NCERT Solutions Class 11 Biology Chapter 16

Q.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) Gastric juice contains pepsin, lipase and rennin

(b) (ii) Succus entericus is the name given to intestinal juice

Q.2 Match column I with column II

Ans-

Q.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 main function of villi is to increase the surface area of intestine for increased and efficient absorption of digested nutrients (monosaccharide and amino acids) through diffusion. Absorption of nutrients through diffusion is only possible after the complete digestion of food. Complete digestion of food occurs only in the small intestine, not in the stomach. That’s why villi are present in the intestine and not in the stomach.

(b) Hydrochloric acid (HCl), secreted by parietal or oxyntic cells of the gastric gland in stomach, converts pepsinogen into its active form pepsin.

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

• Serosa: It is made up of a thin mesothelium and some connective tissues.
• Muscularis: It consists of smooth muscle cells and is arranged into inner circular and outer longitudinal layers. Circular and longitudinal layers are responsible for mixing and propelling of food respectively along the gastrointestinal tract.
• Sub-mucosal layer: It consists of loose connective tissues which contain nerves, blood and lymph vessels. Glands are also found in duodenal mucosa.
• Mucosal layer: It forms the innermost layer. It is irregular with folds (rugae) in stomach and villi in the small intestine. Villi increase the surface area for the absorption of digested nutrients. Villi are further covered with microscopic projection called microvilli which gives a brush border appearance to the epithelial lining of villi.

(d) Fats are insoluble in water and aggregate to form a large mass. Lipases cannot act on these aggregates. Bile is amphipathic in nature with both hydrophilic and hydrophobic groups. Bile breaks these large fat globules into small droplets by the process of emulsification and makes them water-soluble. Hydrophobic parts of bile interact with fat and hydrophilic region interacts with water molecules that make these fat droplets suspended in an aqueous environment. Now, these water-soluble fat droplets are easily accessible to lipases that hydrolyse them into monoglycerides.

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

Ans-

Pancreatic juice contains several pro-enzymes required for the digestion of proteins – trypsinogen, chymotrypsinogen and procarboxypeptidases. First trypsinogen is converted into active trypsin by an enzyme called enterokinase present in intestinal mucosa. Trypsin further activates other protein digestive enzymes.

Trypsin, Chymotrypsin and carboxypeptidases further act on proteins, proteases and peptones and convert it into dipeptides.

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

Ans-

The stomach stores up to 2 litres of partially digested food. Food remains in the stomach for 4-5 hours. The muscular walls of the stomach contract vigorously and mix food with juices that are secreted whenever food enters the stomach. Gastric juice secreted by glands of the stomach contains HCl and inactive proteolytic enzyme- pepsinogen. HCl activates the conversion of pepsinogen into active pepsin which hydrolyses proteins into smaller peptides (proteoses and peptones). HCl also provides an acidic environment (pH of about 2.0) required for optimal activity of pepsin. Rennin is another proteolytic enzyme found only in infants that helps in the digestion of milk by hydrolysing milk protein, casein. At the end of this process, the contents of the stomach acquire a thick, soupy consistency called chyme. The chyme goes into the first part of the small intestine whenever the sphincter, a narrow opening at the base of the stomach, relaxes.

Q.6 Give the dental formula of human beings.

Ans-

The dental formula represents the number and types of teeth found in half lower and half upper jaw of any species. They are multiplied by two to give the total number of teeth present in that species. Teeth are represented in a specific order of incisor (I), canine (C), premolar (PM) and molar (M). The dental formula of an adult human being is

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

Ans-

Bile juice is digestive secretion released by the gall bladder. It helps in the digestion of fat. It does not contain any digestive enzyme, but contains amphipathic bile salts- bilirubin and biliverdin that are essential for solubilisation of large fat globules by breaking them into smaller droplets which can mix with water. This process is called emulsification. Emulsified fat is further hydrolysed by lipases which are otherwise not able to act on large fat globules. Bile juice is alkaline, so it also activates lipases.

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

Ans-

Chymotrypsin is synthesised in pancreas and secreted in the small intestine with other proteolytic enzymes in the form of pancreatic juice. Chymotrypsin is synthesized in an inactive form as chymotrypsinogen which is cleaved by trypsin to give active chymotrypsin. Chymotrypsin helps in digestion of proteins. It hydrolyses proteins into dipeptides. Other digestive enzymes secreted by same source glands are trypsinogen and procarboxypeptidase which also help in digestion of proteins.

Q.9 How are polysaccharides and disaccharides digested?

Ans-

Digestion of polysaccharides (carbohydrates) starts in the mouth and completes in the small intestine. Enzymes required for the digestion of polysaccharides are called amylases. Approximately 30% of the polysaccharides are digested in the mouth by salivary amylases which are secreted into the mouth by salivary glands. Salivary amylases work at pH 6.8; however when they reach stomach, they become inactive due to the low pH of the stomach. Digestion of polysaccharides again starts in the small intestine where pancreatic amylase breaks down large polysaccharides into small disaccharides.

Disaccharides are further digested into monosaccharides by other enzymes present in pancreatic juice.

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

Ans-

HCl is secreted by parietal cells of the stomach which maintains the acidic pH of the stomach. HCl is required for the activation of pepsin which is required for the digestion of proteins. Pepsin is secreted in the stomach as inactive pepsinogen. HCl triggers the conversion of pepsinogen into pepsin. Further, pepsin needs low pH (1.5 to 2.0) for its optimal activity which is provided by HCl. Pepsin becomes inactive at pH above 6.5. Therefore, if HCl is not secreted by the stomach, pepsin will be activated and this will affect protein digestion.

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

Ans-

Butter mainly consists of fat (fat 80%, water 15% and protein 5%). Butter is water-insoluble and forms large fat globules in the aqueous environment of stomach.

Digestion: When these large fat globules reach small intestine, they are first converted into small fat droplets by the action of bile juice. This process is called emulsification. Bile juice also activates pancreatic lipases which are present in the small intestine. Emulsified fat droplets are hydrolysed into triglycerides by pancreatic lipases in the small intestine. These triglycerides are further hydrolyzed into diglycerides and monoglycerides by intestinal lipases.

Absorption: Fats are absorbed by membrane of enterocytes, present in intestinal mucosa, by simple diffusion as they are hydrophobic and can cross the lipid bilayer of cell membrane. Cytoplasm of enterocytes and blood are aqueous in nature and these monoglycerides are water immiscible. So these monoglycerides are packaged into water-soluble particles, called chylomicrons. Chylomicrons are lipo-proteins in nature. Chylomicrons are exocytosed by enterocytes. Exocytosed chylomicrons enter lymph vessel (lacteal). Lymph vessel transports chylomicrons to blood stream.

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

Ans-

Digestion of proteins starts in the stomach and completes in the small intestine. As proteins travel along the gastro-intestinal tract, they are hydrolyzed into smaller peptides and finally into amino acids. Enzymes which hydrolyze proteins are called proteases. There are several proteases which are secreted by the glands of mucosa of stomach and small intestine. Most of these proteases are secreted in an inactive form and converted into active enzyme in the gastro-intestinal lumen.

Stomach: Stomach secretes gastric juice which contains proteolytic enzymes- pepsinogen and rennin along with HCl. HCl makes the gastric environment acidic which is required for the conversion of inactive pepsinogen into active pepsin. Pepsin hydrolyses proteins into smaller peptides. Stomach also secretes another protease rennin that helps in digestion of milk proteins in infants.

Small Intestine: As the proteins enter the small intestine, they are acted upon by the proteases present in pancreatic juice. Pancreatic juice contains several inactive enzymes required for the digestion of proteins –trypsinogen, chymotrypsinogen and procarboxypeptidases. First trypsinogen is converted into active trypsin by an enzyme, enterokinase present in intestinal mucosa. Trypsin further activates other protein digestive enzymes.

Q.13 Explain the term thecodont and diphyodont.

Ans-

Thecodont is a condition in which teeth are embedded in the sockets. These sockets are also called alveoli of the jawbones. These teeth have cylindrical roots. Mammals and crocodiles have thecodont teeth.

Diphyodont is a condition in which teeth appears in two sets in the entire life. The first set of teeth are called deciduous or temporary teeth. The deciduous teeth are replaced by the second and the final set of permanent teeth. For example, humans are diphyodont. They have 20 deciduous teeth which are replaced by 32 permanent teeth.

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

Ans-

There are 32 teeth in an adult human. They are of four different types and this type of dentition is called heterodont dentition. These four types of teeth are Incisors, canines, premolars and molars.

Incisors: Incisors are front teeth. There are eight incisors, four in the upper jaw and four in the lower jaw.

Canines: Canines are situated on both sides of the incisors. There are 4 canines, two in the upper jaw and two in the lower jaw.

Premolars: Premolars are situated next to canines. They are eight in number, four in the upper jaw and four in the lower jaw.

Molars: Molars are situated next to premolars. They are twelve in numbers, six in the upper jaw and six in the lower jaw.

Q.15 What are the functions of liver?

Ans-

The functions of the liver are as follows:

• Liver cells secrete bile juice which helps in the digestion of fat. Fat forms large insoluble fat globules in the aqueous environment of the stomach which are inaccessible to pancreatic lipases. Bile juice contains amphipathic bile salts (bilirubin and biliverdin) which break down large fat globule into small fat droplets. This process is called emulsification. These fat droplets are hydrolyzed into glycerides by pancreatic lipases.
• Bile juice secreted from the liver is also required for activation of pancreatic lipases.
• It detoxifies the blood by removing and metabolizing poisonous substances.
• It produces heparin protein which prevents blood clotting inside the blood vessels.
• It stores glucose as glycogen and breaks down glycogen to glucose between meals to maintain constant blood glucose levels by the process of glycogenesis.
• It produces urea from amino groups and ammonia.
• Liver is the site of vitamin-A synthesis from the carotenes of carrot.