCl.
  • Rennin: Proteolytic enzyme found in gastric juice of infants for digesting milk protein (casein).
  • Gastric lipases: Contribute very little to fat digestion.

• In the Small Intestine: (Major site of digestion)

  • Movements generate thorough mixing.
  • Bile: Emulsifies fats (breaks large fat globules into small micelles), increasing surface area for lipase action. Also activates lipases.
  • Pancreatic Juice:
    • Trypsinogen ---(Enterokinase from intestinal mucosa)---> Trypsin.
    • Trypsin activates other pancreatic proenzymes (Chymotrypsinogen -> Chymotrypsin, Procarboxypeptidase -> Carboxypeptidase).
    • Proteins, Peptones, Proteoses ---(Trypsin, Chymotrypsin, Carboxypeptidase)---> Dipeptides.
    • Polysaccharides (Starch) ---(Pancreatic Amylase)---> Disaccharides.
    • Fats ---(Lipases, aided by bile)---> Diglycerides ---> Monoglycerides.
    • Nucleic acids ---(Nucleases)---> Nucleotides ---> Nucleosides.
  • Intestinal Juice (Succus Entericus): Acts on the end products of pancreatic digestion.
    • Dipeptides ---(Dipeptidases)---> Amino acids.
    • Maltose ---(Maltase)---> Glucose + Glucose.
    • Lactose ---(Lactase)---> Glucose + Galactose.
    • Sucrose ---(Sucrase)---> Glucose + Fructose.
    • Nucleotides ---(Nucleotidases)---> Nucleosides ---(Nucleosidases)---> Sugars + Bases.
    • Di- and Monoglycerides ---(Lipases)---> Fatty acids + Glycerol.
  • Digestion is completed in the duodenum and jejunum.

5. Absorption of Digested Products

• Absorption is the process by which end products of digestion pass from the intestinal lumen into the blood or lymph.

• Mechanisms:

  • Passive Transport (Simple Diffusion): Movement down concentration gradient (e.g., small amounts of monosaccharides like glucose, amino acids, some electrolytes like Cl-).
  • Facilitated Transport: Movement with the help of carrier proteins down concentration gradient (e.g., glucose, some amino acids).
  • Active Transport: Movement against concentration gradient, requires energy (ATP) (e.g., most glucose, amino acids, electrolytes like Na+).
  • Osmosis: Water transport down its potential gradient.

• Sites of Absorption:

  • Mouth: Certain drugs.
  • Stomach: Water, simple sugars, alcohol, some drugs.
  • Small Intestine: Principal organ for absorption. Glucose, fructose, fatty acids, glycerol, amino acids are absorbed through the mucosa into the bloodstream and lymph.
    • Fat Absorption: Fatty acids and glycerol are insoluble. First incorporated into small droplets called micelles (with bile salts). They move into the intestinal mucosa. Here, they are re-esterified into triglycerides and coated with proteins to form chylomicrons (small protein-coated fat globules). Chylomicrons are transported into the lacteals (lymph vessels) in the villi, eventually entering the bloodstream.
  • Large Intestine: Absorption of water, some minerals, and drugs.

• Assimilation: The absorbed substances finally reach the tissues and are utilized for their activities.

6. Egestion

• Undigested, unabsorbed substances (faeces) enter the caecum of the large intestine.
• Water is absorbed in the large intestine. Mucus lubricates the waste for easy passage.
• Faeces are stored temporarily in the rectum.
• Defecation: Voluntary process of eliminating faeces through the anal opening, initiated by a neural reflex (mass peristaltic movement).

7. Disorders of the Digestive System

• Jaundice: Liver affected. Skin and eyes turn yellow due to the deposit of bile pigments (bilirubin). Caused by hepatitis, gallstones, etc.
• Vomiting: Ejection of stomach contents through the mouth. Controlled by the vomit centre in the medulla oblongata. Feeling of nausea precedes vomiting.
• Diarrhoea: Abnormal frequency of bowel movement and increased liquidity of faecal discharge. Reduces absorption of food.
• Constipation: Faeces are retained within the colon/rectum as bowel movements occur irregularly.
• Indigestion: Food is not properly digested, leading to a feeling of fullness. Causes include inadequate enzyme secretion, anxiety, food poisoning, overeating, spicy food.

Multiple Choice Questions (MCQs)

1. Which of the following represents the correct dental formula for an adult human?
   (a) 2102/2102
   (b) 2123/2123
   (c) 2023/2023
   (d) 2122/2122

2. The enzyme Enterokinase, responsible for activating trypsinogen, is secreted by:
   (a) Pancreas
   (b) Gastric mucosa
   (c) Intestinal mucosa
   (d) Liver

3. Bile salts produced by the liver are primarily involved in:
   (a) Digestion of proteins
   (b) Emulsification of fats
   (c) Digestion of carbohydrates
   (d) Absorption of amino acids

4. Parietal cells (Oxyntic cells) of the gastric glands secrete:
   (a) Pepsinogen and Mucus
   (b) HCl and Intrinsic factor
   (c) Mucus and HCl
   (d) Pepsinogen and Intrinsic factor

5. The finger-like projections called villi, which increase the surface area for absorption, are characteristic of:
   (a) Stomach
   (b) Oesophagus
   (c) Small Intestine
   (d) Large Intestine

6. Which enzyme initiates the digestion of carbohydrates in the human alimentary canal?
   (a) Pancreatic amylase
   (b) Salivary amylase (Ptyalin)
   (c) Sucrase
   (d) Lactase

7. Fatty acids and glycerol are absorbed into the intestinal cells, re-synthesized into triglycerides, and then coated with proteins to form structures called:
   (a) Micelles
   (b) Lacteals
   (c) Chylomicrons
   (d) Villi

8. Which part of the alimentary canal is the primary site for the absorption of water and some minerals?
   (a) Stomach
   (b) Small Intestine
   (c) Large Intestine
   (d) Oesophagus

9. A condition where the liver is affected, causing yellowing of the skin and eyes due to the deposition of bile pigments, is known as:
   (a) Vomiting
   (b) Indigestion
   (c) Jaundice
   (d) Diarrhoea

10. Brunner's glands, which secrete alkaline mucus, are found specifically in the:
    (a) Sub-mucosa of the duodenum
    (b) Mucosa of the ileum
    (c) Sub-mucosa of the stomach
    (d) Mucosa of the jejunum

Answer Key:

1. (b)
2. (c)
3. (b)
4. (b)
5. (c)
6. (b)
7. (c)
8. (c)
9. (c)
10. (a)

Revise these notes thoroughly. Understand the sequence of events, the specific roles of enzymes and glands, and the processes of absorption. Good luck with your preparation!