Cell Membranes

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  • Cell Membranes
    • Roles of membranes
      • Separating cell contents from the outside environment
      • Separating cell components from cytoplasm
      • Cell recognition and signalling
      • Holding the components of some metabolic pathways in place
      • Regulating the transport of materials into or out of cells
    • Phosphlipid  bilayer
      • Phosphate  'head'
        • Hydrophilic -water loving
        • Phosphate head stick into the water
      • Fatty acid (lipid) 'tail'
        • Hydrophobic - water hating
      • If phosholipid molecules are completely surrounded by water, a bilayer can form.
        • Phosphate heads on each side of the bilayer stick into the water, while the hydrophobic fatty acid tails point towards each other
          • Means that the hydrophobic tails are away from water
          • Phospholipid molecules can move freely -  just like fluid molecules do.
          • Gives the bilayer stability
      • Basic structural component of all biological membranes
        • Hydrophobic layer formed by the phosphlipid tails creates a barrier to many molecules and separates the cell contents from the outside world.
      • Differentiation of membranes
        • Phosphlipid bilayer would be to fragile to function as a barrier without other components
        • Examples
          • Plasma membranes of the cells in a growing shoot contain receptors the molecules that regulate growth
          • Muscle cell membranes contain a large number of channels that allow rapid uptake of glucose
    • The fluid mosaic model
      • Used to describe the molecular arrangemnts in membranes.
      • Feature: a bilayer of phospholipid molecules forming the basic structure
      • Feature: various protein molecules floating in the phospholipid bilayer, some completly freely, some bound to other components or to sturctures within the cell
      • Feature: some proteins partially embedded in the bilayer on the inside or the outside face;other protiens completely spanning the bilayer
    • Glycoproteins and glycolipids
      • Where phosphlipid molecules have a carbohydrate part attached they are called glycolipids
      • Where protein molecules have a carbohdrate part attached they are called glycoproteins
    • Membrane components and their roles
      • Membranes stability and fluidity
        • Cholesterol gives the membranes some mechanical stability. Fits between the lipid tails and makes the barrier more complete
      • Membrane transport functions
        • Channel proteins allow the movement of large/charged substances across the membrane.
        • Carrier proteins actively move large/charged substances across the membrane
      • Recognition and communication
        • Receptor sites - some allow hormones to bind with the cell so that a cell 'response' can be carried out. A cell can respond to a hormone only if it has a receptorfor that hormone on its cell surface membrane
        • Glycoproteins/lipids are involved in cell signalling that they are 'self' to allow recognition to by the immune system
    • Temperature
      • Increasing temperature gives molecules more kinetic energy - they will move faster.
        • Increased movement of phospholipids and other components makes membranes leaky
          • Allows subsutances that would normally not do so to enter or leave the cell.
      • Organisms that live in very hot or very cold environments need differntly adapted molecular components of their membranes so that their membranes can perform the functions needed to maintain life

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