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

Food is an essential product for the survival of all living beings. The food that living organisms consume is composed of various components, of which the major ones are proteins, fats, and carbohydrates. Additionally, vitamins and minerals are also necessary, but in smaller quantities.

Food is the source of energy and organic materials for living organisms to properly grow and repair body tissues. The water we drink has a significant role in metabolic processes occurring inside the body and prevents the body from getting dehydrated. Biomacromolecules inside the food that living organisms consume are of no use to their body in the matter's original form. Such molecules need to be broken down and changed into simpler substances in the digestive system

This process of conversion or breakdown of complex food materials into small absorbable substances is known as digestion that is performed by the body's digestive system through biochemical and mechanical methods. To learn more about this breakdown of food articles, read the below description.

• Digestive System in Living Organisms
• How is food digested?
• How are digested products absorbed?
• Drawbacks of the Digestive System

The process of food digestion and absorption, which is a critical function in living entities have been enlightened in this chapter. The chapter has been put under Unit V - 'Human Physiology' of Biology syllabus for class 11. Mainly, the topics discussed under this chapter are the types of bodily functions that occur at the same time when digestion and absorption of food take place, like locomotion, exchange of gases, secretion of juices, etc.

Other topics explained here are different types of enzymes released during the process, the role of different organs and microorganisms in the process, and the use of various microprocessors. The concluding part of this chapter talks about the importance of each food material that is included in our diet.

Q1. 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
Answer: Gastric juice contains pepsin, lipase, and rennin. Pepsin is secreted as pepsinogen  (inactivated), which is activated by HCI. Pepsin digests proteins into peptones. Lipase breaks down fats into fatty acids. Rennin is a photolytic enzyme present in gastric juice. It helps in the coagulation of milk.

(b) Succus entericus is the name given to
(i) a junction between the ileum and large intestine
(ii) intestinal juice
(iii) swelling in the gut
(iv) appendix
Answer: Correct option is ii. Intestinal juice. Succus entericus is another name for intestinal juice.  It is secreted by the intestinal gland and contains a variety of enzymes such as maltase,  lipases, nucleosidases, dipeptidases, etc. 

Q2. 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
Answer: Correct matching is (a)- ii, (b)- iv, (c)- iii, (d)- i

Q3. 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 the alimentary canal?
(d) How does bile help in the digestion of fats.
Answer:
(a) The Intestine is involved in the absorption of digested food. So, to increase the surface area of the absorption, villi are present on the surface of the intestine. Villi are small finger-like structures. that increase the surface area of absorption of digested food into the blood.
(b) pepsinogen is a precursor of pepsin stored in the stomach walls. It is converted into pepsin by the action of hydrochloric acid. Pepsin is the activated form of pepsinogen.
(c) The wall of the alimentary canal consists of four concentric layers. Beginning from the outside, these layers are the visceral peritoneum, muscular coat, submucosa, and mucosa.
(d) Bile refers to the digestive juice secreted by the liver and stored in the gallbladder. Bile juice has bile salts such as bilirubin and biliverdin. These break down large fat globules into smaller globules so that the pancreatic enzymes can easily act on them. This process is known as the emulsification of fats. Bile juice also makes the medium alkaline and activates lipase.

Q4. State the role of pancreatic juice in the digestion of proteins.
Answer: The pancreatic juice is secreted by the pancreas and it is a mixture of enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases. These enzymes are inactive and are required for the process of digestion of proteins. The role of these enzymes in protein digestion is depicted below
1. Enzyme trypsinogen gets activated into trypsin by an enzyme called enterokinase. This enzyme is secreted by the intestinal mucosa. Trypsin further activates the other enzymes of pancreatic juice such as chymotrypsinogen and carboxypeptidase.
2. Chymotrypsinogen is a milk-coagulating enzyme that converts proteins into peptides.
3. Carboxypeptidase 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.

Q5. Describe the process of digestion of protein in the stomach.
Answer: The stomach is the first organ where the digestion of proteins starts while the small intestine is the part where protein digestion ends. The stomach possesses gastric glands which secret gastric juices containing enzymes that act on the food. Gastric juice mainly contains hydrochloric acid, pepsinogen, mucus, and rennin. Firstly, the food that enters the stomach becomes acidic when it mixes with the gastric juice. The function of these  components in protein digestion is as follows:
1. Hydrochloric acid dissolves the food particle and creates an acidic medium inside the stomach. Acidic medium is a prerequisite for the conversion of inactive enzyme pepsinogen into active pepsin.
2. Pepsin is a protein-digesting enzyme that converts proteins into proteases and peptides.
3. Rennin which plays an important part in the coagulation of milk is a proteolytic enzyme that is released as prorenin i.e. inactive renin.

Q6. Give the dental formula of human beings.
Answer: The dental formula refers to the arrangement of teeth in each half of the upper jaw and the lower jaw. The dental formula for milk teeth in humans is: 2102/2102 * 2 = 20
- Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, and 2 molars. Premolars are absent in milk teeth. It is represented as 2102/2102.
- 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 2123/2123

Q7. Bile juice contains no digestive enzymes, yet it is important for digestion. Why?
Answer: The bile juice secreted by the liver does not contain any digestive enzymes, however, it plays an important role in the digestion of fats. It contains bile salts such as bilirubin and biliverdin which break down large fat globules into smaller globules. This makes sure that the pancreatic enzymes can easily act on the fat globules. This process is known as the emulsification of fats. Bile juice also makes the medium alkaline and activates lipase.  Hence, bile juice contains no digestive enzymes, yet it is important for digestion.

Q8. Describe the digestive role of chymotrypsin. Which two other digestive enzymes of the same category are secreted by their source gland?
Answer: Chymotrypsinogen is an inactive enzyme that gets activated into chymotrypsin by the action of the enzyme trypsin present in the pancreatic juice. Chymotrypsin, when activated plays an important role in the breakdown of the hydrolyzed peptides partially.
The other digestive enzymes of the same category, are trypsinogen and carboxypeptidase. These are also secreted by the same source-gland, the pancreas.  Trypsinogen gets activated via enterokinase into trypsin.

Q9. How are polysaccharides and disaccharides digested?
Answer: The process of digestion of carbohydrates occurs in the mouth and small intestine. Digestion of carbohydrates in the mouth- After entering into the mouth, the food gets mixed with saliva which breaks down the starch into maltose, isomaltose, and limits dextrins. Saliva contains an enzyme called salivary amylase which is involved in the break down of starch into its constituents at a particular pH of 6.8. This enzyme continues to act in the esophagus, but its action stops in the stomach as the contents become acidic. Therefore, carbohydrate digestion stops in the stomach.
Digestion in the small intestine- In the small intestine, carbohydrate digestion resumes.  The food is acted upon by pancreatic juice and intestinal juice. Pancreatic juice contains the pancreatic amylase that hydrolyses the polysaccharides into disaccharides.  On the other hand, intestinal juice contains a variety of enzymes (disaccharidases such as maltase, lactase, sucrase, etc.) that help in the digestion of disaccharides. The digestion of carbohydrates is completed in the small intestine.
 

Q10. What would happen if HCl were not secreted in the stomach?
Answer: The function of hydrochloric acid in the stomach is to dissolve food particles and maintain an acidic medium. The acidic medium is required for the pepsinogen to be converted into pepsin which is essential for the process of digestion of proteins. Hence, if HCL were not secreted in the stomach, then pepsin would not be activated leading to impaired protein digestion.
 

Q11. How does butter in your food get digested and absorbed in the body?
Answer: Butter is a fat-based product.
Digestion of fats: Fats are digested in the small intestine. The liver secretes bile juice which contains bile salts such as bilirubin and biliverdin. These break down large fat globules into smaller globules, to increase their surface area for the action of lipase. This process is called the emulsification of fats. Later, the pancreatic lipase present in the pancreatic juice and the intestinal lipase present in the intestinal juice hydrolyzes the fat molecules into triglycerides, diglycerides, monoglycerides, and finally into glycerol.
Absorption of fats: During the digestion of fats, the fats are broken down into fatty acids and glycerol. These are water-insoluble substances, hence, they cannot be directly absorbed by the blood.  Therefore, they are first incorporated into small droplets called micelles and then transported into the villi of the intestinal mucosa. These are then converted into small microscopic particles called chylomicrons, which are small, protein-coated fat globules.  These chylomicrons are transported to the lymph vessels in the villi. From the lymph vessels, the absorbed food is finally released into the bloodstream and from the bloodstream, to each and every cell of the body.

Q12. Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal.
Answer: Protein digestion begins in the stomach and completes in the small intestine. The digestion of protein involves the following steps:
Proteolysis in the stomach:
a. Hydrolysis of the peptide bond by water present in the stomach.
b. Pepsin is the protease present in the stomach that starts digestion by secreting pepsinogen.
c. Pepsinogen got activated by gastric juice due to its acidic nature (pH is about 2).
Proteolysis in the small intestine:
a. Enterokinase produced from the wall of the small intestine activates subsequent pancreatic juice by converting trypsinogen to trypsin. Pancreatic digestive juice constitutes enzymes: trypsin, chymotrypsin, carboxypolypeptidase, and proelastase.
b. Amino end is cleaved by enzyme aminopeptidase and other dipeptidases. Polypeptides are cleaved into tripeptides, dipeptides, or amino acids for their absorption.
 

Q13. Explain the terms thecodont and diphyodont.
Answer: Thecodont: It refers to a type of dentition in which the teeth are embedded in the deep sockets of the jaw bone. This type of dentition is common in mammals.
Diphyodont: It refers to a specialized kind 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 permanent adult teeth. This type of dentition can be seen in humans.
 

Q14. Name different types of teeth and their number in an adult human.
Answer: In an adult human, four different types of teeth are found. These include incisor, canine,  premolar, and molar teeth.
a. Incisors: The eight teeth in the front are incisors. There are four incisors each in the upper jaw and the lower jaw. The function of incisors is the cutting of food particles.
b. 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.
c. Premolars: These are the teeth next to the canines. They are eight in number, four each placed in the upper jaw and the lower jaw. Their function is grinding.
d. Molars: These are found 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.

Q15. What are the functions of the liver?
Answer: Functions of the liver are as follows:
1. The liver secretes bile juice which plays an important role in the digestion of fats.
2. Conversion of poisonous ammonia to urea, to clear the blood of drugs and other poisonous substances.
3. Conversion of excess glucose into glycogen for storage, which can later be converted to glucose for energy.