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Digestion and Absorption NCERT Textbook With Solutions PDF Free Download
Chapter 16: Digestion and Absorption
16.1 DIGESTIVE SYSTEM
The human digestive system consists of the alimentary canal and the associated glands.
16.1.1 Alimentary Canal
The alimentary canal begins with an anterior opening – the mouth and opens out posteriorly through the anus. The mouth leads to the buccal cavity or oral cavity. The oral cavity has a number of teeth and a muscular tongue.
Each tooth is embedded in a socket of the jaw bone (Figure16.2). This type of attachment is called thecodont.
The majority of mammals including human beings form two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. This type of dentition is called diphyodont An adult human.
The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx.
A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing.
The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag-like structure called the stomach.
A muscular sphincter (gastro-oesophageal) regulates the opening of the oesophagus into the stomach.
The stomach, located in the upper-left portion of the abdominal cavity has four major parts – a cardiac portion into which the oesophagus opens a fundic region, a body (main central region) and a pyloric portion which opens into the first part of the small intestine.
16.1.2 Digestive Glands
The digestive glands associated with the alimentary canal include the salivary glands, the
liver and the pancreas. Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the submaxillary/sub-mandibular (lower jaw) and the sub-linguals (below the tongue).
These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
The liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm and has two lobes.
The hepatic lobules are the structural and functional units of the liver containing hepatic cells arranged in the form of cords.
Each lobule is covered by a thin connective tissue sheath called he Glisson’s capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.
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NCERT Solutions Class 11 Biology Chapter 16 Digestion and Absorption
(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
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.
(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 that 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.
NCERT Class 11 Biology Textbook Chapter 16 Digestion and Absorption With Answer PDF Free Download