Question

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

Answer 1

Hint: Protein digestion occurs in different organs by different enzymes acting on the polypeptides. They are broken down from complex polypeptides to amino acids through various steps and enzymes.

Complete answer:
Protein digestion begins in the stomach and completes in the small intestine.
The enzymes that act upon and cause the breakdown of proteins are known as proteases.
Digestion in the stomach
The digestive secretions from the gastric glands present on the inner lining of the stomach walls is called gastric juice. The main components of the gastric juice include Hydrochloric acid ($\text{ HCl }$), pepsinogen, and rennin. The food entering the stomach becomes acidic on mixing with the gastric juice.
The acidic pH converts inactive pepsinogen into active pepsin which then converts the proteins into proteases and peptides.
${\ Proteins\xrightarrow { Pepsin } }{ Proteases }+{ Peptides } $
The enzyme rennin in the gastric juice plays an important role in the coagulation of milk.
Digestion in the small intestine
The partly digested food mixed with gastric juices from the stomach which enter the small intestine is known as chyme. The chyme is acted upon by three enzymes in the small intestine- the pancreatic juice, intestinal juice or succus entericus, and bile juice.
The action of pancreatic juice:
Pancreatic juice contains many inactive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases. The enzyme enterokinase that is secreted by the intestinal mucosa activates trypsinogen into trypsin.
${ { Trypsinogen }\xrightarrow { Enterokinase } }{ Trypsin }+{ Inactive\quad peptide }$
The activated trypsin, in turn, activates the other enzymes of the pancreatic juice.
Chymotrypsinogen gets converted to chymotrypsin, which is a proteolytic enzyme that breaks down proteins into peptides.
${ { Chymotrypsinogen }\xrightarrow { Trypsin } }{ Chymotrypsin }$
${ Proteins\xrightarrow { { Chymotrypsin } } }Peptides$
Carboxypeptidases are the enzymes that act on the carboxyl end of the peptide chain and release the last amino acids.
${ Peptides }{ \xrightarrow { { Carboxypeptidases } } }Smaller\quad peptide\quad chain+{ Amino\quad acids }$
The action of intestinal juice:
Intestinal juice contains many different enzymes. Pancreatic amylases digest polysaccharides into disaccharides which are further broken down to monosaccharides.
The proteases in intestinal juice hydrolyze peptides into dipeptides and finally into amino acids.
${ Dipeptides }{ \xrightarrow { Dipeptidases } }{ Peptides }$

Note:
Pancreatic juice also has lipase which breaks down fats into diglycerides and monoglycerides.
The nucleases are the enzymes that break down nucleic acids into nucleotides and nucleosides.
Bile juice that is seen in the intestine enters from the gallbladder and it is necessary for the emulsification of fat.