Digestion & Absorption (NCERT Class 11)

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1. Digestive System: Overview Consists of the alimentary canal and associated digestive glands . Alimentary Canal: From mouth to anus. Digestive Glands: Salivary glands, liver, pancreas. 2. Alimentary Canal: Structure 2.1 Mouth and Oral Cavity Mouth: Anterior opening. Oral Cavity (Buccal Cavity): Contains teeth and a muscular tongue . Teeth: Thecodont: Embedded in sockets of jaw bones. Diphyodont: Two sets during life (temporary/milk teeth, permanent teeth). Heterodont: Different types of teeth (Incisors, Canines, Premolars, Molars). Dental Formula (adult human): $\frac{2123}{2123}$ (for one half of each jaw). Tongue: Freely movable muscular organ, attached to the floor of oral cavity by frenulum . Papillae: Small projections on upper surface of tongue, some bearing taste buds . 2.2 Pharynx and Oesophagus Pharynx: Common passage for food and air. Epiglottis: Cartilaginous flap that prevents food entry into glottis (opening of trachea) during swallowing ( deglutition ). Oesophagus: Thin, long tube extending posteriorly through the neck, thorax, and diaphragm. Opens into the stomach. Gastro-oesophageal Sphincter: Muscular sphincter regulating the opening of oesophagus into the stomach. 2.3 Stomach J-shaped organ, located in the upper left portion of the abdominal cavity. Has four main parts: Cardiac: Where oesophagus opens. Fundic: Dome-shaped. Body (Corpus): Main central region. Pyloric: Opens into the duodenum. Pyloric Sphincter: Regulates the opening of the stomach into the duodenum. 2.4 Small Intestine Longest part of the alimentary canal. Divided into three regions: Duodenum: C-shaped first part. Jejunum: Long coiled middle portion. Ileum: Highly coiled terminal part. Ileum opens into the large intestine . 2.5 Large Intestine Divided into three parts: Caecum: Small blind sac, hosts some symbiotic microorganisms. Vermiform Appendix: Narrow finger-like tubular projection, a vestigial organ, arises from the caecum. Colon: Divided into ascending, transverse, descending, and sigmoid colon. Rectum: Descending colon opens into it. Anus: Rectum opens out through it. Ileo-caecal Valve: Prevents the backflow of faecal matter from the large intestine into the small intestine. 2.6 Histology of Alimentary Canal Wall Four layers from outer to inner: Serosa: Outermost layer, made of thin mesothelium (visceral peritoneum) with some connective tissues. Muscularis: Smooth muscles, usually arranged into an inner circular and outer longitudinal layer. An oblique muscle layer may be present in some regions (e.g., stomach). Sub-mucosa: Loose connective tissue layer containing nerves, blood, and lymph vessels. Duodenal glands (Brunner's glands) are present in this layer. Mucosa: Innermost layer, forms irregular folds ( rugae ) in the stomach and small finger-like projections ( villi ) in the small intestine. Villi: Covered with microvilli (brush border appearance) on epithelial cells, increasing surface area for absorption. Contain a network of capillaries and a large lymph vessel called the lacteal . Crypts of Lieberkuhn: Glands formed between the bases of villi in the small intestine. 3. Digestive Glands 3.1 Salivary Glands Three pairs: Parotids (cheeks), Sub-maxillary/Sub-mandibular (lower jaw), Sub-linguals (below tongue). Secrete saliva into the oral cavity. 3.2 Liver Largest gland of the body, weighs about 1.2-1.5 kg in an adult human. Located in the upper right portion of the abdominal cavity. Two lobes. Hepatic lobules: Structural and functional units of the liver, containing hepatic cells (hepatocytes) arranged in cords. Glisson's capsule: Thin connective tissue sheath covering each hepatic lobule. Bile: Secreted by hepatic cells, stored and concentrated in the gall bladder . Cystic duct: Duct of the gall bladder. Hepatic duct: From the liver. Common Bile Duct: Formed by the union of cystic and hepatic ducts. 3.3 Pancreas Compound (both exocrine and endocrine) elongated organ. Located between the limbs of the C-shaped duodenum. Exocrine part: Secretes pancreatic juice containing digestive enzymes. Endocrine part: Secretes hormones (insulin, glucagon). Hepato-pancreatic Duct: Common bile duct and pancreatic duct open together into the duodenum. Sphincter of Oddi: Guards the opening of the hepato-pancreatic duct into the duodenum. 4. Digestion of Food 4.1 In the Oral Cavity Mastication: Chewing of food by teeth. Saliva: Contains electrolytes ($Na^+, K^+, Cl^-, HCO_3^-$), salivary amylase ( ptyalin ), and lysozyme . Salivary amylase: Hydrolyses starch into disaccharide ( maltose ). Starch $\xrightarrow{\text{Salivary Amylase, pH 6.8}}$ Maltose Lysozyme: Acts as an antibacterial agent. Food mixed with saliva forms a bolus . Deglutition (swallowing): Bolus passes into the pharynx, then oesophagus. No digestion occurs in the oesophagus. 4.2 In the Stomach Food (bolus) mixes thoroughly with acidic gastric juice by churning movements, forming chyme . Gastric Glands: Three major types of cells: Mucus neck cells: Secrete mucus . Peptic or Chief cells: Secrete proenzyme pepsinogen . Parietal or Oxyntic cells: Secrete HCl and intrinsic factor (essential for vitamin B12 absorption). HCl: Provides acidic pH (1.8) optimal for pepsin. Kills bacteria. Pepsinogen: Converted to active pepsin by HCl. Pepsinogen $\xrightarrow{HCl}$ Pepsin Pepsin: Proteolytic enzyme, digests proteins into proteoses and peptones (peptides). Proteins $\xrightarrow{\text{Pepsin}}$ Proteoses + Peptones Rennin: Proteolytic enzyme found in gastric juice of infants, helps in digestion of milk proteins (casein). Small amounts of gastric lipase are also secreted. 4.3 In the Small Intestine Chyme enters duodenum. Receives secretions from liver ( bile ), pancreas ( pancreatic juice ), and its own intestinal wall ( intestinal juice /succus entericus). Pancreatic Juice: Contains trypsinogen , chymotrypsinogen , procarboxypeptidases , amylases , lipases , and nucleases . Trypsinogen: Activated by enteropeptidase (secreted by intestinal mucosa) into trypsin . Trypsinogen $\xrightarrow{\text{Enteropeptidase}}$ Trypsin Trypsin activates other enzymes in pancreatic juice. Bile: Contains bile pigments (bilirubin, biliverdin), bile salts , cholesterol , and phospholipids . No enzymes. Emulsification of fats: Breaks down large fat globules into smaller ones. Activates lipases . Intestinal Juice (Succus Entericus): Secreted by goblet cells and brush border cells of mucosa. Contains disaccharidases (maltase, lactase, sucrase), dipeptidases , lipases , nucleosidases . Mucus along with bicarbonate from pancreas protects intestinal mucosa from acid and provides alkaline medium (pH 7.8). Digestion of Proteins: Proteins, Proteoses, Peptones $\xrightarrow{\text{Trypsin, Chymotrypsin, Carboxypeptidases}}$ Dipeptides Digestion of Carbohydrates: Polysaccharides (Starch) $\xrightarrow{\text{Amylase}}$ Disaccharides Digestion of Fats: Fats $\xrightarrow{\text{Lipases, Bile}}$ Diglycerides $\longrightarrow$ Monoglycerides Digestion of Nucleic Acids: Nucleic Acids $\xrightarrow{\text{Nucleases}}$ Nucleotides $\longrightarrow$ Nucleosides Final Steps of Digestion (in small intestine): Dipeptides $\xrightarrow{\text{Dipeptidases}}$ Amino acids Maltose $\xrightarrow{\text{Maltase}}$ Glucose + Glucose Lactose $\xrightarrow{\text{Lactase}}$ Glucose + Galactose Sucrose $\xrightarrow{\text{Sucrase}}$ Glucose + Fructose Di and Monoglycerides $\xrightarrow{\text{Lipases}}$ Fatty acids + Glycerol Nucleotides $\xrightarrow{\text{Nucleotidases}}$ Nucleosides $\xrightarrow{\text{Nucleosidases}}$ Sugars + Bases These final products are absorbed in the same region of the small intestine. 4.4 In the Large Intestine No significant digestive activity occurs. Functions: Absorption of water, some minerals, and certain drugs. Secretion of mucus for lubricating faecal matter and adhering undigested waste particles. Faeces: Undigested and unabsorbed substances, temporarily stored in the rectum. Defaecation: Egestion of faeces through the anus, a voluntary process. 5. Absorption of Digested Products Process by which end products of digestion pass through the intestinal mucosa into the blood or lymph. Occurs by passive , facilitated , or active transport mechanisms. 5.1 Absorption of Monosaccharides Glucose, Fructose, Galactose. Glucose, Galactose: Absorbed by active transport and facilitated diffusion. Fructose: Absorbed by facilitated diffusion only. Enter the blood capillaries in the villi. 5.2 Absorption of Amino Acids Absorbed by active transport. Enter the blood capillaries in the villi. 5.3 Absorption of Fatty Acids and Glycerol Being insoluble, cannot be absorbed directly into the blood. Incorporated into small droplets called micelles , which move into the intestinal mucosa. Inside mucosal cells, they are re-formed into very small protein-coated fat globules called chylomicrons . Chylomicrons are transported into the lacteals (lymph vessels) in the villi. Lacteals eventually release the absorbed substances into the bloodstream. 5.4 Absorption of Water Largely dependent on the osmotic gradient. Absorbed throughout the alimentary canal but maximum in the small intestine and large intestine. 5.5 Summary of Absorption Sites Mouth: Certain drugs. Stomach: Water, simple sugars, alcohol. Small intestine: Major site for absorption of all digested nutrients (carbohydrates, proteins, fats, minerals, vitamins, water). Large intestine: Water, some minerals, certain drugs. 6. Disorders of Digestive System Jaundice: Liver affected, skin and eyes turn yellow due to deposition of bile pigments. Vomiting: Ejection of stomach contents through the mouth, a reflex action controlled by the vomit centre in the medulla. Diarrhoea: Abnormal frequency of bowel movement and increased liquidity of faecal discharge. Reduces absorption of food. Constipation: Faeces retained within the rectum as bowel movements occur irregularly. Indigestion: Food not properly digested, leading to a feeling of fullness. Causes: inadequate enzyme secretion, anxiety, food poisoning, overeating, spicy food. PEM (Protein Energy Malnutrition): Marasmus: Severe protein and calorie deficiency. Impairs growth, leads to emaciation, dry and wrinkled skin. Kwashiorkor: Protein deficiency only. Leads to wasting of muscles, swelling of legs, failure of growth, brain development.