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This page provides complete NCERT Solutions for CBSE Class 11 Biology Chapter 16 — Digestion and Absorption. All intext questions (organised by page number) and chapter-end exercise questions are answered in detail. Use the section headings to jump directly to the question you need help with, or read through the full page for a comprehensive revision of the chapter.
Key Concepts (Quick Recap)
- Alimentary Canal: Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Small intestine (duodenum, jejunum, ileum) → Large intestine (caecum, colon, rectum) → Anus
- Digestive Glands: Salivary glands (parotid, submandibular, sublingual), gastric glands, intestinal glands, liver (largest gland), pancreas
- Digestive Enzymes: Salivary amylase, pepsin, trypsin, chymotrypsin, carboxypeptidase, pancreatic amylase, lipase, maltase, sucrase, lactase, nucleotidases, nucleosidases
- Digestion in Mouth: Salivary amylase converts starch to maltose; bolus formation
- Digestion in Stomach: HCl activates pepsinogen to pepsin; pepsin digests proteins; gastric lipase acts on fats
- Digestion in Small Intestine: Pancreatic juice (trypsin, chymotrypsin, amylase, lipase), bile emulsifies fats, intestinal enzymes complete digestion
- Absorption: Passive diffusion, facilitated diffusion, active transport; villi and microvilli increase surface area; absorbed nutrients enter blood and lymph
- Disorders: Jaundice (bilirubin accumulation), vomiting (reverse peristalsis), diarrhoea (excessive bowel movement), constipation (hard stools), indigestion (incomplete digestion)
Intext Questions
Page 261
The alimentary canal is a continuous tube that starts at the mouth and ends at the anus. It includes the mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, and anus.
All these glands are associated with the alimentary canal. The salivary glands secrete saliva in the mouth. Gastric glands are present in the stomach wall. The liver secretes bile. The pancreas secretes pancreatic juice. Intestinal glands are present in the small intestine wall.
Saliva performs multiple functions: it lubricates food for easy swallowing (mucin), digests starch (salivary amylase converts starch to maltose), and protects from bacteria (lysozyme kills bacteria).
Page 267
The gastric glands in the stomach secrete three components: HCl (creates acidic environment and kills bacteria), pepsin (digests proteins), and mucus (protects the stomach lining from the corrosive action of HCl).
HCl in the stomach kills harmful bacteria present in food, converts inactive pepsinogen into active pepsin, facilitates the absorption of minerals like iron and calcium, and creates an acidic pH (1.5–2.5) optimal for pepsin activity.
Page 272
The small intestine is the principal organ for absorption of digested nutrients. The presence of villi and microvilli greatly increases the surface area for absorption. Most nutrients are absorbed in the small intestine, though some absorption (of water, some minerals, and drugs) also occurs in the large intestine.
Egestion (or defecation) is the process of eliminating undigested food waste from the body through the anus. It is the final step in the digestive process. The undigested matter forms faeces, which is stored in the rectum before being expelled.
Chapter-End Exercise
The parotid gland is the largest salivary gland and is located near the ear, below the zygomatic arch. The submandibular gland is located below the mandible, and the sublingual gland is located below the tongue.
- Salivary amylase is secreted in the mouth and acts on starch
- Pepsin is secreted by gastric glands in the stomach and acts on proteins
- Trypsin is secreted by the pancreas and acts on proteins in the small intestine
- Lipase is secreted by the pancreas (pancreatic lipase) and also by intestinal glands, acting on fats in the small intestine
Mechanical digestion: The muscular walls of the stomach churn and mix the food thoroughly with gastric juice, forming a semi-solid paste called chyme.
Chemical digestion: The gastric glands secrete gastric juice which contains:
- HCl: Creates an acidic environment (pH 1.5–2.5), kills bacteria, converts pepsinogen to pepsin, and helps in mineral absorption.
- Pepsin: The active proteolytic enzyme that digests proteins into smaller peptides called peptones and proteoses. Pepsin is secreted as inactive pepsinogen, which is activated by HCl.
- Mucus: Protects the stomach lining from the corrosive action of HCl and prevents autodigestion of the stomach wall.
- Gastric lipase: Acts on fats in a limited way, mainly digesting emulsified fats like butter.
Proteins → Pepsin → Peptones + Proteoses
Liver: The liver is the largest gland in the body. Its digestive role is:
- Secretes bile (600–1100 mL/day), which is stored in the gallbladder and released into the duodenum.
- Bile has no digestive enzymes, but it emulsifies fats — breaking large fat globules into smaller droplets, increasing the surface area for lipase action.
- Bile neutralises the acidic chyme coming from the stomach, creating an alkaline medium (pH 7–8) favourable for pancreatic and intestinal enzymes.
- Bile also activates pancreatic lipase.
Pancreas: The pancreas is both an exocrine and endocrine gland. Its exocrine portion secretes pancreatic juice containing:
- Trypsinogen (activated to trypsin by enterokinase): digests proteins into peptides and amino acids.
- Chymotrypsinogen (activated to chymotrypsin): digests proteins.
- Procarboxypeptidase (activated to carboxypeptidase): digests peptides into amino acids.
- Pancreatic amylase: digests starch into maltose.
- Pancreatic lipase: digests emulsified fats into fatty acids and glycerol.
- Nucleases: digest nucleic acids into nucleotides and nucleosides.
Peristalsis helps in digestion by: 1. Propelling food forward along the alimentary canal — from mouth to anus. 2. Mixing food thoroughly with digestive enzymes and juices. 3. Ensuring that food is in contact with the absorptive surface of the intestine for sufficient time. 4. Preventing backward flow of food (the sphincters and directional peristaltic waves ensure one-way movement).
Without peristalsis, food would not move through the digestive tract, and digestion and absorption would be impossible.
1. Bile juice (from liver): Emulsifies fats, neutralises acidic chyme, and activates lipase.
2. Pancreatic juice (from pancreas): Contains trypsin, chymotrypsin, carboxypeptidase, pancreatic amylase, lipase, and nucleases. These enzymes act on proteins, starch, fats, and nucleic acids.
3. Intestinal juice (succus entericus): Contains maltase, sucrase, lactase, dipeptidases, lipase, nucleotidases, and nucleosidases.
Digestion of carbohydrates:
- Starch → Pancreatic amylase → Maltose
- Maltose → Maltase → Glucose
- Sucrose → Sucrase → Glucose + Fructose
- Lactose → Lactase → Glucose + Galactose
Digestion of proteins:
- Proteins → Trypsin/Chymotrypsin → Peptones + Peptides
- Peptides → Carboxypeptidase/Dipeptidases → Amino acids
Digestion of fats:
- Fats → Bile (emulsification) → Small fat droplets
- Fat droplets → Lipase → Fatty acids + Glycerol
Digestion of nucleic acids:
- Nucleic acids → Nucleases → Nucleotides → Nucleotidases → Nucleosides → Nucleosidases → Nitrogenous bases + Sugars
Villi: The inner lining of the small intestine has numerous finger-like projections called villi. Each villus is about 0.5–1.5 mm long. Structurally, each villus has:
- A central lacteal (lymph vessel) for fat absorption
- A network of blood capillaries for absorption of amino acids and sugars
- Smooth muscle fibres that help in movement
- A covering of epithelial cells with microvilli on their surface
Microvilli: The epithelial cells of the villi have numerous microscopic projections called microvilli on their apical surface. These are about 1 micrometre in length.
Functions: 1. Villi and microvilli together increase the surface area of the small intestine by about 600-fold, greatly enhancing absorption. 2. Villi contain blood vessels that carry absorbed nutrients (amino acids, sugars) into the bloodstream. 3. The lacteal in each villus absorbs fatty acids and glycerol, which are then transported through the lymphatic system. 4. The rich blood supply in villi helps maintain a concentration gradient for efficient absorption by active transport and facilitated diffusion.
1. Passive diffusion (simple diffusion): Small molecules move from a region of higher concentration to lower concentration without energy expenditure. Examples: small amounts of monosaccharides and amino acids.
2. Facilitated diffusion: Specific carrier proteins in the cell membrane help transport molecules from higher to lower concentration. No energy is required. Example: absorption of fructose.
3. Active transport: Molecules are transported against the concentration gradient using ATP energy. Examples: absorption of amino acids like alanine, glucose, and some ions.
4. Osmosis: Water is absorbed by osmosis, moving from a region of lower solute concentration to higher solute concentration.
5. Co-transport (symport): Some nutrients are absorbed along with ions. Example: glucose is co-transported with Na⁺ via the sodium-glucose cotransporter.
Absorbed nutrients enter either the blood capillaries (amino acids, monosaccharides) or the lacteals (fatty acids and glycerol as chylomicrons).
1. Jaundice: Yellowing of skin and eyes due to excessive bilirubin in the blood. It occurs when the liver is damaged and cannot process bilirubin properly, or when bile ducts are blocked.
2. Vomiting: The forceful expulsion of stomach contents through the mouth. It is caused by reverse peristalsis of the stomach and oesophagus. It can be triggered by infections, food poisoning, overeating, or motion sickness.
3. Diarrhoea: Frequent and excessive discharge of watery faeces. It occurs when the large intestine cannot absorb sufficient water from the intestinal contents. It can be caused by infections, food intolerance, or stress. Prolonged diarrhoea can lead to dehydration.
4. Constipation: Difficulty in defecation, producing hard, dry stools. It results from slow movement of food through the large intestine, leading to excessive water absorption. It can be caused by low fibre diet, inadequate water intake, or lack of physical activity.
5. Indigestion: Discomfort or pain in the abdomen after eating. It results from incomplete digestion of food, overeating, spicy food, or anxiety. Symptoms include bloating, nausea, and abdominal pain.
Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Small intestine (duodenum, jejunum, ileum) → Large intestine (caecum, colon, rectum) → Anus
(a) Starch: Salivary amylase (in mouth), Pancreatic amylase (in small intestine)
(b) Proteins: Pepsin (in stomach), Trypsin, Chymotrypsin, Carboxypeptidase (in small intestine)
(c) Fats: Gastric lipase (in stomach, limited action), Pancreatic lipase (in small intestine)
(d) Nucleic acids: Nucleases (in small intestine, from pancreatic juice), Nucleotidases, Nucleosidases (in small intestine, from intestinal juice)
1. Bile secretion: The liver secretes bile (500–1000 mL/day), which is stored in the gallbladder and released into the duodenum. Bile contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol, and phospholipids.
2. Fat emulsification: Bile salts emulsify large fat globules into smaller droplets, increasing the surface area for pancreatic lipase action. Without bile, fat digestion would be very inefficient.
3. Neutralisation of acid: Bile neutralises the acidic chyme coming from the stomach, creating an alkaline pH (7–8) that is optimal for pancreatic and intestinal enzyme activity.
4. Activation of enzymes: Bile salts help activate pancreatic lipase.
5. Absorption of fat-soluble vitamins: Bile helps in the absorption of vitamins A, D, E, and K by forming micelles that facilitate their uptake by intestinal cells.
6. Excretion: The liver eliminates waste products like bilirubin (from haemoglobin breakdown) through bile into the intestine.
1. In the mouth: Salivary amylase (ptyalin) in saliva acts on starch and converts it into maltose (a disaccharide). About 30% of starch is digested in the mouth.
Starch → Salivary amylase → Maltose
2. In the stomach: No carbohydrate digestion occurs in the stomach because the acidic pH inactivates salivary amylase.
3. In the small intestine: This is the main site of carbohydrate digestion.
- Pancreatic amylase acts on remaining starch and converts it to maltose.
- Intestinal enzymes (maltase, sucrase, lactase) act on disaccharides:
- Maltose → Maltase → Glucose + Glucose
- Sucrose → Sucrase → Glucose + Fructose
- Lactose → Lactase → Glucose + Galactose
The end products of carbohydrate digestion are monosaccharides — primarily glucose, along with fructose and galactose — which are absorbed by the intestinal villi.
Internal Links
- [Chapter 16 Guide — Digestion and Absorption](/school/cbse/class/11/biology/cbse-digestion-and-absorption)
- [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
- [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)