Biology Class 11 NCERT Solutions: Chapter 16 Digestion and Absorption Part 4

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


The digestion of proteins begins in the stomach and is completed in the small intestine. The enzymes that act on proteins are known as proteases.

Digestion in the stomach:

The digestive juice secreted in the gastric glands present on the stomach walls is called gastric juice. The main components of gastric juice are HCl, pepsinogen, and rennin. The food that enters the stomach becomes acidic on mixing with this gastric juice.

The acidic medium converts inactive pepsinogen into active pepsin. The active pepsin then converts proteins into proteases and peptides.

The enzyme rennin plays an important role in the coagulation of milk.

Digestion in the small intestine:

The food from the stomach is acted upon by three enzymes present in the small intestine - pancreatic juice, intestinal juice (known as Succus entericus), and bile juice.

Action of pancreatic juice

Pancreatic juice contains a variety of inactive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases. The enzymes are present in an inactivated state. The enzyme enterokinase secreted by the intestinal mucosa activates trypsinogen into trypsin.

The activated trypsin then activates the other enzymes of pancreatic juice. Chymotrypsinogen is a proteolytic enzyme that breaks down proteins into peptides.

Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids.

Action of bile juice

Bile juice has bile salts such as bilirubin and biliverdin which break down large, fat globules into smaller globules so that pancreatic enzymes can easily act on them. This process is known as emulsification of fats. Bile juice also makes the medium alkaline and activates lipase. Lipase then breaks down fats into diglycerides and monoglycerides.

Action of intestinal juice

Intestinal juice contains a variety of enzymes. Pancreatic amylase digests polysaccharides into disaccharides. Disaccharidases such as maltase, lactase, sucrose, etc., further digest the disaccharides.

The proteases hydrolyse peptides into dipeptides and finally into amino acids.

Image shows the protein digestion

Image Shows the Protein Digestion

Pancreatic lipase breaks down fats into diglycerides and monoglycerides.

The nucleases break down nucleic acids into nucleotides and nucleosides.

Q: 13. Explain the term thecodont and diphyodont.


Thecodont is a type of dentition in which the teeth are embedded in the deep sockets of the jaw bone. Ankylosis is absent and the roots are cylindrical. Examples include living crocodilians and mammals. Diphyodont is a type of dentition in which two successive sets of teeth are developed during the lifetime of the organism. The first set of teeth is deciduous and the other set is permanent.

The deciduous set of teeth is replaced by the permanent adult teeth.

This type of dentition can be seen in humans.

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


There are four different types of teeth in an adult human. They are as follows:

Image of four different types of teeth

Image of Four Different Types of Teeth

(i) Incisors

The eight teeth in the front are incisors. There are four incisors each in the upper jaw and the lower jaw. They are meant for cutting.

(ii) Canines

The pointy teeth on either side of the incisors are canines. They are four in number, two each placed in the upper jaw and the lower jaw. They are meant for tearing.

(iii) Premolars

They are present next to the canines. They are eight in number, four each placed in the upper jaw and the lower jaw. They are meant for grinding.

(iv) Molars

They are present at the end of the jaw, next to the premolars. There are twelve molars, six each placed in the upper jaw and the lower jaw.

Image of the number of teeth in an adult human

Image of the Number of Teeth

Hence, the dental formula in humans is

This means each half of the upper jaw and the lower jaw has incisors, canine, premolars, and molars. Hence, an adult human has permanent teeth.

Q: 15. What are the functions of liver?


Liver is the largest and heaviest internal organ of the body. It is not directly involved in digestion, but secretes digestive juices. It secretes bile which plays a major role in the emulsification of fats.

Image of the functions of liver

Image of the Functions of Liver

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