Cell Membranes

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  • Cell Membranes
    • Role of membranes
      • holding the components of metabolic pathways in place, regulating the transport of materials into or out of cells
      • separating cell contents from the outside environment, separating cell contents from cytoplasm, cell recognition/ signalling,
    • Phospholipid Bilayer
      • Phosphate head is hydrophillic, lipid tails are hydrophobic. This comes from the way charges are distributed across the molecule
      • If phospholipid molecules are mixed with water they form a layer at the surface
      • If phospholipid molecules are surrounded with water a bi-layer can form. In this state the molecules can move freely
        • rarely some may 'flip flop' from one mono-layer to the other
        • the head cannot easily pass through the hydrophobic region of the bi-layer - giving it some stability
        • the phospholipid molecules are not bonded together
      • Is the basic structure of all biological membranes
        • the  hydrophobic layer formed creates a barrier to many molecules and separates the cell contents from the outside
        • the thin layer of oil is an ideal boundary - where metabolic reactions take place
      • 7-10nm thick
    • Performing the Functions
      • number and type of various components varies according to the function of the particular membrane. This specialisation is part of the differentiation process
        • some membranes are up to 1000 times more permeable to water because they contain aquaporins
      • all membranes are permeable to water molecules because water molecules can diffuse through the lipid bilayer
    • A partially permeable membrane - is permeable to water and some solutes
    • Fluid Mosaic Model
      • Cholesterol - gives the membranes of some eukaryotic cells mechanical stability. Fits between fatty acid tails and helps make the barrier more complete
        • substances like large molecules and ions cannot pass easily and directly through the membrane
      • Channel Proteins - allow the movement of some substances across the membrane. Some molecules are too large and too hydrophilic to pass directly through the bilayer
      • Carrier Proteins - actively move substances across the membrane
      • Receptor Sites - some allow hormones to bind with the cell so that a cell can carry out a cell response
      • Glycoproteins /lipids - cell recognition, receptor sites and cell adhesion
    • Temperature
      • increasing gives molecules more kinetic energy, so they move faster. The increased movement makes membranes leaky, allowing  things to leave/enter the cell
      • As temperature increases, the proteins in the membrane can become denatured


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