Chapter 16 Digestion and Absorption Solutions
Question - 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
Answer - 1 : -
Answer (a): (i) Pepsin, lipase, and rennin
Gastric juice contains pepsin, lipase, and rennin. Pepsin is secreted in an inactive form as pepsinogen, which is activated by HCl. Pepsin digests proteins into peptones. Lipase breaks down fats into fatty acids. Rennin is a photolytic enzyme present in the gastric juice. It helps in the coagulation of milk.
Answer (b): (ii) Intestinal juice
Succus entericus is another name for intestinal juice. It is secreted by the intestinal gland. Intestinal juice contains a variety of enzymes such as maltase, lipases, nucleosidases, dipeptidases, etc.
Question - 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 |
Answer - 2 : -
| 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 |
Question - 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?
Answer - 3 : - 
(a) The mucosal wall of the small intestine forms millions of tiny finger-like projections known as villi. These villi increase the surface area for more efficient food absorption.
Within these villi, there are numerous blood vessels that absorb the digested products of proteins and carbohydrates, carrying them to the blood stream. The villi also contain lymph vessels for absorbing the products of fat-digestion. From the blood stream, the absorbed food is finally delivered to each and every cell of the body.
The mucosal walls of the stomach form irregular folds known as rugae. These help increase the surface area to volume ratio of the expanding 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 in the form of pepsinogen.
Pepsinogen
Pepsin + Inactive peptide
(Inactive) (Active)
(c) The walls of the alimentary canal are made up of four layers. These are as follows:
(i) Serosa is the outermost layer of the human alimentary canal. It is made up of a thin layer of secretory epithelial cells, with some connective tissues underneath.
(ii) Muscularis is a thin layer of smooth muscles arranged into an outer longitudinal layer and an inner circular layer.
(iii) Sub-mucosa is a layer of loose connective tissues, containing nerves, blood, and lymph vessels. It supports the mucosa.
4. Mucosa is the innermost lining of the lumen of the alimentary canal. It is mainly involved in absorption and secretion.
(d) Bile is a digestive juice secreted by the liver and stored in the gall bladder. 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 emulsification of fats. Bile juice also makes the medium alkaline and activates lipase.
State the role of pancreatic juice in digestion of proteins.
Answer - 4 : -
Pancreatic juice contains a variety of inactive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases. These enzymes play an important role in the digestion of proteins.
Physiology of protein-digestion
The enzyme enterokinase is secreted by the intestinal mucosa. It activates trypsinogen into trypsin.
Trypsinogen
Trypsin + Inactive peptide
Trypsin then activates the other enzymes of pancreatic juice such as chymotrypsinogen and carboxypeptidase.
Chymotrypsinogen is a milk-coagulating enzyme that converts proteins into peptides.
Chymotrypsinogen
Chymotrypsin(Inactive) (Active)
Proteins
PeptidesCarboxypeptidase acts on the carboxyl end of the peptide chain and helps release the last amino acids. Hence, it helps in the digestion of proteins.
Peptides
Smaller peptide chain + Amino acidsThus, in short, we can say that the partially-hydrolysed proteins present in the chyme are acted upon by various proteolytic enzymes of the pancreatic juice for their complete digestion.
Proteins, peptones
Dipeptides and proteases
Describe the process of digestion of protein in stomach.
Answer - 5 : -
The digestion of proteins begins in the stomach and is completed in the small intestine. The digestive juice secreted in the gastric glands present on the stomach walls is called gastric juice. The food that enters the stomach becomes acidic on mixing with this gastric juice.
The main components of gastric juice are hydrochloric acid, pepsinogen, mucus, and rennin. Hydrochloric acid dissolves the bits of food and creates an acidic medium so that pepsinogen is converted into pepsin. Pepsin is a protein- digesting enzyme. It is secreted in its inactive form called pepsinogen, which then gets activated by hydrochloric acid. The activated pepsin then converts proteins into proteases and peptides.
Proteins
Proteases + Peptides
Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Rennin plays an important role in the coagulation of milk.
Question - 6 : - Given the dental formula of human beings
Answer - 6 : -
The dental formula expresses the arrangement of teeth in each half of the upper jaw and the lower jaw. The entire formula is multiplied by two to express the total number of teeth.
The dental formula for milk teeth in humans is:
Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, and 2 molars. Premolars are absent in milk teeth.
The dental formula for permanent teeth in humans is:
Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars, and 3 molars. An adult human has 32 permanent teeth.
Question - 7 : - Bile juice contains no digestive enzymes, yet it is important for digestion. Why?
Answer - 7 : -
Bile is a digestive juice secreted by the liver. Although it does not contain any digestive enzymes, it plays an important role in the digestion of fats. Bile juice contains bile salts, bile pigments like bilirubin, biliverdin and phospholipids. Bile salts break down large fat globules into smaller globules so that the 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.
Question - 8 : - Describe the digestive role of chymotrypsin. What two other digestive enzymes of the same category are secreted by its source gland?
Answer - 8 : -
The enzyme trypsin (present in the pancreatic juice) activates the inactive enzyme chymotrypsinogen into chymotrypsin.
Chymotrypsinogen
Chymotrypsin(Inactive) (Active)
The activated chymotrypsin plays an important role in the further breakdown of the partially-hydrolysed proteins.
Proteins
PeptidesThe other digestive enzymes of the same category are trypsinogen and carboxypeptidase. These are secreted by the same source-gland, pancreas.
Trypsinogen is present in an inactive form in the pancreatic juice. The enzyme enterokinase – secreted by the intestinal mucosa – activates trypsinogen into trypsin.
Trypsinogen
Trypsin + Inactive peptideThe activated trypsin then further hydrolyses the remaining trypsinogen and activates other pancreatic enzymes such as chymotrypsinogen and carboxypeptidase. Trypsin also helps in breaking down proteins into peptides.
Proteins
PeptidesCarboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids.
Peptides
Small peptide chain + Amino acids
Question - 9 : - How are polysaccharides and disaccharides digested?
Answer - 9 : -
The digestion of carbohydrates takes place in the mouth and the small intestine region of the alimentary canal. The enzymes that act on carbohydrates are collectively known as carbohydrases.
Digestion in the mouth:
As food enters the mouth, it gets mixed with saliva. Saliva – secreted by the salivary glands – contains a digestive enzyme called salivary amylase. This enzyme breaks down starch into sugar at pH 6.8.
Starch
Maltose + Isomaltose + Limit dextrinsSalivary amylase continues to act in the oesophagus, but its action stops in the stomach as the contents become acidic. Hence, carbohydrate-digestion stops in the stomach.
Digestion in the small intestine:
Carbohydrate-digestion is resumed in the small intestine. Here, the food gets mixed with the pancreatic juice and the intestinal juice. Pancreatic juice contains the pancreatic amylase that hydrolyses the polysaccharides into disaccharides.
Starch
Disaccharides(Polysaccharides)
Similarly, the intestinal juice contains a variety of enzymes (disaccharidases such as maltase, lactase, sucrase, etc.). These disaccharidases help in the digestion of disaccharides. The digestion of carbohydrates is completed in the small intestine.
Maltose
2GlucoseLactose
Glucose + GalactoseSucrose
Glucose + Fructose
Question - 10 : - What would happen if HCl were not secreted in the stomach?
Answer - 10 : -
Hydrochloric acid is secreted by the glands present on the stomach walls. It dissolves bits of food and creates an acidic medium. The acidic medium allows pepsinogen to be converted into pepsin. Pepsin plays an important role in the digestion of proteins. Therefore, if HCl were not secreted in the stomach, then pepsin would not be activated. This would affect protein digestion. A pH of about 1.8 is necessary for proteins to be digested. This pH is achieved by HCl.