Cell Membrane

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  • Cell Membrane
    • structure
      • double membrane
      • phospholipid bilayer
      • barrier is 7-8nm across
      • made of phospholipids, proteins and other macromolecules
    • Diagram
    • function
      • protects living cell from non-living surrounding
      • takes up nutrients
      • secretes chemicals
      • allows cell recognition
      • selectively permeable to water and some solutes
    • why a phospholipid bilayer?
      • hydrophilic heads of phospholipids
      • hydrophobic tails of phospholipids
      • tails = inside heads = outside
    • Fluid because the phospholipid molecules are free to move within their layer
    • Mosaic because the proteins embedded in the layer are different shapes and sizes
    • Membrane proteins
      • extrinsic
        • occur only on the surface/ partly embedded
        • polar/charged and associate with polar phosphate heads
        • provide structural support, recognition sites for hormones and cell surface markers (antigens)
      • intrinsic
        • penetrate lipid bilayer, normally through whole membrane
        • transport proteins
        • channel proteins and carrier proteins
    • Membrane Permeability
      • lipid soluble, gasses, hydrophobic molecules and small polar molecules
        • Dissolve in the phospholipids and diffuse across the membrane
        • vitamin A, oxygen, carbon dioxide, ethanol, water ect...
      • water soluble
        • pass through intrinsic proteins (hydrophilic lining) as they cannot diffuse through phospholipids
        • glucose, ions, amino acids ect..
    • factors affecting permeability
      • temperature: at low temps the molecules have low KE making the membrane rigid. when temperature is higher the membrane is more fluid so gaps between lipids are larger allowing more through
        • but when temperature is too high the proteins denature leaving huge gaps in the membrane and controll of what comes in and out is lost
      • Ethanol: ethanol will dissolve the lipids making the membrane porous allowing things to leak out. if there's enough ethanol the membrane will dissintagrate completely. acid can also do this
      • detergent: reduces surface tension and disperses the phospholipids in membrane, increasing permeability
      • NaCl: sodium ions will bond to phospholipids oxygen in the heads reducing their movement


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