Digestion in stomach:
The chief organic molecules in food that are digested in the stomach are the proteins (native) and short chain simple lipids. The cells of the mucosal wall produce the gastric juice and it consists of the following enzymes apart from HCI and mucin.
- Pepsin- proteolytic enzyme(endopeptidase)
- Renin – milk curdling enzyme (Calf and infants of humans)
- Gastric lipase – acts on simple lipids. Pepsin acts on native protein molecules of food in the lumen of the stomach converting it into proteoses, peptones and polypeptides.
protetoses,peptones and polypolypeptides.
In Infants & calf, where the chief source of food is milk, renin is an additional enzyme which helps in curdling of milk protein casein and later this curdled casein is digested by the action of pepsin and converted to proteoses, peptones and polypeptides.
protetoses,peptones and polypolypeptides.
Digestion in small Intestine:
Two types of juices are secreted into the intestinal lumen namely the pancreatic juice and intestinal juice. Both there juices possesses enzymes digesting all types of complex organic molecules namely carbohydrates, proteins, lipids & nucleic acids.
The digestion by these enzyme in the small intestine proceeds as follows:
Carbohydrates:
Polysaccharides starch (plant source) and glycogen (animal source) are acted upon by pancreatic amylase (amylopsin) in the small intestinal lumen and converted to a pool of maltose, a disaccharide.
The disaccharides, in the intestinal lumen namely maltose (obtained from digestion of polysaccharides or direct source); sucrose and Lactose (from diet) are acted upon disaccharide digesting (disaccharases) enzyme into a pool of monosaccharides, glucose, Fructose and galactose.
Proteins:
The proteins reaching the small intestinal lumen are in the form of native proteins, proteoses, peptones and polypeptides. They are acted upon by following enzymes.
Thus resulting in a final pool of absorbable amino acids.
Lipids:
The Lipids are first emulsified by bile salts in the intestinal lumen. The emulsified lipids are then worked upon by the pancreatic lipase to a pool of glycerol, fatty acids and monoglycerides.
Nucleic acids:
Nucleases from the pancreas” acts upon the RNA & DNA nucleic acids breaking them into nucleotides and these nucleotides are later acted upon by nucleotidases & nucleosidases of the intestinal juice.
deoxyribose sugar; ribose sugar; nucleic acid bases.
(ii) The acid chyme released from the stomach into the intestinal lumen may contain digested protein (by pepsin) or native protein (untouched by pepsin if any) apart from carbohydrates and fats. The acid chyme is first neutralised by bicarbonates of pancreatic juice and then acted upon by various enzymes. The proteins in the chyme are acted upon by the proteolytic enzymes of the
- Pancreatic juice – namely trypsin, chymotryr.in and carboxypeptidase.
- ‘Intestinal juice – aminopeptidase, tripeptidase and dipeptidases.
Among these trypsin and chymotrypsin are endopeptidases (attacking internal peptide bonds) acting on native protein and the carboxypeptidases, aminopeptidases, tri and dipeptidases are exopeptidases (attacking superficial peptide bonds at the carbxyl end or amino end respectively) acting on protein fragments produced from whole or native proteins.
The action of the above enzymes on proteins is as follows;
(i) Trypsin and Chymotrypsin act on native proteins splitting or reducing them into shorter protein chains namely proteoses, peptones and polypeptides.
(ii) Carboxypeptidases and amino peptidases act on the protein molecules got from the digestion of native proteins converting them into tripeptides, dipeptides and amino acids.
(iii) The tri and dipeptides resulted from the above reactions are acted upon by tri and dipeptidases and split into amino acids.
Finally, resulting in a pool of amino acids which can be easily absorbed.
