Digestion and Absorption
Welcome to HSLC Guru. This page provides complete notes, important questions and answers, MCQs, fill in the blanks, true/false items and a glossary for Class 11 Biology Chapter 16 — Digestion and Absorption following the latest ASSEB (Assam State School Education Board) syllabus. The material is designed for English medium learners to build a strong foundation in human digestive physiology, prepare for half-yearly and final examinations, and revise quickly before the test day.
Summary
Food is essential for growth, repair and energy supply. Complex food substances such as carbohydrates, proteins and fats are converted into simpler, soluble and absorbable forms through the process of digestion. The human digestive system consists of the alimentary canal and the associated digestive glands. The alimentary canal is a long muscular tube extending from the mouth to the anus and includes the mouth (buccal cavity), pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus. The accessory digestive glands are the salivary glands, the liver with the gallbladder, and the pancreas. The liver secretes bile which is stored in the gallbladder, while the pancreas releases pancreatic juice into the duodenum.
Human teeth are thecodont (embedded in sockets), diphyodont (two sets — milk and permanent) and heterodont (four types — incisors, canines, premolars and molars). The dental formula of an adult human is 2(2123/2123) = 32 teeth. Mechanical digestion begins in the buccal cavity by chewing, while chemical digestion starts with salivary amylase (ptyalin) that converts starch into maltose. In the stomach, gastric juice contains HCl, pepsinogen and mucus. Pepsin digests proteins into peptones and proteoses, while rennin (in infants) curdles milk. In the small intestine, bile emulsifies fats and pancreatic enzymes — trypsin, chymotrypsin, carboxypeptidase, pancreatic amylase and pancreatic lipase — act on proteins, carbohydrates and fats. Intestinal juice (succus entericus) contains maltase, sucrase, lactase, dipeptidases, lipases and nucleosidases that complete the digestion of food.
Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. It occurs by passive diffusion (fructose, some amino acids), facilitated transport (glucose with carrier proteins), active transport (against concentration gradient using energy — glucose, amino acids, electrolytes) and simple diffusion (water, alcohol, monoglycerides). Different parts of the alimentary canal absorb different substances — alcohol and certain drugs are absorbed in the mouth and stomach; monosaccharides, amino acids, vitamins and most water are absorbed in the small intestine through villi and microvilli; and water, some minerals and drugs are absorbed in the large intestine. Fats are absorbed as chylomicrons through the lacteals of villi.
The absorbed nutrients are transported to body tissues for utilisation, a process called assimilation. Undigested and unabsorbed substances reach the rectum and are expelled as faeces through the anus by a controlled process called egestion or defaecation. Common disorders of the digestive system include jaundice (yellowing of skin and eyes due to bile pigments), vomiting (reverse expulsion of stomach contents controlled by the vomit centre in the medulla), diarrhoea (frequent watery stools), constipation (difficulty in passing stool due to retention of faeces), and indigestion (feeling of fullness due to inadequate enzyme secretion, food poisoning or anxiety). Knowledge of these processes is essential for maintaining a healthy lifestyle.
Very Short Answer Type Questions (1 Mark)
Q1. What is digestion?
Answer: Digestion is the process by which complex food substances are broken down into simpler, soluble and absorbable molecules with the help of enzymes.
Q2. Write the dental formula of an adult human.
Answer: The dental formula of an adult human is 2(2123/2123) = 32 teeth.
Q3. Name the enzyme present in saliva.
Answer: Salivary amylase (ptyalin) is present in saliva. It converts starch into maltose.
Q4. Where is bile produced and stored?
Answer: Bile is produced by the liver and stored in the gallbladder.
Q5. What is the role of HCl in the stomach?
Answer: Hydrochloric acid (HCl) provides an acidic medium for pepsin to act, kills harmful microorganisms in food and converts inactive pepsinogen into active pepsin.
Q6. Name the three parts of the small intestine.
Answer: The three parts of the small intestine are duodenum, jejunum and ileum.
Q7. What is egestion?
Answer: Egestion or defaecation is the process of expelling undigested and unabsorbed food materials (faeces) from the body through the anus.
Q8. What is jaundice?
Answer: Jaundice is a disorder in which the skin and eyes appear yellow due to deposition of bile pigments caused by liver dysfunction.
Q9. Define assimilation.
Answer: Assimilation is the process by which absorbed nutrients are utilised by body cells for the synthesis of complex substances and for energy production.
Q10. Name the enzyme that digests milk protein in infants.
Answer: Rennin is the enzyme that curdles and digests milk protein (casein) in infants.
Short Answer Type Questions (2-3 Marks)
Q1. Differentiate between thecodont, diphyodont and heterodont teeth.
Answer: Thecodont means teeth are embedded in sockets of the jaw bone. Diphyodont means an organism develops two sets of teeth in its lifetime — temporary milk teeth (20) and permanent adult teeth (32). Heterodont means teeth are of different types performing different functions, namely incisors (cutting), canines (tearing), premolars (grinding) and molars (crushing). Humans display all three features.
Q2. List the functions of saliva.
Answer: Saliva (i) moistens and lubricates food for easy swallowing, (ii) contains the enzyme salivary amylase that digests starch into maltose, (iii) keeps the buccal cavity clean and slightly antimicrobial through lysozyme, and (iv) helps in taste perception by dissolving food substances. Daily output of saliva is about 1.0–1.5 litres.
Q3. Distinguish between passive transport and active transport in absorption.
Answer: Passive transport takes place along the concentration gradient (from high to low) and does not require energy; for example, the absorption of fructose. Active transport occurs against the concentration gradient, requires ATP energy and uses carrier proteins; for example, the absorption of glucose, amino acids and electrolytes from the small intestine.
Q4. What is the role of bile in digestion?
Answer: Bile is secreted by the liver and stored in the gallbladder. Although it does not contain digestive enzymes, it (i) emulsifies fats by breaking large fat globules into smaller droplets, increasing the surface area for lipase action, (ii) neutralises the acidic chyme arriving from the stomach, (iii) helps in the absorption of fat-soluble vitamins (A, D, E, K) and (iv) activates pancreatic lipase.
Q5. Differentiate between diarrhoea and constipation.
Answer: Diarrhoea is the abnormal frequency of bowel movements with watery stools, caused by reduced absorption of water in the large intestine; it leads to dehydration. Constipation is the difficulty in passing stool due to faeces being retained in the rectum, often resulting from low fibre intake, dehydration or lack of physical activity.
Q6. Mention the products formed after the complete digestion of carbohydrates, proteins and fats.
Answer: Carbohydrates are completely digested into monosaccharides (glucose, fructose and galactose). Proteins are digested into amino acids. Fats are digested into fatty acids and glycerol (with monoglycerides as intermediates). These are the absorbable end products of digestion.
Long Answer Type Questions (5-7 Marks)
Q1. Describe the structure of the human alimentary canal with the help of a labelled outline.
Answer: The human alimentary canal is a coiled muscular tube about 8–9 metres long extending from the mouth to the anus. It consists of the following parts:
- Mouth (Buccal cavity): Bounded by lips; contains the tongue and 32 teeth. It is the site for ingestion, mastication and partial digestion of starch.
- Pharynx: A common passage for food and air. The epiglottis prevents the entry of food into the trachea during swallowing.
- Oesophagus: A muscular tube about 25 cm long that carries the bolus to the stomach by peristalsis. The cardiac sphincter regulates entry into the stomach.
- Stomach: A J-shaped sac with three regions — cardiac, fundic and pyloric. Gastric juice acts here.
- Small intestine: The longest part (about 6 m), divided into duodenum (C-shaped), jejunum and ileum. It is the principal site of digestion and absorption.
- Large intestine: Comprises caecum (with vermiform appendix), colon (ascending, transverse, descending and sigmoid) and rectum. It absorbs water and electrolytes.
- Anus: The terminal opening through which faeces are eliminated, controlled by the anal sphincters.
The wall of the alimentary canal from the oesophagus to the rectum is composed of four layers — serosa, muscularis, submucosa and mucosa.
Q2. Explain the digestion of food in the stomach and small intestine.
Answer: In the stomach, food is mixed with gastric juice secreted by gastric glands. Gastric juice contains HCl, pepsinogen, prorennin and mucus. HCl converts pepsinogen into active pepsin which digests proteins into proteoses and peptones. In infants, prorennin is converted to rennin which curdles milk. The semi-digested food called chyme then passes into the duodenum through the pyloric sphincter.
In the small intestine, three secretions act on the chyme — bile from the liver, pancreatic juice from the pancreas, and intestinal juice (succus entericus) from intestinal glands. Bile emulsifies fats. Pancreatic juice contains trypsin and chymotrypsin (which digest proteins to peptides), pancreatic amylase (starch to maltose), pancreatic lipase (fats to fatty acids and glycerol) and nucleases. Intestinal juice contains maltase, sucrase and lactase (which split disaccharides into monosaccharides), dipeptidases (which split dipeptides to amino acids), lipases and nucleosidases. Thus, complete digestion of carbohydrates, proteins and fats takes place in the small intestine.
Q3. Describe the different mechanisms of absorption in the alimentary canal.
Answer: Absorption is the process of transfer of digested products from the lumen of the gut into the blood or lymph. It occurs through four mechanisms:
- Simple/Passive Diffusion: Some substances pass through the intestinal mucosa from higher to lower concentration without using energy. Example — fructose, some amino acids, water, alcohol.
- Facilitated Diffusion: Some substances cannot pass freely through the membrane and require carrier proteins, but no energy is consumed. Example — fructose and some amino acids.
- Active Transport: Some substances are absorbed against the concentration gradient with the help of carrier proteins and energy in the form of ATP. Example — glucose, amino acids, electrolytes (Na⁺, K⁺, Ca²⁺).
- Absorption of fats: Fatty acids and glycerol enter the intestinal cells where they are re-synthesised into triglycerides, packed into chylomicrons and absorbed into the lymph through lacteals of villi.
Absorption occurs in different parts — alcohol/drugs in mouth and stomach; nutrients and vitamins in the small intestine; water and minerals in the large intestine.
Q4. Write a note on the common disorders of the digestive system.
Answer: The common disorders of the digestive system are:
- Jaundice: The liver is affected; the skin and eyes appear yellow due to deposition of bile pigments. It may be caused by hepatitis, gallstones or alcohol.
- Vomiting: The reverse ejection of stomach contents through the mouth, controlled by the vomit centre in the medulla. It may be triggered by infection, food poisoning or motion sickness.
- Diarrhoea: Abnormal frequency of bowel movement with watery stools that reduces absorption and causes dehydration. Common causes include bacterial or viral infection.
- Constipation: Faeces are retained in the rectum and bowel movement is irregular due to low intake of fibre, water or lack of exercise.
- Indigestion (Dyspepsia): Feeling of fullness or pain in the stomach due to inadequate enzyme secretion, anxiety, food poisoning, overeating or spicy food.
Q5. Explain the structure and function of the liver and pancreas.
Answer: The liver is the largest gland of the body weighing about 1.2–1.5 kg. It is situated in the upper right side of the abdomen below the diaphragm and consists of two lobes — right and left. The structural and functional unit of the liver is the hepatic lobule. It produces bile, stores glycogen, detoxifies harmful substances, synthesises plasma proteins and converts excess amino acids into urea. Bile produced by the liver is stored in the gallbladder.
The pancreas is a heterocrine (mixed) gland situated between the limbs of the duodenum. Its exocrine part secretes pancreatic juice containing trypsinogen, chymotrypsinogen, pancreatic amylase, pancreatic lipase and nucleases that act on all major nutrients. Its endocrine part — Islets of Langerhans — secretes the hormones insulin (from beta cells) and glucagon (from alpha cells) that regulate blood glucose level.
Multiple Choice Questions (MCQs)
Q1. The dental formula of an adult human is —
(a) 2(2123/2123)
(b) 2(2133/2133)
(c) 2(1123/1123)
(d) 2(2120/2120)
Answer: (a) 2(2123/2123)
Q2. The enzyme present in saliva is —
(a) Pepsin
(b) Trypsin
(c) Salivary amylase
(d) Lipase
Answer: (c) Salivary amylase
Q3. Bile is produced by —
(a) Pancreas
(b) Liver
(c) Gallbladder
(d) Stomach
Answer: (b) Liver
Q4. The longest part of the alimentary canal is —
(a) Stomach
(b) Small intestine
(c) Large intestine
(d) Oesophagus
Answer: (b) Small intestine
Q5. Pepsin is secreted by —
(a) Salivary glands
(b) Liver
(c) Gastric glands
(d) Pancreas
Answer: (c) Gastric glands
Q6. Fats are absorbed through —
(a) Blood capillaries
(b) Lymph (lacteals)
(c) Hepatic vein
(d) Bile duct
Answer: (b) Lymph (lacteals)
Q7. The Islets of Langerhans are present in —
(a) Liver
(b) Stomach
(c) Pancreas
(d) Spleen
Answer: (c) Pancreas
Q8. Which enzyme digests milk protein in infants?
(a) Pepsin
(b) Rennin
(c) Trypsin
(d) Lactase
Answer: (b) Rennin
Q9. Vermiform appendix is associated with —
(a) Stomach
(b) Ileum
(c) Caecum
(d) Rectum
Answer: (c) Caecum
Q10. Which of the following is the end product of fat digestion?
(a) Glucose and fructose
(b) Amino acids
(c) Fatty acids and glycerol
(d) Peptones
Answer: (c) Fatty acids and glycerol
Fill in the Blanks
Q1. The largest gland of the human body is the __________.
Answer: liver
Q2. The semi-digested food in the stomach is called __________.
Answer: chyme
Q3. The enzyme that converts maltose into glucose is __________.
Answer: maltase
Q4. The finger-like projections of the small intestinal mucosa are called __________.
Answer: villi
Q5. Bile is stored in the __________.
Answer: gallbladder
True or False
Q1. Saliva digests proteins in the buccal cavity.
Answer: False (Saliva digests starch, not proteins.)
Q2. Trypsin is secreted by the pancreas.
Answer: True
Q3. The small intestine is the main site of absorption.
Answer: True
Q4. Humans have monophyodont dentition.
Answer: False (Humans are diphyodont — they have two sets of teeth.)
Q5. Faeces are stored in the rectum before egestion.
Answer: True
Glossary
| Term | Meaning |
|---|---|
| Digestion | Breakdown of complex food into simpler absorbable molecules. |
| Alimentary canal | The continuous tube extending from the mouth to the anus. |
| Bolus | Soft, rounded mass of chewed food formed in the buccal cavity. |
| Chyme | Semi-digested acidic food formed in the stomach. |
| Chyle | Alkaline fluid containing digested food in the small intestine. |
| Peristalsis | Rhythmic contraction of muscles that pushes food along the gut. |
| Heterodont | Animals having different types of teeth for different functions. |
| Diphyodont | Animals that develop two sets of teeth in life — milk and permanent. |
| Thecodont | Teeth embedded in sockets of the jaw bone. |
| Villi | Finger-like projections of the small intestinal mucosa increasing absorption surface. |
| Lacteal | Lymph capillary in a villus that absorbs digested fats. |
| Bile | Greenish-yellow alkaline secretion of the liver that emulsifies fats. |
| Pepsin | Protein-digesting enzyme of the stomach. |
| Trypsin | Protein-digesting enzyme of the pancreas. |
| Lipase | Fat-digesting enzyme. |
| Sucrase | Enzyme that splits sucrose into glucose and fructose. |
| Lactase | Enzyme that splits lactose into glucose and galactose. |
| Maltase | Enzyme that converts maltose into glucose. |
| Absorption | Transfer of digested food from the gut into blood or lymph. |
| Assimilation | Use of absorbed nutrients by body cells. |
| Egestion | Elimination of undigested food as faeces through the anus. |
| Jaundice | Yellowing of skin and eyes due to liver disorder. |
| Diarrhoea | Frequent watery bowel movements with reduced absorption. |
| Constipation | Difficulty in passing stools due to retention of faeces. |
| Indigestion | Discomfort or fullness in the stomach due to improper digestion. |