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Biochemistry of Gastrointestinal tract

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Presentation on theme: "Biochemistry of Gastrointestinal tract"— Presentation transcript:


2 Biochemistry of Gastrointestinal tract

3 What is Food Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is ingested by an organism and assimilated by the organism's cells in an effort to produce energy, maintain life, or stimulate growth.

4 Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones. Proteins are hydrolyzed to Amino-acids Polysaccharides into mono-saccharaides / hexoses like glucose/fructose/galactose

5 Lipids into fatty acids, mono and diglycerides and even glycerol.
Is brought by GI juices like saliva, gastric, pancreatic, intestinal juices and the bile. GI also produce hormones regulating activity of secretion of juices. Gastrin, cholecystokinin, secretin, glicentin, pancreatic polypeptides,

6 Digestion may be Mechanical Chewing : mouth teeth Churrning : stomach Segmentation: intstines Chemical By various secretions/ enzymes present in various part of GIT.

7 Absorption: normal assimilation by the tissues of the products of digestion. Absorption starts in the proximal portion of the small intestine(jejunum): but Vitamin B12 and bile salts are absorbed in the distal part of the small intestine (ileum). Water , uncombined vitamins and minerals do not go undergo digestion and are absorbed as such.

8 Purpose of digestion/absorption
under nutrition i- Impaired growth ii-Defective immune mechanism iii-Reduced work capacity Over nutrition i- Obesity ii-CV diseases iii-Cancers





13 Saliva



16 Spitting : eject saliva forcibly from one's mouth, sometimes as a gesture of contempt or anger.
The act of expectorating.

17 Other oral cavity small glands (5%)
Secretions of three pairs of glands and provide saliva Parotid (20%) Submaxillary (70%) Sublingual (5%) Other oral cavity small glands (5%)

18 Remaining 5% comes from multiple glands in oral cavity
20% 5% 70%


20 Ebner's glands, also Von Ebner's glands are exocrine glands found in the mouth. More specifically, they are serous salivary glands which reside adjacent to the moats surrounding the circumvallate papillae in the posterior one-third of the tongue, anterior to the terminal sulcus They secrete lingual lipase


22 Histological types Serous glands Mucus glands Mixed serous and mucus glands secretion varies according to gland Parotid: watery, rich in amylase Sublingual: viscous, rich in mucin and glycoprotein Submandibular : mixed gland serous and mucous glands

23 Glands are acini lined by serous or mucus cells.
Acini pour secretion via intercellular canalculi into intercalated ducts which drain into striated duct ending into excretory ducts opening into oral cavity



26 Composition : varies at different time Water 99.5% Inorganic constituents : 1/3 of solids –Na+ ,K+, Ca++, Mg++ , Cl- ,HCO3- , sulfate , phosphate, nitrate, iodide and thiocynate (smoker). at normal flow rate Na+,Cl- concentration is less , more K+

27 With increased secretion rate :
K+ falls, Na+ Cl-, HCO3 increased greatly Na+ absorption is carried out through Na+ K+ ATPase pump. Excess of aldosterone decreases absorption of Na, Cl- but reverse for K+

28 Volume: at rest 20 ml /hour, increased at meal time 1000-1500 ml/day
Specific gravity: (hypotonic) , osmolality increased in dehydration Reaction : resting glands- acidic 6.4 – 6.9 , pH determined by HCO3 content. In mouth ,it loses CO2 and becomes alkaline,

29 Salivary amylase(ptylin) Lingual lipase (Ebner,s glands)
Organic constituents: Salivary amylase(ptylin) Lingual lipase (Ebner,s glands) mucins (mucoproteins) Small amount of lactic acid, ascorbic acid, choline, phenol, urea, cholesterol, Blood group substances (secretors) Kallikerin in saliva acts on kininogen to liberate bradykinin (vasodilator) Ig A (defense factor)

30 Salivary Amylase (ptyalin)
pH is around neutrality(6.9) it is α-amylase (endoamylase) Action of this enzyme only lasts for a short time. Hydrolyses polysaccharides at α(1—4) linkages not 1—6 linkages, terminal α (1—4) and 1—4 α linkages next to branching points. Activity of amylase is increased by Cl-

31 Ptylin has no β-(1—4) glucosidase and hence cellulose is not digested in man and passes as such.
End products are: oligosacharides, disach (maltose) trisach (maltotriose), some larger polymers with 1—4 linkages. glucose formation is almost negligible.

32 Resultant product is limit dextrins (short branched and un-branched oilgosaccharides)
Saliva moistens the food and even dissolve it partially. Teeth and tongue helps in chewing (mechanical digestion) and moistened and masticated food is converted into semi-sold mass “bolus”




36 alkaline favor tartar deposition in gums (CaCO3 or calcium phosphate)

37 Salivary and gastric LIPASES are acid lipases (pH 4-6).
TAGs of short and medium chain length (as in milk fat) are the primary target. Thus this salivary/ gastric lipase play a role in neonatal lipid digestion, which is primary source of energy. They(both salivary and gastric lipases) are important in disease of pancreas like cystic fibrosis when strong pancreatic lipase is not available

38 Lipases act on position 3 of TAG liberating 1,2-diglycerol(DAG) and free FA.
Lingual lipase is active in stomach and brings about 30% hydrolysis.


40 Functions of Saliva: 1. Lubrication /moistening: due to content of mucin-food moistened, chewing facilitate mixing of mucin and swallowing is made easy. 2. Solvent action:- by dissolution of food it stimulate taste buds, increases appetite , and further saliva secretion

41 3.Cleansing action: Continuous flow of saliva maintains good oral hygeine, by inhibiting bacterial growth. Lysozyme enzyme present is natural anti-bactrial agent. 4. Secretary functions: saliva contains haptocorrin (glyco-protein) aids vitamin B12 absorption.

42 5. Digestive function: salivary amylase and lingual lipase present in saliva exert digestive function. Catalyzes hydrolysis of polysaccharides (α-1-4 linkages) (not of 1-6 linkages). End products oligosaccharides, disaccharides (maltose) tri-saccharides (maltotriose). Glucose formation negligible.

43 7.Regulation of water metabolism:
6. Excretory function: antibiotics, metals-(Pb,Hg),drugs (iodides), alkaloids(morphine) viruses(polio-rabies-hepatitis-HIV-mumps), urea in chronic cases 7.Regulation of water metabolism: in dehydration secretion is decreased-dryness of mouth –produce sensation of thirst and forcing to drink water.

44 8. Esophageal clearing: 9. Deiodination : of thyroid hormones and take part in regulation of hormone.

45 Salivary glands supplied by
Parasympathetic cholinergic : primarily concerned with secretion of saliva, stimulation result vasodilatation and secretion by acinar cells (acetylcholine-blocked by atropine), vasodilatation is due to bradykinin and VIP. Vasoactive intestinal peptides

46 Sympathetic adrenergic: when stimulated ,vasoconstriction, contraction of myo-epithelial cells secretion is quite small amount of saliva

47 Regulation of saliva: Spontaneous secretion-resting : continuous without any factor (continuous release of acetylcholine), keep mouth-pharynx moist Unconditioned reflexes: material placed in mouth, movement of tongue, neural mechano-chemical receptors stimulated by dryness of mucosa ,chewing, chemical in food

48 Reflexes stimulate salivation center in medulla, near to it lie respiratory-vomiting centers.
Conditioned reflexes : occur in empty mouth impulses arise in special senses organs like sight and smell, salivation center present in brain nausea/vomiting associated with salivation.


50 Xerostomia  (also termed dry mouth as a symptom or dry mouth syndrome as a syndrome) is dryness in the mouth (xero- +stom- + -ia), which may be associated with a change in the composition of saliva, or reduced salivary flow (hyposalivation), or have no identifiable cause.

51 Causes: 1. Medication-older person use excess medicine for various diseases 2. Mouth breathing 3. Dehydration 4. Radiation to salivary glands 5. Psychogenic


53 Chewing is the most efficient way to stimulate salivary flow by causing muscles to compress the salivary glands and release saliva. Through internal feedback mechanisms the taste and consistency of food also affect the quantity of saliva produced. It is difficult to exhaust the salivary glands, so chewing throughout the day to maintain stimulated salivary flow is possible.

54 Measure of Saliva normal resting state stimulated state

55 The person chews a piece of paraffin until it becomes soft
The person chews a piece of paraffin until it becomes soft. Before the collection is started the first portion of saliva is swallowed.   Start timer and the chewing is continued for another 5 minutes (for subjects with high secretion rate, 3 minutes may be enough).

56 The person sits in an upright position with his head inclined forward so that the production of saliva is collected in the floor of the mouth and then flows out over the lip. Saliva formed is let to drip into the graduated test tube or a measure-cup for 15 minutes. The result of this collection is expressed as milliliters per minute.

57 The saliva is spat out at short intervals in a graduated test tube or a measure-cup during the collection period. The collected saliva is then measured. The measurement should not include the foam which is formed during the collection. The result is expressed as mililiters per minute.


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