NCERT Class 11-Biology: Chapter –16 Digestion and Absorption Part 5

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Long Answer Type Questions

Question 1:

A person had roti and dal for his lunch. Trace the changes in those during its passage through the alimentary canal.


The following changes takes place during the passage of roti and dal through the alimentary canal:

i. The human alimentary canal begins with mouth i.e., buccal cavity or oral cavity. Here the roti and dal is chewed finely changing the food to semisolid paste and mixed with saliva. This makes the food easy to swallow and saliva has salivary amylase which helps in digestion of starch to Maltose.

Starch Maltose

ii. The oral cavity then leads into short pharynx followed by oesophagus and stomach. The semisolid food with salivary amylase passes through this path without any reaction and reaches the stomach.

iii. Next in stomach food is further churned into finer paste. The stomach has HCl in it. The churned food is mixed with the HCl. Mixing of HCl kills the germs left in the food. HCl also make the pH of food too acidic in nature so that pepsin can act optimally.

The roti has carbohydrates and fibres in it and dal has protein and little fat in it.

In stomach partial digestion of protein takes place. The following reaction takes place in stomach.

iv. Stomach is followed by small intestine. The pancreas releases pancreatic juice on the way. The pancreatic juice contains pancreatic amylase.

The pancreatic amylase converts polysaccharides into disaccharides. This is processing of carbohydrates present in the roti, i.e.

Polysaccharides (starch) Disaccharides

Pancreatic juice also contains Chymotrypsin which helps in digestion of peptones and proteoses, i.e.

Lipase helps in digestion of fats as follows:

FatsDiglycerides Monoglycerides

Intestinal juice contains various enzymes which help in digestion of all the nutrients.

After all the nutrients are converted into simple substances, these are absorbed by small intestine walls.

The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine and finally to rectum.

Question 2:

What are the various enzymatic types of glandular secretions in our gut helping digestion of food? What is the nature of end products obtained after complete digestion of food?


The various enzymatic types of glandular secretions in our gut helping digestion of food are as follows:

- Secretion from gastric glands: gastric juice is secreted in stomach which contains HCl and Pepsin enzyme. Gastric juice contains hydrochloric acid and Proenzyme - pepsinogen and prorennin. Maintains a strongly acidic pH which converts these proenzymes into pepsin. Rennin is secreted in infants that helps in digestion of milk.

- Secretions in Liver: here bile juice is secreted. Bile helps in emulsification of fats. Bile also provides alkaline medium which is useful for working of enzymes present in the small intestine.

- Secretions from pancreas: here pancreatic juice is secreted. The pancreatic juice contains inactive enzymes Trypsinogen, Chymotrypsinogen, Procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by an enzyme, Enterokinase, (secreted by the intestinal mucosa) into active trypsin, which in turn activates the other enzymes in the pancreatic juice. Amylase digests carbohydrates and trypsin/chymotrypsin digests protein.

Pancreatic juice and bile are released through the hepato-pancreatic duct. The bile released into the duodenum contains bile pigments (bilirubin and billiverdin), bile salts, cholesterol and phospholipids but no enzymes. Bile also activates lipases.

- Secretions from small intestine: The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or success entericus. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities.

The nature of end products obtained after complete digestion of food is as follows

Dipeptides Amino acids

Maltose Glucose + Glucose

Lactose Glucose Fructose

Nucleotides Nucleosides Sugars + bases

Di and monoglycerides Fatty acids Glycerols