Question

Explain the digestion of carbohydrates and proteins in the alimentary canal.

Answer 1

Alimentary canal begins with an opening at the anterior side (mouth) and posteriorly opens out through the anus. Biomolecules in the food cannot be utilized by our body in its original sorm. It should be broken down and converted into simple substances in the digestive system.

Complete answer:
Before discussing digestion of carbohydrates and proteins, we should explain the definition of digestion.
Digestion is the process of conversion of complex food substances into simple absorbable forms and is carried out through the digestive system by mechanical and biochemical methods.

The mouth in the alimentary canal leads to the buccal cavity, which leads to the pharynx. Pharynx is the common passage for food and air. From pharynx, oesophagus passing through our neck to the stomach. Stomach opens to the Small intestine. Small intestine is divided into duodenum, jejunum and ileum. Ileum opens to the large intestine, which is divided into caecum, colon and rectum. Rectum opens to the anus.

Buccal cavity performs two functions, mastication of food and swallowing. The teeth and tongue with help of saliva thoroughly masticate and mix up the food. After this process, food is known as bolus. Bolus then passed into the pharynx and oesophagus by swallowing and deglutition. Bolus then passed down by successive waves of muscular contraction known as peristalsis. The saliva secreted into the oral cavity contains $N{a^ + }$, ${K^ + }$, $C{l^ - }$, $HC{O^{3 - }}$ and enzymes salivary amylase and lysozyme. The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of salivary amylase. Salivary amylase hydrolysed about 30 percent of the starch into maltose, which is a disaccharide. Lysozyme prevents infection through food, which is an antibacterial agent.

$starch\xrightarrow[{pH6.8}]{{s.amylase}}maltose$
For 4-5 hours, food is stored in the stomach. Food in the stomach is called chyme. It mixes thoroughly with gastric juice of the stomach by churning movements of muscular walls. The pro-enzyme pepsinogen present in the stomach is converted to active enzyme pepsin, which convert proteins into proteoses and peptones. Acidic pH 1.8 of HCl is optimal for pepsin. In infants, milk protein is digested by proteolytic enzyme rennin found in gastric juice.

Muscularis layer of the small intestine generates various movements. These movements mix up the food with different secretions and facilitate digestion. The pancreatic juice secreted into the small intestine. Pancreatic juice contains trypsinogen, chymotrypsinogen, procarboxypeptidase, amylases, lipases and nucleases; they are inactive enzymes. Trypsinogen is activated by enterokinase secreted by intestinal mucosa into active trypsin, which in turn activates other enzymes of pancreatic juice. Goblet cells present in intestinal mucosal epithelium secrete mucus. Secretions of goblet cells and secretions of brush border cells of mucosa constitute succus entericus or intestinal juice. Disaccharidases,
dipeptidase, lipases, nucleosidases etc. are present in succus entericus.

Proteins, proteoses and peptones are fully digested by pancreatic enzymes of pancreatic juice:

$proteins \\\ peptones\xrightarrow[{carboxypeptidase}]{{tryp\sin /chymotryp\sin }}dipeptides \\\ proteoses \\\$

Carbohydrates present in the chyme are hydrolysed by pancreatic amylase into disaccharides:
$Polysaccharides\xrightarrow{{Amylase}}Disaccharides$
The enzymes in the succus entericus act on the end products to form simple molecules:

$Dipeptides\xrightarrow{{Dipeptidases}}a\min oacids$
$Maltose\xrightarrow{{Maltase}}Glu\cos e + Glu\cos e$
$Lactose\xrightarrow{{Lactase}}Glu\cos e + Galactose$
$Sucrose\xrightarrow{{Sucrase}}Glu\cos e + Fructose$
Here, all the undigested or partially digested carbohydrates and proteins present in the chyme are completely digested by the enzymes in the intestine.

Note: The mucus and bicarbonates present in the gastric juice help in the lubrication and protection of mucosal epithelium from destruction by concentrated $HCl$. Gastric glands present in stomach mucosa are of three types, namely mucous neck cells, peptic or chief cell, parietal or oxyntic cell.