NCERT Solutions for Class 11 Biology Chapter 16: Digestion and Absorption

1. Choose the correct answer among the following:

(a) Gastric juice contains:

(i) pepsin, lipase and rennin

(ii) trypsin, lipase and rennin

(iii) trypsin, pepsin and lipase

(iv) trypsin, pepsin and renin

(b) Succus entericus is the name given to:

(i) a junction between ileum and large intestine

(ii) intestinal juice

(iii) swelling in the gut

(iv) appendix

Ans. (a) (i) pepsin, lipase and rennin

1. Pepsin is secreted in an inactive form as pepsinogen, which is activated by HCl. Pepsin digests proteins into peptones.

2. Lipase breaks down fats into fatty acids.

3. Rennin is a proteolytic enzyme present in the gastric juice. It helps in the coagulation of milk.

(b) (ii) Intestinal juice.

Succus entericus is secreted by the intestinal gland. Intestinal juice contains a variety of enzymes such as maltase, lipases, nucleosidases, dipeptidases, etc.

2. Match column I with column II.

Column I Column II
(a) Bilirubin and biliverdin (i) Parotid
(b) Hydrolysis of starch (ii) Bile
(c) Digestion of fat (iii) Lipases
(d) Salivary gland (iv) Amylases

Ans. The correct match is as follows:

Column I Column II
(a) Bilirubin and biliverdin (ii) Bile
(b) Hydrolysis of starch (iv) Amylases
(c) Digestion of fat (iii) Lipases
(d) Salivary gland (i) Parotid

3. Answer briefly:

(a) Why are villi present in the intestine and not in the stomach?

(b) How does pepsinogen change into its active form?

(c) What are the basic layers of the wall of alimentary canal?

(d) How does bile help in the digestion of fats?

Ans. (a) The villi and microvilli are present in small intestine to increase the surface area for digested nutrient absorption. The more would be the surface area of intestine, more will be the amount of absorbed food.

The mucosal walls of stomach forms irregular folds known as rugae. These help increase the surface area to volume ratio of the expanding stomach. So, the villi present in the intestine and not in the stomach.

(b) Pepsinogen is a precursor of pepsin stored in the stomach walls. It is converted into pepsin by hydrochloric acid. Pepsin is the activated form of pepsinogen.

(c) The wall of alimentary canal consists of following four layers:

1. Serosa is the outermost layer and made up of mesothelium and connective tissues.

2. Muscularis is made up of smooth muscles that are arranged in an inner circular and outer longitudinal layer.

3. Sub mucosal layer is made up of loose connective tissues.

4. Mucosa consists of layers of loose connective tissues.

(d) Bile secreted by liver contains bile pigments such as bilirubin and biliverdin. These pigments break down large fat globules into smaller particles so that the pancreatic enzymes can easily act on them. The process is known as emulsification of fats.

4. State the role of pancreatic juice in digestion of proteins.

Ans. The pancreatic juice is secreted by the pancreas and contains enzymes such as trypsinogen, chymotrypsinogen and carboxypeptidases. These enzymes are inactive and they are required for the process of digestion of proteins.These enzymes play following role in protein digestion:

(a) Enzyme Trypsinogen: This enzyme gets activated into trypsin by an enzyme called enterokinase. Trypsinogen is secreted by the intestinal mucosa. Trypsin further activates the other enzymes of pancreatic juice such as chymotrypsinogen and carboxypeptidase.

(b) Chymotrypsinogen: It is a milk-coagulating enzyme responsible for converting proteins into peptides.

(c) Carboxypeptidase: It is an enzyme that acts on the carboxyl end of the peptide chain and helps in the release of the last amino acid from the polypeptide chain, thus, aiding protein digestion.

5. Describe the process of digestion of protein in stomach.

Ans. The digestion of protein starts in the stomach and completed in the small intestine. The stomach wall contains gastric glands which secrete digestive juice. The gastric juice contains hydrochloric acid (HCl), pepsinogen, mucus, and rennin. HCl dissolves the bits of food and creates an acidic medium so that pepsinogen is converted into pepsin. Pepsin is a proteolytic enzyme. It is secreted in its inactive form called pepsinogen, which then gets activated by HCl. The activated pepsin then converts proteins into proteases and peptides.

Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Rennin plays an important role in the coagulation of milk.

6. Give the dental formula of human beings.

Ans. The dental formula is the arrangement of teeth in each half of the upper jaw and the lower jaw. The dental formula for milk teeth (number 20) is:

Incisors = 2, Canines = 1, Molar = 2 (in each half of jaw)

$$=\frac{212}{212}ร—2=20$$

An adult human has 32 permanent teeth. Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars, and 3 molars. The dental formula for permanent teeth in humans represented as:

Incisors = 2, Canines = 1, Premolars = 2, Molars = 3

$$=\frac{2123}{2123}ร—2=32$$

7. Bile juice contains no digestive enzymes, yet it is important for digestion. Why?

Ans. Liver secretes bile juice. It does not contain any digestive enzymes, but is important for the digestion of fats in the digestive system. Bile juice contains bile pigments such as bilirubin and biliverdin. The main function of bile juice is to convert big fat globules into small globules by the process called emulsification. In addition, bile juice also makes the medium alkaline and activates enzyme lipase (conducts hydrolysis of fats). Hence, bile juice contains no digestive enzymes, yet it is important for digestion.

$$\text{Fat} (\text{triglycerides})\xrightarrow{\text{Bile}}\text{Emulsified fat}$$

8. Describe the digestive role of chymotrypsin. Which two other digestive enzymes of the same category are secreted by its source gland?

Ans. Chymotrypsin completes the digestion of protein. It is a proteolytic enzyme of pancreatic juice secreted by exocrine part of pancreas. It converts proteins, peptones and proteoses into oligopeptides and dipeptides. The other two proteolytic enzymes present in pancreatic juice are trypsinogen and procarboxypeptidase.

9. How are polysaccharides and disaccharides digested?

Ans. The digestion of carbohydrates starts in the mouth and completes in the small intestine region of the alimentary canal. The enzymes responsible for carbohydrate digestion are called carbohydrase.

(a) In mouth: Food that enters mouth is first mixed with saliva secreted by the salivary glands. Saliva contains a digestive enzyme called salivary amylase or ptyalin. This enzyme breaks down starch into sugar at pH 6.8.

$$\text{Starch}\xrightarrow[\text{pH}\space6.8]{\text{Salivary amylase}}\text{Maltose + Isomaltose+ Limit dextrins}$$

(b) In stomach: The digestion of carbohydrates does not occur in stomach because gastric juice itself has no carbohydrase.

(c) In small intestine: The food mixes with two juices in small intestine. These are pancreatic juice and intestinal juice.

(i) Pancreatic juice contains a carbohydrase named pancreatic amylase. This enzyme hydrolyses more starch and glycogen.

(ii) Intestinal juice contains a variety of enzymes, such as carbohydrases; maltase, isomaltase, a-dextrinase, sucrase and lactase which act on disaccharides as follows:

These enzymes help in the digestion of disaccharides. Thus, the digestion of carbohydrates gets completed in the small intestine.

$$\text{Malstose}\xrightarrow{\text{Maltase}}\text{Glucose + Glucose}\\\text{Lactose}\xrightarrow{\text{Lactase}}\text{Glucose + Galactose}\\\text{Sucrose}\xrightarrow{\text{Sucrase}}\text{Glucose + Fructose}$$

The sugars like glucose, fructose and galactose are monomers of carbohydrates. These are absorbed by intestinal mucosa.

10. What would happen if HCl were not secreted in the stomach?

Ans. Hydrochloric acid (HCl) is secreted by the parietal cells located in stomach epithelial wall. Its function in the stomach is to dissolve food particles and maintain an acidic medium. The acidic medium is important for the pepsinogen to be converted into pepsin. It is an essential process of digestion of proteins. Hence, if HCl were not secreted in the stomach, the pepsin would not be activated leading to disturbance in protein digestion.

11. How does butter in your food get digested and absorbed in the body?

Ans. Digestion and absorption of butter (fat) occurs in the small intestine.

Digestion of fat: The digestion of fats occurs in small intestine.

(a) The bile juice secreted by the liver contains bile salts that break down large fat globules into smaller globules. This process is referred to as emulsification of fats.

(b) The emulsified fat globules are hydrolysed by the pancreatic lipase present in the pancreatic juice and the intestinal lipase present in the intestinal juice into triglycerides, diglycerides, monoglycerides and ultimately into glycerol and fatty acids.

$$\text{Fats}\xrightarrow[\text{Lipase}]{\text{Pancreatic}}\text{Triglycerides + Diglycerides}\\\text{Diglycerides and monoglycerids}\xrightarrow[]{\text{Lipases}}\text{Fatty acids + Glycerol}$$

Absorption of fat: The digested fat is hydrolysed into fatty acids and glycerol first. These products are insoluble water so cannot be absorbed by the blood directly. To facilitate their absorption, they are first changed into small droplets called micelles. The lipase acts on micelles to break down fat molecules into fatty acids and monoglycerides. These fatty acids and monoglycerides, pass through the small intestine, these are converted to triglycerides, which combine with cholesterol, phospholipids and protein to form a structure called a chylomicron. The protein coating of the chylomicron makes it water-soluble so it can travel through the lymph vessels and eventually the bloodstream and finally delivered to each cell of the body.

12. Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal.

Ans. Digestion of protein in the alimentary canal:

(a) Stomach is the organ where main protein digestion takes place. The enzyme pepsin is produced in the stomach. It plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids.

(b) Small intestine contains enzymesโ€” trypsin, elastase, and chymotrypsin which act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme.

(c) Further breakdown of peptides to single amino acids is aided by enzymes called peptidases.

(d) Enzymes like carboxypeptidase, dipeptidase and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines.

Table: Steps in digestion of protein in the digestive system

Enzyme Parts that
Produce it
Organ where Digestion occurs Substrate to act on End Products
Pepsin Stomach chief cells Stomach Proteins Peptides
Trypsin, Elastase, Chymotrypsin Pancreas Small intestine Proteins Peptides
Carboxypeptidase Pancreas Small intestine Peptides Amino acids and peptides
Aminopeptidase, Dipeptidase Lining of intestine Small intestine Peptides Amino acids

13. Explain the term thecodont and diphyodont.

Ans. Thecodont: It is a type of dentition in which the base of the tooth is enclosed in jaw sockets. The tooth is embedded in a socket of jawbone. This type of dentition is present in humans.

Diphyodont: It is a type of dentition in which two successive sets of teeth are developed during the lifetime of the living organism. The first set of teeth is deciduous (milk teeth) and the other set is permanent. The deciduous set of teeth is replaced by the permanent adult teeth. This type of dentition is found in most mammals including humans.

14. Name different types of teeth and their number in an adult human.

Ans. Humans have heterodont dentition. It means they have four different types of teeth. The number of different types of teeth in human are as follows:

Incisors = 8, Canines = 4, Premolars = 8, Molars = 12

Thus, adult human consists of 32 teeth.

15. What are the functions of liver?

Ans. Liver is the largest and associated gland with the digestive system. It carries out many important functions in the body:

(a) It secretes bile which is important for the digestion of fat.

(b) It is important for the metabolism of proteins, fats and carbohydrates.

(c) It is responsible to excrete cholesterol, bilirubin and hormones, etc.

(d) It synthesizes and activates enzymes.

(e) It stores vitamins, minerals and glycogen.

(f) It is responsible to synthesize plasma proteins namely albumin and clotting factors in blood.

(g) It detoxifies the blood and purifies it.