CBSE Class 11 Biology Chapter 18: Digestion and Absorption NCERT Solutions

(a) The correct option is (i) pepsin, lipase, and rennin.

Gastric juice, secreted by the stomach lining, is a complex mixture containing several components crucial for digestion. It includes pepsin, which is initially secreted as the inactive zymogen pepsinogen and activated by hydrochloric acid (HCl) to digest proteins into smaller peptides called peptones. Lipase is also present to aid in the breakdown of fats. Rennin is an enzyme particularly important in infants for the coagulation of milk proteins (casein), facilitating their digestion.

(b) The correct option is (ii) intestinal juice.

Succus entericus is the term used for the intestinal juice, which is secreted by the glands located in the wall of the small intestine. This juice is rich in various enzymes, such as disaccharidases (like maltase, sucrase, lactase), peptidases (like dipeptidases), and lipases, which are essential for the final stages of digestion of carbohydrates, proteins, and fats, respectively.

The correct matching of Column I with Column II is as follows:

• (a) Bilirubin and biliverdin are the principal bile pigments, which are waste products from the breakdown of hemoglobin. They are found in (ii) Bile.
• (b) Hydrolysis of starch is primarily carried out by amylase enzymes. Salivary amylase in the mouth and pancreatic amylase in the small intestine are responsible for this process. Therefore, it matches with (iv) Amylases.
• (c) Digestion of fat is accomplished by lipase enzymes, which break down triglycerides into fatty acids and glycerol. This matches with (iii) Lipases.
• (d) Parotid is a major salivary gland. Salivary glands, including the parotid, submandibular, and sublingual glands, produce saliva which contains salivary amylase. Thus, it relates to (i) Parotid as an example of a salivary gland.

The complete matching is: (a) - (ii), (b) - (iv), (c) - (iii), (d) - (i).

1. Why are villi present in the intestine and not in the stomach?

Villi are microscopic, finger-like projections that line the mucosal wall of the small intestine. Their presence dramatically increases the surface area available for absorption of digested food molecules. The small intestine is the primary site for nutrient absorption due to these villi and the microvilli on their epithelial cells. In contrast, the stomach's primary functions are food storage, mechanical churning, and the initial digestion of proteins. While the stomach lining has folds called rugae that allow for expansion, it lacks the specialized villi structure because significant absorption does not occur there. The absorbed nutrients pass into the bloodstream and lymph vessels within the villi.

2. How does pepsinogen change into its active form?

Pepsinogen is an inactive zymogen precursor of the enzyme pepsin. It is synthesized and secreted by the chief cells in the stomach lining. The conversion of pepsinogen into its active form, pepsin, is triggered by the highly acidic environment of the stomach, which is maintained by the secretion of hydrochloric acid (HCl) from the parietal cells. HCl partially breaks down pepsinogen, and the resulting pepsin molecules then catalyze the conversion of the remaining pepsinogen into more pepsin. This process is known as autocatalysis.

3. What are the basic layers of the wall of the alimentary canal?

The wall of the alimentary canal, from the lumen outwards, is composed of four fundamental layers:

1. Mucosa: This is the innermost layer lining the lumen. It is responsible for secretion and absorption and often contains folds, villi, or microvilli to increase surface area.
2. Submucosa: Located outside the mucosa, this layer is made of loose connective tissue and contains blood vessels, lymphatic vessels, nerves, and glands.
3. Muscularis: This layer consists of smooth muscle, typically arranged in an inner circular layer and an outer longitudinal layer. These muscles contract rhythmically to mix food with digestive juices and propel it along the tract (peristalsis).
4. Serosa: This is the outermost protective layer, a thin sheet of connective tissue covered by a single layer of squamous epithelium.

4. How does bile help in the digestion of fats?

Bile, produced by the liver and stored in the gallbladder, plays a crucial role in fat digestion, although it does not contain digestive enzymes itself. Its primary function is emulsification. Bile salts in the bile break down large fat globules into much smaller droplets. This process, called emulsification, significantly increases the total surface area of the fats exposed to the action of lipase enzymes. Lipases, secreted by the pancreas and small intestine, can then more efficiently hydrolyze these smaller fat droplets into absorbable fatty acids and glycerol.