Biological membrane & its chemical composition

Plasma Membrane

1. BIOLOGICAL MEMBRANE & ITS CHEMICAL COMPOSITION PRADEEP SINGH, MANSI MODI M.Sc. MEDICAL BIOCHEMISTRY HIMSR, JAMIA HAMDARD

2. INTRODUCTION Fundamental unit of life. Self replicating structure. The first cells were observed and named by Robert Hooke in 1665 from slice of cork. Cell membrane is a protective sheath, also known as plasma membrane. The region of cell lying between the plasma membrane and the nucleus is the cytoplasm.

3. CONTENTS Plasma Membrane and its Functions Chemical Composition Of Membranes Lipids Bilayer and Micelles Mobility of Lipid Components In Membrane Fluid Mosaic Model Asymmetry Of L ipid B ilayer Membrane Proteins Fluidity Of L ipid B ilayer Lipid Rafts Dynamic Nature Of Membranes

4. Plasma Membrane Plasma Membrane of a eukaryotic cells is a dynamic, fluid structure It forms the external boundaries of cell. All membranes consist of a double layer of lipid molecules in which proteins are embedded. Thickness of cell membrane varies from 75-110 Å STRUCTURE .

5. FUNCTION’S Protective:- Forms outermost boundary of the cell organelles. Digestive:- Takes in food and excretes waste products. Selective Permeability:- a) Non-Polar Molecules ( gases like O 2,CO 2, N 2, lip ids, steroid hormones, alcohol) can dissolve in the polar region of the membrane and move rapidly across the membrane. b ) Polar molecules( Water soluble substances: ions, glucose, urea etc ) have much lower solubility , Therefore, penetrate the membrane much more slowly. Chemical and Physical Properties of membrane control the free passage of ions in and out of cell. This property helps in maintaining components in i ntracellular fluid and extracellular fluid. Insulating Properties:- It acts as dielectric material of a charged condenser, thus cell membrane has a very high insulating value.

6. Chemical Composition O f Plasma Membrane Plasma membrane have –54% lipids 46%protein 5-10%carbohydrates

7. Chemical Composition O f Plasma M embrane Lipids and proteins are the two major components of all membrane. Carbohydrates are present in outer leaflets. But in inner leaflets, do not have a significant role. Carbohydrates bound either to proteins as constituents of glycoproteins , Or bound to lipids as constituents of glycolipids.

8. Lipids Are The Major Components:- 3 major components of eukaryotic membrane are glycerophospholipids sphingolipids cholesterol

9. Glycerophospholipids Glycerophospholipids have a glycerol molecule with a phosphate esterified at the ɑ-carbon. And two long chain fatty acids esterifies to the remaining carbon atoms. Structure of major alcohols esterified to phosphatidic acid to form the glycerophosphospholipid . For ex:-if alcohol is choline ,molecules is called phosphotidylcholine and if serine, then is called phosphotidylserine .

10. Glycerophospholipids Glycerophospholipids contain two fatty acyl groups esterified to C1 and C2 of glycerol. A saturated fatty acid is usually found on C1 and unsaturated fatty acid on C2. The presence of unsaturated fatty acids has a marked effect on the fluidity of the membrane. Phosphotidylcholine and phosphotidylethanolamine are the most common glycerophospholipids .

11. Glycerophospholipids Ether phospholipids contain an alkyl group (alkyl acyl glycerophospholipid ) termed as plasmogens . Plasmogens containing ethanolamine or choline plasmogens esterified to the phosphate are abundant in nervous tissue and heart . High levels of ether linked lipids in plasma membrane of very metastatic cancer cells have been reported. Suggested a role for the lipids in the invasive properties of these cells.

12. Glycerophospholipids A re Amphipathic

13. Glycerophospholipids A re Amphipathic The basic structure of the plasma membrane is the lipid bilayer. Glycerophospholipids contain both a polar and non polar end. Polar head group are in contact with intra or extracellular aquous phase. Whereas non polar tails face each other.

14. Sphingolipids The amino alcohols sphingosine and dihydrosphingosine are the basis for the sphingolipids . Ceramide have a saturated or unsaturated long chain fatty acyl group in amide linkage with the amino group of sphingosine . The sphingomyelins , the most abundant sphingolipids in mammalian tissues ,have phosphorylcholine esterified at C1.

15. Glycosphingolipids contain a sugar linked by a ẞ- glycosidic bond to C1OH group of a ceramide . A subgroup of cerebroside which contain either glucose or galactose . Galactocerebroside predominate in brain and nervous tissue, whereas glucocerebroside occur in small quantities of non neuronal tissues.

16. The plasma membrane of animal cells contains 4 major phospholipids, such as – phosphotidylcholine , phosphotidylserine , phosphotidylethanolamine and sphingomyelin . At neutral pH, the polar head group may have no net charge, Or it may have net negative charge.

17. Cholesterol Cholesterol is the third major lipid in membrane Cholesterol is a compact rigid hydrophobic molecule. It has a polar hydroxyl group at C3. Cholesterol alters the fluidity of membranes .

18. The major lipid components of plasma membranes:-PC-21% PE+PS-29% SM-21% CHOLESTEROL-26%

19. Glycoproteins or Glycolipids Carbohydrates are present in membranes as oligosaccharides. They are covalently attached to the lipids or proteins. The carbohydrate is on the extracellular surface of plasma membrane and the luminal surface of endoplasmic reticulum.

20. Role of carbohydrates Carbohydrates mainly serve as cell to cell recognition m olecules, membrane receptor.

21. LIPID BILAYER

22. Lipid bilayer and Micelles The amphipathic compound with a hydrophilic head and a hydrophobic tail interact with aqueous systems in vitro to form a sphere. Micelles have only one polar surface, which is side presented to the aqueous phase..

23. The structure of a micelles is very stable . Because of hydrophobic interactions between hydrocarbon chains and attraction of polar head group to water . Micelles are important in intestinal digestion and absorption of lipids

24. Mobility of lipid components in membrane Lipid bilayer not rigid and static structure. Lipid molecules can rotate freely around their long axis and diffuse laterally within each leaflets. All phospholipids of plasma membrane are mobile, they are not fixed. They are moving laterally . The transition of a lipid molecule from one leaflet to the other is called transverse diffusion.

25. FLUID MOSAIC MODEL In 1972 SJ Singer and GL Nicolson proposed fluid mosaic model.

26. Asymmetry Of L ipid B ilayer Outer leaflets Lots of carbohydrates. Sphingomyelin and phosphotidylcholine . Cho moities are present. Floppase is an outward-directed ATP-dependent transporter. Inner leaflets Carbohydrates does not have significant role. Phosphotidylserine and phosphotidylethanolamine . Absent Flippase is an inward-directed ATP-dependent lipid class of transporters.

27. Membrane P rotein Integral protein Peripheral protein Integral proteins are permanently attached to the membrane. Embedded in the whole membrane. Serve as carrier proteins, channels, &enzymes. Detergents should be used to remove integral proteins. Glycophorin are the example of integral proteins. Peripheral proteins are temporarily attached to the membrane. Located on the inner or outer surface of the phospholipid bilayer. Serve as receptors and surface antigens. Peripheral proteins removed by salt, pH changes Erythrocyte spectrin are the example of peripheral proteins.

28. Integral proteins INTEGRAL PROTEINS Integral membrane proteins are embedded are asymmetric in the membrane Integral proteins are mostly transmembrane proteins. All transmembrane proteins are amphipathic Protein that are held in lipid bilayer very tightly .

29. Types Of I ntegral P roteins S tructure 6 different types of ɑ-helix protein structure primarily of hydrophobic amino acids. TYPE-1 TYPE-2 C N N C

30. TYPE-3 Multiple trends in single polypeptide. TYPE-4 Consists of several different polypeptides assemble together in a channels through the membrane. TYPE-5 Proteins are anchored to lipid bilayer through covalently linked lipids. Type-6 Proteins have both transmembrane domains & lipid anchors.

31. Have specific protein which serve as channels carrier protein, enzymes and receptors . A+B C+D

32. Fluidity of a lipid bilayer Although the lipid bilayer structure is quite stable its individual phospholipid & sterol molecules have much freedom of motion which is main reason of fluidity property of lipid bilayer. Membrane fluidity is influenced by: -Temperature -Fatty acid composition -sterol content

33. Fluidity O f Lipid B ilayer Low temperature Fluid like organization Polar head loosely packed Tailed disordered Membrane thinner Fatty acid unsaturated High temperature Phase transition Fluid like organization. Polar head loosely packed Tails disordered. Membrane thinner. Gel like organization. Polar head tightly packed. Tails regular Membrane thicker

34. Abnormalities of Cell Membrane Fluidity in Disease A major factor in the fluidity of the membrane in higher organisms and mammals in the presence of cholesterol. In case of spur cell anaemia , erythrocyte membrane of individuals have increased cholesterol content & a spiny shaped. Cells are destroyed prematurely in the spleen. This condition occurs in severe liver disease such as alcoholic cirrhosis. Increase intracellular membrane cholesterol which affect their fluidity. The intoxicating effect of ethanol on the nervous system is probably due to modification of membrane fluidity, altering membrane receptors and ions channels.

35. Lipid Rafts Lipid rafts are membrane microdomains enriched in sphingolipids,cholesterol and certain lipid- linkedproteins .

36. Rafts is mainly responsible for signaling and communication. LIPID RAFTS Caveolin Non caveolar

37. Caveolins Caveolin ( caveolae”little caves’) Integral membrane protein. Two globular domains connected by hairpin like hydrophobic structures. Bound inward to cytoplasmic l eaflet. FUNCTIONS- - Membrane trafficking - Signal transduction

38. Dynamic Nature of Membrane Diagram indicate the relationship of for membrane-associated proteins with the lipid bilayer.

39. Some membrane protein movement is restricted. Cellular membrane are in a constantly changing state, with movement of proteins and lipids laterally, changes in association of lipid anchored proteins and molecules into and out of the membrane.

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