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Lipids digestion and absorption
This document discusses the digestion and absorption of lipids. It notes that lipids are hydrophobic and insoluble in water. It describes how saliva, pancreatic juice, and bile all play roles in emulsifying and breaking down lipids through various lipases. This allows lipids to be broken into smaller components like fatty acids and monoglycerides that can be absorbed. It explains that absorbed lipids are recombined into triglycerides and phospholipids within mucosal cells and transported via chylomicrons to tissues.
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Lipids digestion and absorption
- 1. Lipids Digestion and Absorption R.C. Gupta Professor and Head Department of Biochemistry National Institute of Medical Sciences Jaipur, India
- 2. Lipids are chemicallya heterogeneous group of compounds Their common property is insolubility in water and solubility in organic solvents Secondly, they are actually or potentially related to fatty acids
- 3. Unlike carbohydrates andproteins, lipids can be stored in large quantities Stored lipids constitute an important reservoir of energy About four-fifths of the stored energy is in the form of lipids
- 4. The major storageform of lipids is triglycerides (triacylglycerols) Triglycerides are stored in specialized cells called adipocytes A tissue rich in adipocytes is known as adipose tissue
- 5. The other lipidspresent in the body are: Phospholipids Glycolipids Cholesterol Free fatty acids Fat-soluble vitamins
- 6. All the lipidsexcept vitamins can be synthesized in the body However, all these lipids are present in diet also The dietary lipids have to be digested and absorbed
- 7. Major dietary lipidsare triglycerides, phospholipids and esterified cholesterol These have to be hydrolysed before their absorption from the alimentary tract Digestion and absorption
- 8. Bile also playsan important role in digestion and absorption of lipids Saliva Digestive juices which participate in digestion of lipids are: Succus entericus Pancreatic juice
- 9. Saliva contains alipase which is secreted by the dorsal surface of tongue Food stays in mouth for a very short period But salivary lipase remains active even in the acidic environment of stomach Salivary lipase converts triglycerides into diglycerides and free fatty acids
- 10. Salivary lipase acts preferentiallyon: Triglycerides containing short-chain fatty acids The ester bond at position 3
- 11. CH2‒ O ‒C ‒R1 CH ‒ O ‒ C ‒ R2 CH2‒ O ‒ C ‒R3 Salivary lipase O II O II O II
- 12. Upon reaching smallintestine, food comes in contact with pancreatic juice Pancreatic juice contains a lipase which is far more powerful than salivary lipase However, the hydrophobic nature of lipids poses a problem
- 13. Being hydrophobic, lipidsare insoluble in the aqueous environment of the gut They are present in the gut as large globules Pancreatic lipase can act only at the surface of fat globules
- 14. For efficient digestion,the lipids need to be emulsified Emulsification is breaking up of fat globules into small droplets Bile salts, present in bile, play a key role in emulsification
- 15. Bile salts arepowerful emulsifying agents They have both hydrophilic and hydro- phobic domains The hydrophobic portion dips into the fat globule The hydrophilic domain attracts water
- 16. OH OHHO Sodium glycocholate (bilesalt) II O C−NH−CH2−COONa Hydrophilic portion Hydrophobic portion
- 17. Peristaltic movement ofthe gut breaks up fat globules into small droplets Presence of bile salts on their surface prevents their re-association Phospholipids which are amphipathic molecules also help in emulsification
- 18. Emulsification greatly increasesthe surface area for pancreatic lipase to act on Pancreatic lipase requires co-lipase to become active Co-lipase is a protein present in pancreatic juice
- 19. Pancreatic lipase caneasily hydrolyse the ester bonds at positions 1 and 3 The ester bond at position 2 is resistant to the action of pancreatic lipase Pancreatic lipase converts triglycerides into 2-monoacylglycerol and fatty acids
- 20. CH2‒ O ‒C ‒R1 CH ‒ O ‒ C ‒ R2 CH2‒ O ‒ C ‒R3 O II O II O II CH2‒ OH CH ‒ O ‒ C ‒ R2 CH2‒ OH 2-Monoacylglycerol O II R1‒COOH + R3‒COOH 2 H2O Pancreatic lipase Triacylglycerol
- 21. Pancreatic juice alsocontains phospho- lipase A2 and cholesterol esterase Phospholipase A2 hydrolyses the ester bond at position 2 of glycerophospholipids Cholesterol esterase hydrolyses the ester bond of esterified cholesterol
- 22. Phospholipase A2 CH2‒O‒C‒R1 CH‒O‒C‒R2 CH2‒O‒‒Choline CH2‒O‒C‒R1 CH‒OH CH2‒O‒‒Choline H2O R2‒COOH Hydrolysis ofa glycerophospholipid O II O II O II
- 23. Cholesterol esterase H2O R‒COOH HO Hydrolysis ofesterified cholesterol R‒C‒O O II
- 24. Succus entericus containsa powerful phospholipase This causes complete hydrolysis of the phospholipids Glycerol, fatty acids, phosphate and inositol/nitrogenous bases are liberated
- 25. Thus, the majorproducts of digestion of lipids are: Glycerol Cholesterol Fatty acids Monoglycerides
- 26. Emulsification also helpsin their absorption Digested lipids are passively absorbed into the mucosal cells of the small intestine Being hydrophobic, they can easily pass through the lipid bilayer of cell membrane
- 27.
- 28. The products ofdigestion are re-combined in the mucosal cells Re-combination produces triglycerides, esterified cholesterol and phospholipids These are combined with a small amount of some proteins (apolipoproteins) The products are chylomicrons, a type of lipoproteins
- 29.
- 30. The chylomicrons enterthe lymphatic vessels (lacteals) They reach the systemic circulation via thoracic duct The lipids present in chylomicrons are taken up by various tissues