Digestion and Absorption

Role of digestive enzymes and gastrointestinal hormones


  • Digestion of carbohydrate takes place in oral cavity, stomach and small intestine.
  • Digestion of carbohydrate in oral cavity : is done by salivary amylase. It is absent in saliva of harbivorous animals like cows and buffaloes, predatory carnivorous mammals like lions and tigers. But saliva of pigs contain salivary amylase.
    \tt Starch \ \ \underrightarrow{salivary \ amylase}\ \ Maltose \ + Isomaltose \ + \alpha Dextrins
  • Gastric juice also contains small amount of gastric amylase but its action is inhibited by highly acidic medium. This takes place in stomach.
  • Digestion of carbohydrates in small intestine : Brunner's gland secrete large amount of viscous, enzyme free, alkaline and watery mucoid fluid which enables the duodenum to withstand the acidic chyme. Pancreatic amylase converts starch into maltose, isomaltose and α - dextrin.
    \tt Starch \ \xrightarrow[\alpha - amylase]{{Pancreatic}} \ Maltose + \ Isomaltose \ + \alpha - Dextrin
  • Intestinal juice contains maltose, isomaltose, sucrose (invertase) lactose and α-dextrinase which acts as follows :
    \tt Maltose \ \ \underrightarrow{Maltase} \ \ glucose + \ glucose
    \tt Isomaltose \ \ \underrightarrow{Isomaltase} \ \ glucose + \ glucose
    \tt Sucrose \ \ \underrightarrow{Sucrase} \ \ Glucose + \ Fructose
    \tt Lactose \ \ \underrightarrow{Lactase} \ \ glucose + \ galactose
    \tt \alpha - Dextrins \ \ \underrightarrow{\alpha \ Dextrinase} \ \ glucose
  • Lactose intolerance is developed in some person where they produce little or no lactase. In such person, the lactose of milk remains undigested and fermented in the intestine producing gases and acids.
  • Proteases (or) peptidases hydrolase protein. Many of these enzymes are secreted in their inactivated forms called pro enzymes. saliva doesn't contain any protein digesting enzyme, so the digestion of protein doesn't occur in oral cavity.
  • The gastric gland of stomach secrete gastric juice. It contains HCl, proenzymes - pepsinogen and prorennin. Rennin is present in infant's stomach, but it is absent in adults.
  • sometimes chyme is squeezed into oesophagus which causes burning of some cells. That is called as heart burn (∼ pyrosis or hyper acidity). semi digested food with acidic mixture of gastric juice is called chyme.
  • Following reactions are takes place in pronein.
    \tt pepsinogen (proenzyme) \ \underrightarrow{HCl} \ Pepsin
    \tt Protein \ \underrightarrow{pepsin} \ peptones \ and \ proteoses
    \tt Pronennin (pro enzyme) \ \underrightarrow{HCl} \ Rennin
    \tt Casein (milk protein) \ \underrightarrow{rennin} \ paracasein
    \tt Paracasein \ + Ca \rightarrow \ Calcium \ paracaseinate \ (curd)
    \tt Calcium \ paracaseinate \ \underrightarrow{pepsin} \ peptones
  • Digestion of pronein in small intestine : pancreatic juice contains pro enzymes - trypsinogen, chymotrypsinogen, procarboxypeptidase  and elastase.
    \tt Tripsinogne \ (pro enzyme) \ \xrightarrow[of \ intestinal \ juice]{{Enterokinase}} \ Trypsin
    \tt Chymotrypsinogen \ (proenzyme) \ \underrightarrow{Trypsin} \ chymotrypsin
    \tt Procarboxypeptidase \ (proenzyme) \ \underrightarrow{Trypsin} \ Carboxypeptidase
    \tt Pronein \ \underrightarrow{Trypsin} \ Large \ peptides
    \tt Proneins \ \underrightarrow{chymotrypsin} \ Large \ peptides
    \tt peprones \ \underrightarrow{chymotrypsin} \ Dipeptides
    \tt Proteoses \ \underrightarrow{Carboxypeptidases} \ Dipeptides
    \tt Large \ peptides \ \underrightarrow{Carbopeptidases} \ Dipeptides \ + Aminoacid
    \tt Elastin \ \underrightarrow{elastase} \ Dipeptides
    \tt Dipeptides \ \underrightarrow{Dipeptidase} \ Aminoacid
  • Digestion of fats is takes place largely in small intestine. Bile salts of bile breakdown fat droplets into many small ones by reducing the surface tension of fat droplet. This process is called as emulsification.
  • \tt Fat \ (triglyceride) \ \underrightarrow{Bile} \ Emulsified \ fat
    Pancreatic lipase is the principal enzyme for the digestion of fat. Lipase is activated by bile.
    \tt Emulsified \ fat \ \xrightarrow[lipase]{{pancreatic}} + \ Fatty \ acid + \ Diglyceride

    \tt Diglyceride \ \xrightarrow[lipase]{{pancreatic}} + \ Fatty \ acid + \ monoglyceride

    \tt Monoglyceride \ \xrightarrow[lipase]{{pancreatic}} + \ Fatty \ acid + \ Glycerol
  • Nucleic acids are digested in the small intestine. Pancreatic juice contain two nucleases : DNase, RNase.
    \tt DNA \ \underrightarrow{DNase} + Deoxyribonucleotides
    \tt RNA \ \underrightarrow{RNase} + Ribonucleotides
    Intestinal juice contains nucleotidases adn nucleosidases.
    \tt Nucleotides \ \underrightarrow{Nucleotidases} \ + Nucleoside \ + Inorganic phosphorus
    \tt Nucleoside \underrightarrow{Nucleosidases} \ + Nitrogenous \ base \ + pentose \ sugar
  • Gastrin is secreted by gastrin cells (G cells) in the pyloric region of the stomach which stimulates gastric gland to secrete and release the gastric juice. It stimulates gastric mobility and HCl secretion.
  • Enterogatron (Gas Inhibitory peptide - GIP) secreted by K cells of duodenum and jejunum which reduces gastric secretion.
  • secretin (first hormone discovered by scientists) secreted by the epithelium of duodenum which releases bicarbonates in the pancreatic juice, it increases the secretion of bile and decreases gastric secretion and motility.
  • cholecystokinin pancreoxymin (cck - pz) secreted by epithelium of entire small intestine, stimulate the gall bladder to release bile and pancreas to secrete and release digestive enzymes in the pancreatic juice.
  • Duocrinin is secreted by the duodenal epithelium and stimulates the Brunner's gland to release mucus and enzymes into intestinal juice.
  • Enterocrinin is secreted by small intestine which stimulates the crypts of lieberkuhn to release enzymes into the intestinal juice.
  • Vasoactive Intestinal peptide (VIP) is secreted by the epithelium of entire small intestine which dilates peripheral blood vessels of the gut and also inhibits gastric acid secretion.
  • Villikinin is secreted by the epithelium of entire small intestine which accelerates movement of villi. Pancreatic polypeptide (PP) is secreted by the pancreatic polypeptide cells of islets of Langerhans. It inhibits the release of pancreatic juice from the pancreas.
  • Somatostatin secreted by the delta cells of islets of Langerhans of pancreas inhibits the secretion of glucagon by alpha cells and insulin by beta cells. Somatostatin produced by argentaffin cells of gastric and intestinal glands supresses the release of hormones from the digestive tract.

Watch this video for the topic  Role of digestive enzymes and gastrointestinal hormones from 0:46 to 53:05

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