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Mechanism of digestion in Small Intestine

Chyme entering the small intestine contains partially digested carbohydrates (maltose), proteins (proteoses and peptides), and lipids (monoglycerides).

  • Completion of the digestion of carbohydrates, proteins, and lipids is a collective effort of pancreatic juice, bile juice, and intestinal juice in the small intestine.
  • Movements of muscularis layer of the small intestine help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.

Pancreatic juice is a clear, colorless liquid, consists of water, some salts, sodium bicarbonate, and several enzymes.

  • Slightly alkaline pH (7.1–8.2) that buffers acidic gastric juice in chyme
  • Sodium bicarbonate stops the action of pepsin from the stomach, and creates the proper pH for the action of digestive enzymes in the small intestine.
  • Enzymes in pancreatic juice include Pancreatic amylase, Trypsin, Chymotrypsin, Carboxypeptidase, Elastase, Pancreatic lipase, Nucleases (Ribonuclease and Deoxyribonuclease).
  • Pancreatic amylase is starch-digesting enzyme.
  • Trypsin, Chymotrypsin, Carboxypeptidase and  Elastase are protein digesting enzymes that digest proteins into peptides.
  • Pancreatic lipase is principal triglyceride–digesting enzyme in adults.
  • Ribonuclease and deoxyribonuclease digest ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) into nucleotides, respectively.
  • Protein-digesting enzymes of the pancreas are produced in an inactive form as Trypsinogen, Chymotrypsinogen, Procarboxypeptidases, and Proelastase, just as pepsin so that enzymes do not digest cells of the pancreas itself.
  • Trypsinogen is activated by an enzyme, Enterokinase, secreted by the intestinal mucosa into active Trypsin.
  • Now, trypsin converts chymotrypsinogen, procarboxypeptidase, and proelastase into chymotrypsin, carboxypeptidase, and elastase, respectively.

Bile juice: Bile released into the duodenum contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.

Intestinal juice:  Clear yellow fluid, contains water, enzymes and mucus and is slightly alkaline (pH 7.8), due to high concentration of bicarbonate ions (HCO3).

  • Intestinal juice is also called as Succus entericus.
  • Absorptive cells of the small intestine synthesize several digestive enzymes, called brush-border enzymes.
  • Brush-border enzymes are four Carbohydrate-digesting enzymes called α-dextrinase, maltase, sucrase, and lactase; Protein-digesting enzymes called peptidases (aminopeptidase and dipeptidase)lipases and two types of nucleotide-digesting enzymes, nucleosidases and phosphatases.
  • Together, pancreatic and intestinal juices provide a liquid medium that aids the digestion and absorption of substances from chyme in the small intestine.
  • Mucus along-with bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide alkaline medium (pH 7.8) for enzymatic activities.
  • Firstly, pancreatic enzymes act on chyme then intestinal enzymes to make simple absorb-able forms.

Digestion of Carbohydrates

  • Starch (left over by action of salivary amylase) cleaved by pancreatic amylase in S.I.
  • Pancreatic amylase acts on both glycogen and starch, but has no effect on cellulose. (Indigestible plant fiber that is commonly referred to as “roughage”). Pancreatic amylase converts starch into disaccharides.

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  • α-dextrinase acts α -dextrins, clipping off one glucose unit at a time.
  • Sucrase breaks sucrose into Glucose + Fructose.
  • Lactase digests lactose into Glucose + Galactose.
  • Maltase splits maltose and maltotriose into two or three molecules of glucose, respectively.
  • These monosaccharides are ready for absorption now.

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Digestion of Proteins

  • Protein digestion starts in the stomach, where proteins are fragmented into peptides by the action of pepsin. Enzymes in pancreatic juice—trypsin, chymotrypsin, carboxypeptidase, and elastase continue to break down proteins into peptides.

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  • Trypsin, chymotrypsin, and elastase all cleave the peptide bond between a specific amino acid and
    its neighbor while carboxypeptidase splits off the amino acid at the carboxyl end of a peptide.
  • Protein digestion is completed by two peptidases in the brush border.
  • Aminopeptidase cleaves off the amino acid at the amino end of a peptide.
  • Dipeptidase splits dipeptides (two amino acids joined by a peptide bond) into single amino acids.

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Digestion of Lipids

  • Emulsification is a process in which the large lipid globule is broken down into several small lipid globules.
  • Bile salts help in emulsification of fats. Bile salts are amphipathic in nature, means they carry both hydrophobic and hydrophilic group. Hydrophobic groups of bile salt interact with large lipid globule while hydrophilic group interact with watery intestinal chyme. This results in tension and break down of large lipid molecule into many small lipid molecules of diameter about 1µm.
  • Small lipid molecules formed from emulsification provide a large surface area that allows pancreatic lipase to function more effectively
  • Fats are broken down by pancreatic lipase with the help of bile into di-and monoglycerides.

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Digestion of Nucleic Acids

  • Two nucleases of pancreatic juice: Ribonuclease and deoxyribonuclease acts on nucleic acids to form nucleotides and nucleosides.

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  • Now intestinal enzymes: Nucleosidases and phosphatases further act on nucleosides to form sugar (pentoses), phosphates, and nitrogenous bases.
  • These products are absorbed via active transport.
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