The cell membrane


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  • The Plasma Membrane 02/09/12 G. Podgorski, Biol. 1010
  • The cell membrane

    1. 2. BIOMEDICAL IMPORTANCE <ul><li>Permit cellular individuality,form close compartments. </li></ul><ul><li>Plasma membrane has selective permeabilities and act as a barrier,thereby maintainig differences in the composition between the inside and outside of the cell. </li></ul><ul><li>Membranes establish and maintain the concentration of electrolytes,nonelectrolytes and water between the exterior and interior of the cell. </li></ul>
    2. 3. <ul><li>Exchange mateerials with the extracellular environment,exocytosis and endocytosis </li></ul><ul><li>Signal transduction. </li></ul><ul><li>Membranes form specialized compartments(organelles) in the cell. </li></ul><ul><li>Membranes localize enzymes. </li></ul><ul><li>Membranes possess antigenic properties. </li></ul>
    3. 4. CHEMICAL COMPOSITION OF MEMBRANE <ul><li>LIPIDS </li></ul><ul><li>Phospholipids </li></ul><ul><li>Glycolipids </li></ul><ul><li>Cholesterol </li></ul><ul><li>These lipids are amphipathic in nature,having polar and non polar regions. </li></ul><ul><li>Lipid content and composition of various membranes is different. </li></ul>
    4. 5. <ul><li>PROTEINS </li></ul><ul><li>Transmembrane,integeral,peripheral proteins </li></ul><ul><li>Glycoproteins. </li></ul><ul><li>Cytoskeletal filaments </li></ul><ul><li>Carbohydrate chains </li></ul>
    5. 6. Structure of the Plasma Membrane
    6. 7. Phospholipids <ul><li>Majpr lipids in the membrane(50-90%). </li></ul><ul><li>Mainly lecithin and cephalin </li></ul><ul><li>Forms the lipid bilayer structure </li></ul>
    7. 8. GLYCOLIPIDS <ul><li>Least abundant(5%),Sphingophospholipids and Glycosphingolipids </li></ul><ul><li>Cerebrosides,contain only one sugar residue(glucose or galactose) </li></ul><ul><li>Gangliosides,contain a branch chain of as many seven residues. </li></ul>
    8. 9. CHOLESTERL <ul><li>2-25% of the membrane.more concentrated in the outer leaflet of the plasma membrane. </li></ul><ul><li>It regulates the membrane fluidity. </li></ul>
    9. 10. <ul><li>CARBOHYDRATES . </li></ul><ul><li>These are either in the form of glycolipids or glycoproteins. </li></ul><ul><li>External to the plasma membrane, all animal cells have glycocalyx . This consists of the carbohydrate moieties of membrane glycolipids and glycoproteins. </li></ul>
    10. 11. <ul><li>Functions(Glycocalyx) </li></ul><ul><li>In digestive tract </li></ul><ul><li>Protection </li></ul><ul><li>Immunity to infection </li></ul><ul><li>Defense against cancer </li></ul><ul><li>Transplant compatibility </li></ul><ul><li>Cell adhesion </li></ul><ul><li>Fertilization : Enables sperm to recognize and bind to eggs </li></ul><ul><li>Embryonic development </li></ul>
    11. 12. PROTEINS <ul><li>Proteins may be globular or fibrous in nature. </li></ul><ul><li>Integral proteins( may be fibrous) are themselves amphipathic in nature,and are bound tightly to the membrane.They generally span the entire lipid bilayer. </li></ul><ul><li>Peripheral proteins(globular) are weakly and reversibly bound to the hydrophilic regions of the specific integral proteins. </li></ul>
    12. 13. <ul><li>e.g Ankyrin(peripheral protein) is bound to band 3 protein(integral) in the RBCs membrane. </li></ul><ul><li>Glycophorin(rich in Sialic acid) is peripheral protein in human erytherocytes. </li></ul>
    13. 14. <ul><li>Integral proteins or transmembrane proteins </li></ul><ul><li>Span the membrane and have a hydrophilic cytosolic domain , which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet protein motifs . </li></ul><ul><li>Ion channels, proton pumps , G protein-coupled receptor </li></ul>
    14. 15. <ul><li>Lipid anchored proteins </li></ul><ul><li>Covalently-bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane. </li></ul><ul><li>G proteins </li></ul>
    15. 16. <ul><li>Peripheral proteins </li></ul><ul><li>Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and, once reacted the molecule, dissociates to carry on its work in the cytoplasm. </li></ul><ul><li>Some enzymes , some hormones </li></ul>
    16. 18. <ul><li>) Transport Proteins </li></ul><ul><li>2) Receptor Proteins </li></ul><ul><li>3) Enzymatic Proteins </li></ul><ul><li>4) Cell Recognition Proteins </li></ul><ul><li>5) Attachment Proteins </li></ul><ul><li>6) Intercellular Junction </li></ul><ul><li>Proteins </li></ul>
    17. 19. 1) Transport Proteins <ul><li>Channel Proteins – channel for lipid insoluble molecules and ions to pass freely through </li></ul><ul><li>Carrier Proteins – bind to a substance and carry it across membrane, change shape in process </li></ul>
    18. 20. 2) Receptor Proteins <ul><li>– Bind to chemical messengers (Ex. hormones) which sends a message into the cell causing cellular reaction </li></ul>
    19. 21. 3) Enzymatic Proteins <ul><li>– Carry out enzymatic reactions right at the membrane when a substrate binds to the active site </li></ul>
    20. 22. 4) Cell Recognition Proteins <ul><li>– Glycoproteins (and glycolipids) on extracellular surface serve as ID tags (which species, type of cell, individual). Carbohydrates are short branched chains of less than 15 sugars </li></ul>
    21. 23. 5) Attachment Proteins <ul><li>Attach to cytoskeleton (to maintain cell shape and stabilize proteins) and/or the extracellular matrix (integrins connect to both). </li></ul>
    22. 24. 6) Intercellular Junction Proteins <ul><li>– Bind cells together </li></ul><ul><ul><li>Tight junctions </li></ul></ul><ul><ul><li>Gap junctions </li></ul></ul>
    23. 25. <ul><li>Different membranes have different protein/lipid ratio (3.6 for mitochondria, 1.1 for erythrocytes plasma membrane, 0.25 for myelin) </li></ul><ul><li>This property can be used to isolate various membrane fractions. </li></ul>
    24. 26. <ul><li>The type of the membranes is identified by the presence of specific protein markers. </li></ul><ul><li>For example: Na,K-ATPase is a marker of plasma membrane, while succinate dehydrogenase ,ATP synthetase is a mitochondria marker,Glucose 6 phosphatase is a marker for ER. </li></ul>
    25. 27. <ul><li>FLUID - because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid. </li></ul><ul><li>MOSAIC - because of the pattern produced by the scattered protein molecules when the membrane is viewed from above. </li></ul>FLUID MOSAIC MODEL copyright cmassengale
    26. 28. Membranes are fluid structures
    27. 29. <ul><li>Movement can be: </li></ul><ul><li>Lateral </li></ul><ul><li>Flip flop </li></ul>
    28. 30. Membrane fluidity is important for: <ul><li>fusion of membranes, i.e. fusion of vesicles with organelles </li></ul><ul><li>diffusion of new lipids and new proteins laterally, so they are equally distributed </li></ul><ul><li>diffusion of proteins and other molecules laterally across the membrane in signaling/reactions </li></ul><ul><li>proper separation of membranes during cell division </li></ul>
    29. 31. <ul><li>Factors that affect membrane fluidity: </li></ul><ul><li>Length of the fatty acyl chains </li></ul><ul><li>Degree of saturation of the fatty acyl chains </li></ul><ul><li>Presence of cholesterol </li></ul><ul><li>Cholesterol affects fluidity </li></ul>
    30. 32. ROLE OF MEMBRANES IN DISEASE. <ul><li>Achondroplasia </li></ul><ul><li>Familial hypercholesterolemia </li></ul><ul><li>Cystic fibrosis </li></ul><ul><li>Wilson’s disease </li></ul><ul><li>Hereditary spherocytosis </li></ul><ul><li>Paroxysmal nocturnal hemoglobinuria et c. </li></ul>