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Dr . Samah Kotb Nasr Eldeen

Cellular Biochemistry and Metabolism (CLS 331). Dr . Samah Kotb Nasr Eldeen. Digestion and Absorption. FOOD. Digestion involves hydrolyzing food molecules into smaller molecules for absorption through the gastrointestinal epithelium.

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Dr . Samah Kotb Nasr Eldeen

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  1. Cellular Biochemistry and Metabolism(CLS 331) Dr. Samah Kotb Nasr Eldeen Dr Samah Kotb

  2. Digestion and Absorption Dr Samah Kotb

  3. FOOD Dr Samah Kotb

  4. Digestion involves hydrolyzing food molecules into smaller molecules for absorption through the gastrointestinal epithelium. • Polysaccharides are absorbed as monosaccharides, triacylglycerols as 2-monoacylglycerols, fatty acids, glycerol and proteins as amino acids. • The diet must provide: • metabolic fuels (carbohydrate and fat) for bodily growth and activity • protein for synthesis of tissue proteins • fiber for roughage • minerals for specific metabolic functions • certain polyunsaturated fatty acids of the n-3 and n-6 families for eicosanoid . • synthesis and other functions • vitamins, organic compounds needed in small amounts for many varied essential functions. Dr Samah Kotb

  5. digestion Dr Samah Kotb

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  8. DIGESTION & ABSORPTION OF CARBOHYDRATES Dr Samah Kotb

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  11. DIGESTION & ABSORPTION OF CARBOHYDRATES • The digestion of complex carbohydrates is by hydrolysis to liberate oligosaccharides, then free mono- and disaccharides. • Amylases Catalyze the hydrolysis of Starch • The hydrolysis of starch by salivary and pancreatic amylases catalyze random hydrolysis of α(1→4) glycoside bonds, yielding dextrins, then a mixture of glucose, maltose, and isomaltose. • Disaccharidases(Brush Border Enzymes) • The disaccharidases—maltase, sucrase-isomaltase, lactase, —are located on the brush border of the intestinal mucosal cells where the resultant monosaccharidesand others arising from the diet are absorbed. Dr Samah Kotb

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  15. Mechanisms for the Absorption of Monosaccharides in the Small Intestine Dr Samah Kotb

  16. Two Separate Mechanisms for the Absorption of Monosaccharides in the Small Intestine • Glucose and galactose are absorbed by a sodium-dependent process. They are carried by the same transport protein sodium-glucose linked transporter (SGLT 1) and compete with each other for intestinal absorption. • Other monosaccharides are absorbed by carrier-mediated diffusion. Because they are not actively transported, fructose and sugar alcohols are only absorbed down their concentration gradient. Dr Samah Kotb

  17. Transport of glucose, fructose, and galactose across the intestinal epithelium. Dr Samah Kotb

  18. The SGLT 1 transporter is coupled to the Na+-K+ pump, allowing glucose and galactose to be transported against their concentration gradients. • The GLUT 5 Na+-independent facilitative transporter allows fructose as well as glucose and galactose to be transported with their concentration gradients. • Exit from the cell for all the sugars is via the GLUT 2 facilitative transporter. Dr Samah Kotb

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  22. DIGESTION & ABSORPTION OF LIPIDS • The major lipids in the diet are triacylglycerols. • Hydrolysis of triacylglycerols is initiated by gastric lipases that attack the ester bond, forming 1,2-diacylglycerols and free fatty acids, aiding emulsification. • Pancreatic lipase is secreted into the small intestine and requires a further pancreatic protein, colipase, for activity. • It is specific for the primary ester links in triacylglycerols resulting in 2-monoacylglycerols and free fatty acids as the major end-products of luminal triacylglycerol digestion. Dr Samah Kotb

  23. Triacylglycerols Stomach gastric lipases 1,2-diacylglycerols + free fatty acids Pancreatic lipase small intestine 2-monoacylglycerols + free fatty acids hydrolysis Secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct . free fatty acids + glycerol Portal vein Dr Samah Kotb

  24. Monoacylglycerols are hydrolyzed with difficulty to glycerol and free fatty acids. • Bile salts, formed in the liver and secreted in the bile, enable emulsification of the products of lipid digestion into micelles and liposomes together with phospholipids and cholesterol from the bile. • Because the micelles are soluble, they allow the products of digestion, including the fat soluble vitamins, to be transported through the aqueous environment of the intestinal lumen and permit close contact with the brush border of the mucosal cells, allowing uptake into the epithelium, mainly of the jejunum. Dr Samah Kotb

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  26. The bile salts pass on to the ileum, where most are absorbed into the enterohepatic circulation. • Within the intestinal epithelium, 1-monoacylglycerols are hydrolyzed to fatty acids and glycerol and 2-monoacylglycerols are re-acylated to triacylglycerols via the monoacylglycerol pathway. Dr Samah Kotb

  27. Glycerol released in the intestinal lumen is not reutilized but passes into the portal vein; glycerol released within the epithelium is reutilized for triacylglycerol synthesis via the normal phosphatidic acid pathway. • All long-chain fatty acids absorbed are converted to triacylglycerol in the mucosal cells and, together with the other products of lipid digestion, secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct . Dr SamahKotb

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  29. Home work • DISCUSS • Mechanisms for the Absorption of Monosaccharides in the Small Intestine??????? • Effect of bile salts in lipid metabolism ?? ????

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  31. Complete the figure A: B: C : D: E: F: G: K: I: H: Dr Samah Kotb

  32. Complete the figure A: B: C : D: E: F: G: Dr Samah Kotb

  33. DIGESTION & ABSORPTION OF PROTEINS Dr Samah Kotb

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  37. DIGESTION & ABSORPTION OF PROTEINS • Enzymes involved in the Digestion of Proteins • Proteases are proteolytic digestive enzymes and are of two types: Endopeptidases and Exopeptidases. • Endopeptidaseshydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, e.g, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas. • Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of polypeptides. Dr Samah Kotb

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  39. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from the free carboxyl terminal. Dr Samah Kotb

  40. Aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. Dr Samah Kotb

  41. Activation of proteases Dr Samah Kotb

  42. The proteases are secreted as inactive zymogens, the active site of the enzyme is masked by a small region of its peptide chain, which is removed by hydrolysis of a specific peptide bond. • For example: • Pepsinogen is activated to pepsin by gastric acid and by activated pepsin. • In the small intestine, trypsinogen, the precursor of trypsin, is activated by enteropeptidase, which is secreted by the duodenal epithelial cells. • Trypsin can then activate chymotrypsinogento chymotrypsin, proelastaseto elastase, procarboxypeptidase to carboxypeptidase, and proaminopeptidaseto aminopeptidase. Dr Samah Kotb

  43. gastric acid pepsin Pepsinogen activated pepsin Enteropeptidase Trypsin Trypsinogen Trypsin Chymotrypsinogen Chymotrypsin Trypsin Elastase Proelastase Trypsin Procarboxypeptidase Carboxypeptidase Trypsin Proaminopeptidase Aminopeptidase Dr Samah Kotb

  44. Mechanisms of Amino Acids Absorption Dr Samah Kotb

  45. Free Amino Acids & Small Peptides Are Absorbed by Different Mechanisms • The end product of the action of endopeptidases and exopeptidases is a mixture of free amino acids, di- and tripeptides, and oligopeptides, all of which are absorbed. • Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are several different amino acid transporters, with specificity for the nature of the amino acid side chain (large or small; neutral, acidic, or basic). Dr Samah Kotb

  46. The various amino acids carried by any one transporter compete with each other for absorption and tissue uptake. • Dipeptides and tripeptides enter the brush border of the intestinal mucosal cells, where they are hydrolyzed to free amino acids, which are then transported into the hepatic portal vein. Dr Samah Kotb

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  48. DIGESTION & ABSORPTION OF VITAMINS & MINERALS Dr Samah Kotb

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  50. DIGESTION & ABSORPTION OF VITAMINS & MINERALS • Vitamins and minerals are released from food during digestion. • The fat-soluble vitamins are absorbed in the lipid micelles that result from fat digestion. • Water-soluble vitamins and most mineral salts are absorbed from the small intestine either by active transport or by carrier-mediated diffusion. Dr Samah Kotb

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