Describe the process of digestion in digestive system.

Explanation

Digestion is the process of convertion of complex non-diffusible food substances into simple diffusible forms. The process of digestion is accomplished by mechanical and biochemical process.
Bigestion in the buccal cavity : Buccal cavity performs two major functions, mastication of food and facilitation of swallowing. Teeth and tongue with the help of saliva masticate and mix up the food thoroughly. Mucus in saliva helps in lubricating and adhering the masticated food particles into bolus. The saliva secreted into oral cavity contains electrolytes such as `Na^(+), K^(+), Cr, HCO_(3)^(-)` and enzymes like salivary amylase (ptyalin) and lysozyme. Carbohydrates digestion starts in the buccal cavity, about 30% of starch is hydrolysed here into a disaccharide called maltose by the enzyme amylase (ptyaline). Lysozyme acts as antibacterial agent that prevents infections.
II. Digestion in the stomach : As the bolus enters into stomach starch digestion stops and protein digestion begins. The food entered into stomach is mixed thoroughly with gastric juice of the stomach by the churning movement of its muscular wall and the product is called chyme. The mucus and bicarbonates present in the gastric juice act as lubricant and protect the mucosal epithelium from HCI. HCL in the stomach provides the acidic pH (1.8)
which is optimal for the action of pepsin. The proenzymes of gastric juice, the pepsinogen and prorennin on exposure to HCl are converted into the active enzymes, pepsin and rennin respectively. Pepsin converts proteins into proteoses and peptones. Rennin found in gastric juice of infants. It acts on the milk protein, the casein in the presence of calcium ions convert into calcium paracaseinate and proteoses. Pepsin acts on paracaseinate and convert it into peptones. The entire process of protein digestion in stomach takes about 4 hours.
Prorennin (inactive)`overset( HCl)to` Rennin (active)
Pepsinogen (inactive) `overset(HCl)to` Pepsin (active)
Casein `underset(Ca ^(+2)) oveerset("Rennin")to` Calcium paracaseinate:
Calciumparacaseinate `overset("Pepsin")to ` Peptones
Proteins`overset("Pepsin")to` Proteoses +Peptones
III. Digestion in the small intestine : Various types of movements are generated by the muscular external layer of small intestine. These movements help in thorough mixing of the food with bile, pancreatic juice and intestinal juice in the intestine and thereby facilitate digestion. The duodenal cells of the proximal part produces large amount of bicarbonates Ito completely neutralize anv gastric acid that passes further down into the digestive tract.
i) Digestion of proteins : Pancreatic juice contains protein hydrolysing enzymes like trypsinogen, chymotrypsin and procarboxy peptidases, but they are inactive enzymes.
Trypsinogen is activated by the enzyme enterokinase secreted by the intestinal mucosa into active trypsin which inturn activate the other enzymes in the pancreatic juice. Trypsin itself can similarly activate trypsinogen into trypsin.
Chymotrypsin, trypsin and carboxy peptidase of pancreatic juice act upon proteins, proteoses and peptones in the chyme, result in the formation of tri and dipeptides which inturn hydrolysed into amino acids by the action of tri and dipeptidases.
`}{:("Proteins"),("Proteoses"),("Peptones"):}pverset("Trypsin/Xhymotrypsin")underset("Carboxy peptidase") to ` Tripeptides + Dipeptides
Tripeptides `overset("Tripetidase") to `Dipeptides + Aminoacids
Dipeptides `overset("Dipeptidase")to` Aminoacids
ii) Digestion of fats : Bile salts of bile help in the emulsification of fat i.e., breakdown of fats into very small micelles. Bile also activates lipases of pancreatic juice (steapsin) and intestinal lipases. These lipases act on emulsified fats and convert them into fatty acids and glycerols.
Fats `overset("Bile salts") to ` Emulsified fats
Emulsified fats `overset(" Lipuses")` Diglycerides `overset("Lipases")` Monoglycerides
Monoglycerides `overset("Lipases")to` Fatty acids + Glycerol
iii) Digestion of Carbohydrates : Carbohydrates in the chyme are hydrolysed by the pancreatic amylase into disaccharides. Intestinal disaccharidases act on the disaccharides and convert them into monosacharides.
Carbohydrates `overset("Amylase")` Maltose, Sucrose, Lactose
Maltose `overset("Maltase")` Glucose + Glucose
Sucrose `overset("Invertase")to` Glucose + Fructose
Lactose `overset("Lactase")` Glucose + Galactose
iv) Digestion of nucleic acids : Nucleases of the pancreatic juice act on the nucleic acids to form nucleotides and nucleosides. Nucleotidases and nucleosidases of the intestinal juice convert the nucleotides and nucleosides into pentose sugar and nitrogen bases.
Nucleic acids `overset("Nucleases")to` Nucleotides (DNA, RNA)
Nucleotides `overset("Nucleotidase")to` Nucleosides + Phosphates
Nucleosides `overset("Nucleosides ")to` Sugars + Nitrogen bases
The end products of digestion pass through the intestinal mucosa into blood (or) lymph is carriedout by passive, active (or) facilitated transport mechanisms.