Class 11-science NCERT Solutions Biology Chapter 16 - Digestion And Absorption

(a) (i) Pepsin, lipase and rennin

(b) (ii) Intestinal juice

(a) - (ii), (b) - (iv), (c) - (iii), (d) - (i)

(a)  The major portion of absorption takes place in the small intestines. In the stomach, simple sugars, water and alcohol are absorbed. Villi are specialised projections in the small intestine which have a large surface area for absorption. Food is fit to be absorbed only after complete digestion, so the function of villi begins after all the digestive processes are complete.

(b)  The proenzyme pepsinogen, on exposure to hydrochloric acid, gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach. Pepsin converts proteins into proteoses and peptones (peptides).

Pepsinogen Pepsin + Protein Proteose + Peptone

(c)  The alimentary canal is composed of the following basic layers:

 i. Serosa: It is the outermost layer and is made of a thin mesothelium with some connective tissues.

 ii. Muscularis: It is formed by smooth muscles usually arranged into an inner circular and outer longitudinal layer.

 iii. Sub-mucosa: It is formed of loose connective tissues containing nerves, blood and lymph vessels.

 iv. Mucosa: It forms irregular folds in the stomach and small finger-like folding called villi in the small intestine.

(d) The bile released into the duodenum contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e. breaking down of fats into small micelles. It also activates lipases which carry out the digestion of fats.

Pancreatic juice contains inactive enzymes - trypsinogen, chymotrypsinogen and procarboxypeptidases. Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn, activates the other enzymes in pancreatic juice.

Chymotrypsinogen and procarboxypeptidases get converted into chymotrypsin and carboxypeptidase in the presence of trypsin. Chymotrypsin converts the proteins into peptides, and carboxypeptidase further converts peptides into smaller peptide chains and amino acids. 

The digestion of protein begins in the stomach and is completed in the small intestine. The digestive juice secreted by the digestive glands is called gastric juice which consists of hydrochloric acid, pepsinogen, mucus and rennin. The proenzyme pepsinogen, on exposure to hydrochloric acid, gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach. Pepsin converts proteins into proteases and peptones (peptides).

An adult human has 32 teeth which are of four different types - incisors (I), canine (C), premolars (PM) and molars (M). The arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM and M is represented by a dental formula which in human is as follows:

Bile emulsifies the fats. Bile juice also helps in activation of lipase, the enzyme which digests fat. Additionally, bile converts the acidic chyme to alkaline so that other enzymes can work on food.

Chymotrypsin digests proteins and converts it into dipeptides. Other digestive enzymes of the same category are trypsin and carboxypeptidase, and they perform a similar function in the digestion of proteins.

Digestion of carbohydrates takes place in the mouth and the small intestine. The enzymes carbohydrases act on carbohydrates. As the food enters the mouth, it gets mixed with saliva and breaks down starch into maltose, isomaltose and limit dextrins.

The digestion of carbohydrates does not take place in the stomach.

In the small intestine, the food gets mixed with pancreatic juice and intestinal juice. Pancreatic juice contains pancreatic amylase which digests polysaccharides into disaccharides.

Intestinal juice also contains a variety of enzymes which digest the disaccharides into glucose, fructose and galactose. 

HCl converts pepsinogen (proenzyme) and prorennin (proenzyme) into pepsin and rennin. It provides the acidic pH (pH 1.8) optimal for pepsin. Rennin is a proteolytic enzyme found in the gastric juice of infants which helps in the digestion of milk proteins. So, without HCl, infants would not be able to digest milk proteins.

HCl is also necessary to kill harmful bacteria which may be present in the food. In the absence of HCl, harmful bacteria will not be killed and may cause diseases.

Butter is a kind of fat which is broken down by lipases present in pancreatic juice and bile into di- and monoglycerides.

Fat is absorbed by villi in the small intestine.

The digestion of proteins begins in the stomach and is completed in the small intestine. The enzymes which act on proteins are called proteases. In the stomach, the proenzyme pepsinogen gets converted into pepsin which acts on protein to convert it into peptone.

The pancreatic juice contains inactive enzymes - trypsinogen, chymotrypsinogen and procarboxypeptidases in the inactive form. 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. Chymotrypsinogen and procarboxypeptidases get converted into chymotrypsin and carboxypeptidase in the presence of trypsin. Chymotrypsin converts the proteins into peptides, and carboxypeptidase further converts peptides into smaller peptide chains and amino acids.

In human beings, each tooth is embedded in a socket of the jaw bone. This type of attachment is called thecodont.

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 has 32 permanent teeth which are of four different types

(Heterodont dentition):

Functions of liver:

 i. The liver produces bile which helps in the digestion of fats.

 ii. The liver separates the excess of sugar from the blood and stores it in its cells as glycogen. This process is called glycogenesis and is aided by the pancreatic hormone called insulin.

 iii. It secretes an anticoagulant called heparin.

 iv. It produces a protein, angiotensinogen, which helps the kidneys in maintaining body fluid osmoregulation.

 v. It is an important seat of lymph formation.

 vi. It produces red blood cells in the embryo.