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Cell membrane. structure AIMC
- 1. Cell Membrane Structure andFunction
- 2. Cell Membrane • Everycell is encircled by a membrane and most cells contain an extensive intracellular membrane system. • Selective Barrier: Membranes fence off the cell's interior from its surroundings (maintain extracellular Na,Ca,Cl,HCO3& intracellular K.Mg,P,levels) . • Transport: Membranes let in water, certain ions and substrates and they excrete waste substances. • Key role: in Cell-cell interaction and transmembrane signalling • Without a membrane: The cell contents would diffuse into the surroundings, information containing molecules would be lost and many metabolic pathways would cease to work.
- 3. Membrane Function-transport • Sometimesthe elements bind to the proteins, which flip over, thus transporting the element into the cell. • Some proteins form a ‘pore’ through which the element can pass from the outside to the inside of the cell membrane. • The movement of the phospholipid and protein components through the plasma membrane permits the membrane to change shape—this is known as fluidity. • This flexibility is crucial to many different types of cells.
- 4. Organelles Membrane The cellis highly organized with many functional units or organelles inside. Each organelles is surrounded by a membrane that contains specific proteins and lipid components that enable it to perform its unique roles.e.g. Help shape organelles- Site of energy transduction-mitochondria Do excitation-response coupling-nerves Localize enzymes-lysosomes In essence membranes are essential for the integrity and function of the cell.
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- 6. Structure - TheCell Membrane The fluid mosaic model of membrane structure • The membrane is a mosaic (mixture) of different protein molecules floating in a bilayer (double layer) of phospholipids • Each phospholipid has a hydrophilic (water- loving) head & hydrophobic (water-hating) tails • Because of this feature of phospholipids, the lipid bilayers assemble themselves spontaneously
- 7. The cell membranceis a complex 3d circular structure The cell membrane is a complex 3 d structure.
- 8. Structure • Fluid-like composition…likesoap bubbles • Composed of: – Lipids in a bi-layer – Proteins embedded in lipid layer (called trans-membrane proteins) – Proteins floating within the lipid sea (called integral proteins) – Proteins associated outside the lipid bi- layer (peripheral proteins).
- 9. The fluid mosaicmodel • Proposed by Singer Nicolson in 1972 is often likened to icebergs (of proteins) floating in a sea (of phospholipids). • This proposes that the cell membrane is made up of 2 main layers – lipids and proteins. • The lipids form themselves into a bi-layer with the water seeking ends (hydrophilic) facing out & the water hating ends (hydrophobic) facing in. • The proteins are embedded in this lipid bilayer but can for the most part move laterally or flip over in the plane of the memb. • Proteins n phospholipids move randomly in plan of memb termed lateral diffusion that depends upon Lipid composition,Temperature,more unsaturated FA.
- 10. The fluid mosaicmodel
- 12. The fluid mosaicmodel Lipids arranged in bi-layer with proteins embedded or associated with them.
- 13. Composition of thecell membrane Phospholipids Peripheral Proteins Integral Proteins Trans – membrane proteins Cell Membrane
- 14. Membrane Lipids form aBi-layer Outside layer Inside Layer
- 15. MEMBRANE LIPIDS A; Glycolipids/Glycosphingolipids: (sphingosine+FA+Sugar) B;Phospholipids: (Alcohol+FA+Phosphoric Acid): • Glycerophospholipid –main compt. (phosphatidyl choline+ph ethanolamine+ph serine+ph inositol) and • Sphingophospholipids (sphyngomyeline)
- 16. Phospholipid Molecule Model glycerol fatty acids(hydrophobic) – hate water phosphate (hydrophilic) – like water Draw This
- 17. • In outermemb mainly: Phosphatidylcholin n Sphingomyeline • In inner memb mainly: Aminophospholipids Phosphotidylinositol (transfer information)
- 18. Phospholipids • Make up75% of cell surface membrane • Amphipathic – hydrophilic head (water- loving and polar) and hydrophobic tail (water- hating and non-polar) • Phospholipid bilayer forms spontaneously in aqueous environment eg. extra-cellular fluid and cytosol • Selectively permeable – small molecules and lipid soluble molecules pass through easily. Hydrophilic substances cannot diffuse and rely on membrane pore/channels to pass
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- 20. TRANSMEMBRANE PROTEIN • Integralproteins/Transmembrane proteins are most common,interact externally with phospholipids n require detergents for their separation. These span the bilayer as a bundle of α-helical once o or many times ,usually globular n amphipathic with two hydrophylic ends protruding on both surfaces of memb.and separated by an intervening hydrophobic region that traverses hydrophobic core of bilayer(GPCR-7pass R).
- 21. PERIPHERAL PROTEINS • Theydo not interact directly with hydrophobic cores of phospholipid bilayer n don’t require detergent for their release.They are bound to hydrophilic region of specific integral proteins.They can be released by salt soln. of high ion strength(e.g.Ankyrin- RBCs).
- 22. Different Membranes HaveDifferent Protein Compositions • Proteins are the major functional molecules of membranes • and consist of enzymes, pumps and channels, • structural components, antigens (eg, for histocompatibility), • and receptors for various molecules.
- 23. Proteins • Variety offunctions: – Transport proteins for ions and polar molecules – Enzymes: eg ATPase in mitochondrial membrane, chloroplast membrane, intestinal wall cells (for hydrolysis of disaccharides)
- 24. Proteins - Glycoproteins •transmembrane proteins span the entire membrane & are usually glycoproteins: • Four main functions – Act as channels: Na+/K+ Pump to maintain ion concentrations either side of the membrane – Transporters: some proteins identify & attach to specific substances eg. nutrients, neurotransmitters – Receptors: recognise & bind to target molecules such as hormones – Enzymes eg. ATPase
- 25. Glycoproteins • Most proteinsin plasma membrane have short carbohydrate molecules attached • H-bond with water to help stabilise membrane structure • May act as receptor for hormones, neurotransmitters • Antigens = glycoproteins that help cells recognise each other. Each cell has it’s own antigen
- 26. Glycolipids • 5% ofmembrane lipids have short carbohydrate chains attached • Occur on external surface of membrane • Maintain stability of cell membrane. • Fascilitate cellular recognition for immune response and for connection of cells to form tissues.
- 27. Cholesterol • In humanscholesterol (a steroid) is present in almost same proportion as phospholipids. • Also amphipathic – fit between phospholipids and maintain membrane fluidity • Important because it makes cell membrane more rigid and prevents membrane being too fluid & breaking up (37degrees = relatively high temp) = mechanical stability • Hydrophobic regions prevent ions & polar molecules passing through – especially important in neurons
- 28. Cholestrol-fluidity of memb. •Hydrophilic-OH part of cholesterol binds with hydrophilic head of memb. lipids. • Hydrophobic-HC chain of USFA-bent n steroid nu bind with hydrophobic tails of memb. lipids. • It prevent hydrophobic chains from packing too closely together as a result protein molecules that are not bound can rotate laterally in the plane of leaflet-so maintaining fluidity of memb.
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- 30. GLYCOCALYX • 5%of wtof cell memb. is carbohydrate part of Glycoproteins and Glycolipids.It is well said that all the cells are sugar coated. • Their presence on outer surface of memb.(The Glycocalyx) has been shown with use of plant Lectins,proteins,that bind specific Glycosyl resid. • Glycophorin is a major integral memb. Glycoprotein of human erythrocytes. Glycocalyx protect cell from digestion.
- 31. MEMBRANES ARE DYNAMIC •Lipid and proteins in membrane undergo turnover just as do in other compartments of the cell.(in endocytosis) • Lipids and certain Proteins show lateral diffusion in the plane of their membranes, slow Flip-Flop movement. • Transverse movement of lipids across the membrane is-extremely slow- flip-flop movement.
- 32. Membranes Are Asymmetric Structures(cho-lipids- proteins,special structers) (((• Outside surface: carbohydrats (glycolipids n glycoproteins),specific proteins & phos,choline n sphingomyeline • Inside surface: specific proteins & aminophospholipids, phos.inositol, • Regional differences: • Villous border of mucosal cell • Lipid Rafts,Caveolae • Gap junctions,Tight junctions n Synapsis
- 33. Lipid rafts These aredynamic areas of the exoplasmic leaflet of the lipid bilayer enriched in Lipids:cholesterol,sphingolipids,phospholipid Proteins:Cytoplasmic proteins covalently modified by saturated FA,e.g.Palmitoyl & Myristoyl,others are Src kinases,G proteins,GF Receptors,PKC. they are involved in signal transduction and possibly other processes.
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- 35. Caveolae • Caveolae mayderive from lipid rafts. Many, if not all, contain the • protein caveolin-1, which may be involved in their formation from rafts. • Caveolae are observable by electron microscopy as flask, or tube- shaped indentations of the cell membrane into the cytosol . • Proteins detected in caveolae include various components of the signal transduction system (eg, the insulin receptor and some G proteins;), the folate receptor, and endothelial nitric oxide synthase (eNOS). • Caveolae and lipid rafts are active areas of research
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- 38. Tight junctions • Theyare often located below the apical surfaces of epithelial cells and prevent the diffusion of macromolecules between cells. • They are composed of various proteins, including occludin, various claudins, and junctional adhesion molecules.
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- 40. Desmosomes • Desmosomes/macula adherens(binding body)is a cell structure specialized for cell- to-cell adhesion.It provide a connection between intermediate filaments of cell cytoskeleton of adjacent cells.
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- 42. Gap Junctions • Gapjunctions are structures that permit direct transfer of small molecules (up to ~1200 Da) from one cell to its neighbor. • Gap junctions are composed of a family of proteins called connexins that form a bihexagonal structure consisting of 12 such proteins. Six connexins form a connexin hemichannel and join to a similar structure in a neighboring cell to make a complete, membrane- spanning connexon channel . • One gap junction contains several connexons. Different connexins are found in different tissues.
- 44. CARDIOLIPIN • Cardiolipin Isa Major Lipid of Mitochondrial • Membranes • Phosphatidic acid is a precursor of phosphatidylglycerol, which in turn gives rise to cardiolipin • This phospholipid is found only in mitochondrial membrane and is essential for the mitochondrial function. • Decreased cardiolipin levels or alterations in its structure or metabolism cause mitochondrial dysfunction in aging and in pathologic conditions including heart failure, hypothyroidism, and Barth syndrome (cardioskeletal myopathy).
- 45. Cardiolipin and Syphlis Cardiolipinis antigenic and is recognized by antibodies raised against Treponema pallidum in patients of syphilis causing demage to all body cells.
- 46. LIPOSOMES • Artificial membranesystems can be prepared from phospholipids which are treated to form spherical vesicals in which lipids form a bilayer. • Such vesicals,surrounded by a lipid bilayer with an aqeous interior are termed liposomes. • USES: For stydy purpose,as vehicle to administer nutrients& drugs to treat tumours and gene therapy.
- 47. Transfer of materialn information across membrane