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Biological Membranes

Biological Membranes. Dr. Sarray Sameh Ph.D. Biological membrane-Function. Membranes are the outer boundary of individual cells and of certain organelles. Serve as barriers separating cell from the environment

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Biological Membranes

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  1. Biological Membranes Dr. Sarray Sameh Ph.D

  2. Biological membrane-Function • Membranes are the outer boundary of individual cells and of certain organelles. • Serve as barriers separating cell from the environment • Defines the difference (gradient) between extracellular fluid (ECF)/ Intracellular fluid (ICF). • Selectively permeable (chooses what may cross the membrane) • Mediate cell-cell communications and define cell identity • Receive and transmit various signal ECF ICF

  3. Membrane Structure • Major component are : Lipid and dispersed • proteinsembeddedin the membrane; Carbohydrates • (do not exist free form in membrane). • Different membranes have different protein/lipid ratio • Lipids areamphipathic: they have both polar and non • polar moities: This organization form a closed bilayer structure in an aqueous medium. • The type of membranes may be identified • by the presence of specific proteins markers. • Membrane have an ‘inside-outside’ asymetry • mainly by proteininserted in asymetric fashion. • Thelipids composition is different in two leaflets. • The oligosaccharides always project towards the exterior

  4. Lipid Composition of Cell Membranes • Acts as permeability barriers and are essential for the maintenance of fluidity membranes Head • Phospholipids: • most abundant (50-90% of total lipid content); • Most of them are derivatives of diacyl-glycerol-3-phosphate(DAG), phosphatidate. • They are amphipathic; the hydrophilic part is in the head group (alcohol (serine, ethanolamine, choline..)+phosphate) which extend outward • The hydrophobic portion is the fatty acid chain (Tail) extend inward • Principal phospholipids: phosphatidylcholine, P.etahnolamine, P.serine, Cardiolipin, P.inositol and sphingomyelin • Cholesterol • 5-25% of membrane content; sterols are usually more concentrated in the plasma membrane; • cholesterol dispersed throughout cell membranes between phospholipids. Its polar group is near the polar head group of phospholipids and the steroid rings and hydrocarbon tails are oriented parallel to those of phopsholipids. Tail • Glycolipids • the least abundant, usually less than 5%. Exp cerebroside (Gal as sugar) • The carbohydrate portion is always oriented toward the outside of the cell

  5. Cell Membrane Proteins • The main types of membrane proteins are: • Integral membrane proteins / Transmembrane protein • intrinsic proteins are embedded deeply in the bilayer. • Peripheral membrane protein proteins bound to the external or internal face of membrane

  6. Functions of membrane proteins • Proteins are responsible for many biological functions of the membrane: • Receptorsfor signals such as hormones, neurotransmitters, growth factor. Binding of a hormone to the protein transmits the signal. • Transporters • Ion channels • pumps • Membrane based enzyme • Structural components (The biconcave discoidal shape of the erythrocyte is due in part to specific membrane proteins). • Tissue specific antigen • Members of all of these classes are often glycosylated enable molecules to enter and exit a cell

  7. Carbohydrates of Cell Membranes • Minor component 5-8% • Never as free form:Theyare attached to protein or lipid as glycoprotein or glycolipid. • Typical sugars in glycoproteins and glycolipids include: • glucose, galactose, mannose and fucose • N-acetylated sugars e.g. N-acetyl glucosamine, N-acetyl galactosamine and N-acetyl neuraminic acid (sialic acid) • Functions: • Cell recognition and communication • Helps in intercellular attachment/adhesion • Act as receptors • Cell identity markers

  8. Lipid Rafts Specialized micordomains within cell membrane rich in cholesterol and sphingolipid Function of lipid raft: cholesterol transport, endocytosis and signal transduction

  9. Fluid Mosaic Model A membrane is a fluid structure with a mosaic of various proteins embedded in it when viewed from the top: • Phospholipids can move laterally a small amount and can flex their tails; • Membrane proteins also move laterally making the membrane fluid

  10. Membrane fluidity • Influenced by two major factors: temperature and lipid composition of the membrane: • At low Temperature-fluidity is less • As the temperature increases the hydrophobic side chains undergo a transition from the ordered state to the disordered state -increase fluidity • Fluidity of membrane is maintained by: • Length of hydrocarbon chain • Degree of unsaturation • Short chain FA increase the fluidity • Long chain FA decrease the fluidity • Unsaturated FA that exist in cis configuration increase the fluidity; Trans FA decrease the fluidity of membrane • More the number of double bond-greater is the fluidity

  11. END!

  12. Major components of cell membranes • Lipids, proteins and CHO are held together by several non covalent interactions. • Different membranes have different protein/lipid ratio • The type of membranes may be identified by the presence of specific proteins markers. E.g. Na,K-ATPase is a marker of plasma membrane, while Succinate- dehydrogenase is a mitochondrial marker.

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