Class 11 Biology Notes Chapter 16 (Chapter 16) – Examplar Problems (English) Book
Detailed Notes with MCQs of Chapter 16: Digestion and Absorption. This is a crucial chapter, not just for your Class 11 understanding, but also forms the basis for many questions in competitive government exams. Pay close attention to the details.
Chapter 16: Digestion and Absorption - Detailed Notes
1. Introduction:
- Food: Provides energy, organic materials for growth, and repair of tissues. Major components are carbohydrates, proteins, and fats (macronutrients). Vitamins and minerals (micronutrients) are also required in small quantities.
- Digestion: The process of converting complex, non-absorbable food substances into simple, absorbable forms. This is achieved through mechanical and biochemical methods.
- Absorption: The process by which digested end products pass through the intestinal mucosa into the blood or lymph.
2. Human Digestive System:
Consists of:
- Alimentary Canal: A long, muscular tube starting from the mouth and ending at the anus.
- Associated Digestive Glands: Salivary glands, Liver, Pancreas.
A. Alimentary Canal:
- Mouth: Leads to the buccal cavity (oral cavity).
- Teeth: Thecodont (embedded in jaw sockets), Diphyodont (two sets: milk/deciduous and permanent/adult), Heterodont (different types: Incisors (I), Canines (C), Premolars (PM), Molars (M)).
- Dental Formula (Adult): 2123/2123 (I 2/2, C 1/1, PM 2/2, M 3/3) x 2 = 32 teeth.
- Tongue: Freely movable muscular organ attached to the floor of the oral cavity by the frenulum. Upper surface has papillae, some bear taste buds. Helps in mastication, mixing food with saliva, swallowing (deglutition), and speech.
- Pharynx: Common passage for food and air. Divided into nasopharynx, oropharynx, and laryngopharynx. The epiglottis (a cartilaginous flap) prevents food entry into the glottis (opening of the windpipe) during swallowing.
- Oesophagus: A thin, long tube extending posteriorly, passing through the neck, thorax, and diaphragm, leading to the stomach. A muscular gastro-oesophageal sphincter regulates the opening into the stomach.
- Stomach: J-shaped muscular bag located in the upper abdominal cavity. Has four parts:
- Cardiac: Where oesophagus opens.
- Fundic: Superior region.
- Body: Main central region.
- Pyloric: Opens into the small intestine via the pyloric sphincter.
- Small Intestine: Longest part of the alimentary canal, distinguished into three regions:
- Duodenum: C-shaped, first part. Receives secretions from the liver and pancreas via the hepato-pancreatic duct.
- Jejunum: Middle part, coiled.
- Ileum: Highly coiled, terminal part. Opens into the large intestine.
- Function: Principal organ for digestion and absorption. Inner surface area increased by folds (plicae circulares), villi, and microvilli (brush border appearance).
- Large Intestine: Wider than the small intestine. Consists of:
- Caecum: Small blind sac, hosts symbiotic microorganisms. Vermiform appendix (vestigial organ) arises from it.
- Colon: Divided into ascending, transverse, descending, and sigmoid colon.
- Rectum: Stores faecal matter temporarily.
- Function: Absorption of water, some minerals, and drugs; formation and storage of faeces.
- Anus: Terminal opening through which faeces are eliminated (egestion/defaecation). Guarded by internal (involuntary) and external (voluntary) anal sphincters.
B. Histology of Alimentary Canal:
Wall consists of four layers (from outer to inner):
- Serosa: Outermost, thin mesothelium with some connective tissue.
- Muscularis: Smooth muscles, usually arranged as outer longitudinal and inner circular layers. Oblique muscle layer present in the stomach. Responsible for peristalsis.
- Sub-mucosa: Loose connective tissue containing nerves, blood vessels, and lymph vessels. Brunner's glands (secrete mucus) are present in the duodenum's sub-mucosa.
- Mucosa: Innermost layer lining the lumen.
- Forms irregular folds (rugae) in the stomach and finger-like projections (villi) in the small intestine.
- Cells lining villi have microscopic projections called microvilli (brush border).
- Villi supplied with a network of capillaries and a large lymph vessel (lacteal).
- Contains goblet cells (secrete mucus).
- Forms glands in the stomach (gastric glands) and crypts between villi bases in the intestine (crypts of Lieberkühn).
C. Digestive Glands:
- Salivary Glands: Three pairs located just outside the buccal cavity:
- Parotids: Cheek.
- Sub-maxillary/Sub-mandibular: Lower jaw.
- Sub-linguals: Below the tongue.
- Saliva: Contains electrolytes (Na+, K+, Cl-, HCO3-), enzymes (salivary amylase/ptyalin and lysozyme), and mucus. pH ~6.8.
- Liver: Largest gland (~1.2-1.5 kg). Located in the upper right side of the abdominal cavity. Has two lobes.
- Hepatic lobules: Structural and functional units, containing hepatic cells arranged as cords. Each lobule covered by Glisson's capsule.
- Bile: Secreted by hepatic cells, stored and concentrated in the gall bladder. Bile duct (from gall bladder) and hepatic duct (from liver) 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.
- Bile Composition: Bile pigments (bilirubin, biliverdin - breakdown products of haemoglobin), bile salts (help in emulsification of fats), cholesterol, phospholipids. No enzymes.
- Pancreas: Compound gland (both exocrine and endocrine). Located between limbs of the 'C' shaped duodenum.
- Exocrine part: Secretes alkaline pancreatic juice containing inactive enzymes: trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, nucleases.
- Endocrine part (Islets of Langerhans): Secretes hormones (insulin, glucagon).
3. Digestion of Food:
- In the Mouth: Mastication mixes food with saliva. Salivary amylase starts carbohydrate digestion (starch into maltose, ~30%). Lysozyme acts as an antibacterial agent. Food formed into a bolus. Swallowing (deglutition) moves the bolus to the oesophagus. Peristalsis (wave-like muscular contractions) pushes food down.
- In the Stomach: Food mixes with acidic gastric juice (pH 1.8-3.0) secreted by gastric glands. Churning movements create chyme.
- Gastric Glands:
- Mucus neck cells: Secrete mucus (protection from HCl).
- Peptic/Chief cells: Secrete proenzyme pepsinogen.
- Parietal/Oxyntic cells: Secrete HCl and intrinsic factor (essential for Vitamin B12 absorption).
- HCl function: Activates pepsinogen to pepsin, provides acidic pH for pepsin action, kills microbes.
- Pepsin: Proteolytic enzyme, converts proteins into proteoses and peptones.
- Rennin (in infants): Milk protein (casein) digestion.
- Small amounts of lipases are also secreted.
- Gastric Glands:
- In the Small Intestine: Chyme enters the duodenum. Digestion is completed here. Requires:
- Bile: Emulsifies fats (breaks large fat globules into smaller micelles). Activates lipases.
- Pancreatic Juice:
- Trypsinogen --(Enterokinase from intestinal mucosa)--> Trypsin.
- Trypsin activates chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase.
- Proteins, proteoses, peptones --(Trypsin, Chymotrypsin, Carboxypeptidase)--> Dipeptides.
- Polysaccharides (starch) --(Pancreatic Amylase)--> Disaccharides.
- Fats (triglycerides) --(Lipases, aided by bile)--> Diglycerides --> Monoglycerides + Fatty acids.
- Nucleic acids --(Nucleases)--> Nucleotides --> Nucleosides.
- Intestinal Juice (Succus Entericus): Secreted by Crypts of Lieberkühn and Brunner's glands (mucus). Contains various enzymes acting on end products:
- Dipeptides --(Dipeptidases)--> Amino acids.
- Maltose --(Maltase)--> Glucose + Glucose.
- Lactose --(Lactase)--> Glucose + Galactose.
- Sucrose --(Sucrase)--> Glucose + Fructose.
- Nucleotides --(Nucleotidases)--> Nucleosides --(Nucleosidases)--> Sugars + Bases.
- Di/Monoglycerides --(Lipases)--> Fatty acids + Glycerol.
- Alkaline medium (pH ~7.8) provided by bicarbonate ions from pancreas and mucus protects the intestinal mucosa and optimizes enzyme activity.
4. Absorption of Digested Products:
- Mechanism: Passive transport (simple diffusion, osmosis, facilitated diffusion) and active transport.
- Sites:
- Mouth: Certain drugs.
- Stomach: Water, simple sugars, alcohol, some drugs.
- Small Intestine: Principal organ for absorption. Glucose, fructose, amino acids, fatty acids, glycerol, electrolytes, vitamins, water.
- Glucose, amino acids, some electrolytes (like Na+): Absorbed mainly by active transport.
- Fructose, some amino acids: Absorbed by facilitated transport.
- Water, some electrolytes (like Cl-): Absorbed by osmosis/simple diffusion.
- Fatty acids and glycerol: Insoluble, first incorporated into small droplets called micelles. Move into intestinal mucosa. Re-esterified into small protein-coated fat globules called chylomicrons. Transported into lacteals (lymph vessels) in the villi, eventually released into the bloodstream.
- Large Intestine: Water, some minerals, and drugs.
- Assimilation: Absorbed substances reach tissues via blood/lymph and are utilized for energy, growth, and repair.
- Egestion (Defaecation): Undigested, unabsorbed substances (faeces) enter the caecum, move through the colon, stored in the rectum. Eliminated through the anus via a voluntary process initiated by a mass peristaltic movement and neural reflex.
5. Regulation of Digestive Activity:
- Under neural (intrinsic - local enteric nervous system; extrinsic - CNS) and hormonal control.
- Sight, smell, or presence of food stimulates saliva secretion.
- Gastric and intestinal secretions also stimulated by neural signals.
- Local hormones produced by gastric and intestinal mucosa regulate secretions:
- Gastrin: Stimulates HCl and pepsinogen secretion.
- Secretin: Stimulates secretion of water and bicarbonate ions from the pancreas.
- Cholecystokinin (CCK): Stimulates secretion of pancreatic enzymes and bile release from the gall bladder.
- Gastric Inhibitory Peptide (GIP): Inhibits gastric secretion and motility.
6. Calorific Value:
- Amount of heat energy liberated from complete combustion of 1g food in a bomb calorimeter.
- Gross Calorific Value: Carbohydrates: 4.1 kcal/g; Proteins: 5.65 kcal/g; Fats: 9.45 kcal/g.
- Physiologic Value (actual energy available): Carbohydrates: 4.0 kcal/g; Proteins: 4.0 kcal/g; Fats: 9.0 kcal/g.
7. Disorders of the Digestive System:
- Jaundice: Liver affected, skin and eyes turn yellow due to deposition 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 retained within the colon due to irregular bowel movements.
- Indigestion: Food not properly digested, leading to a feeling of fullness. Causes include inadequate enzyme secretion, anxiety, food poisoning, overeating, spicy food.
- Protein-Energy Malnutrition (PEM): Dietary deficiencies of proteins and total food calories, widespread in underdeveloped countries. Affects infants and children.
- Marasmus: Deficiency of both proteins and calories. Seen in infants < 1 year. Impaired growth, extreme emaciation, thinning of limbs, dry wrinkled skin, no oedema.
- Kwashiorkor: Deficiency predominantly of protein. Seen in children > 1 year. Wasting of muscles, thinning of limbs, failure of growth, but unlike marasmus, some fat is still present under the skin; extensive oedema and swelling of body parts are seen.
Multiple Choice Questions (MCQs):
-
Which layer of the alimentary canal wall contains Brunner's glands?
a) Serosa
b) Muscularis
c) Sub-mucosa
d) Mucosa -
The enzyme enterokinase, responsible for activating trypsinogen, is secreted by:
a) Pancreas
b) Gastric mucosa
c) Intestinal mucosa
d) Liver -
Which of the following is NOT a function of the large intestine?
a) Absorption of water
b) Absorption of some minerals and drugs
c) Secretion of digestive enzymes
d) Formation and temporary storage of faeces -
Micelle formation is essential for the absorption of:
a) Amino acids
b) Glucose
c) Fatty acids and glycerol
d) Vitamins A and D -
Which hormone stimulates the secretion of pancreatic juice rich in bicarbonate ions?
a) Gastrin
b) Secretin
c) Cholecystokinin (CCK)
d) Gastric Inhibitory Peptide (GIP) -
Kwashiorkor differs from Marasmus in that Kwashiorkor:
a) Is caused by deficiency of both protein and calories
b) Primarily affects infants below one year
c) Shows extensive oedema and swelling
d) Results in extreme emaciation with no subcutaneous fat -
Thecodont, Diphyodont, and Heterodont conditions are characteristic of human:
a) Tongue papillae
b) Salivary glands
c) Dentition
d) Intestinal villi -
Which cells in the gastric glands secrete Hydrochloric acid (HCl) and intrinsic factor?
a) Mucus neck cells
b) Peptic cells (Chief cells)
c) Parietal cells (Oxyntic cells)
d) Goblet cells -
Bile salts primarily function in:
a) Digestion of proteins
b) Emulsification of fats
c) Activation of pepsinogen
d) Absorption of carbohydrates -
The final step in the digestion of proteins occurs when dipeptides are broken down into amino acids by:
a) Pepsin
b) Trypsin
c) Carboxypeptidase
d) Dipeptidases
Answer Key for MCQs:
- c) Sub-mucosa (specifically in the duodenum)
- c) Intestinal mucosa
- c) Secretion of digestive enzymes
- c) Fatty acids and glycerol (and fat-soluble vitamins)
- b) Secretin
- c) Shows extensive oedema and swelling
- c) Dentition
- c) Parietal cells (Oxyntic cells)
- b) Emulsification of fats
- d) Dipeptidases (part of succus entericus)
Make sure you revise these notes thoroughly. Understand the sequence of events, the specific enzymes and their substrates/products, the sites of action, and the regulatory mechanisms. Don't forget the disorders – they are frequently asked. Good luck with your preparation!