Crossing Membranes

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  • Crossing Membranes
    • Diffusion
      • the net movement of molecules from a region of high concentration to a region of low concentration
      • molecules possess kinetic energy so no enery is required from the cell
      • DOWN a concentration gradient
      • EQUILIBRIUM - when molecules are distrubuted evenly, they still move but there is no overall movement
      • Rate of Diffusion
        • Conc. Gradient - greater = increased rate
        • Temperature - increase= more kinetic energy to molecules = increased rate
        • Stirring - increases the movement of molecules and rate of diffusion
        • Surface Area - greater = increased rate
        • Distance/Thickness - thick membranes - greater distance for molecules to travel - decreased rate
        • Size of molecule - smaller = quicker rate
      • In Membranes
        • lipid molecules can pass through the phospholipid bilayer by diffusing down the conc. gradient
        • CO2 and O2 are small enough to pass through the bilayer between phospholipid molecules
        • Facilitated Diffusion
          • Channel Proteins - form pores in the membrane, often complementry shaped/gated - can be opened-closed
            • Ions .E.G. Na/Ca
          • Carrier Proteins - complementry shape- proteins changes shape when the specific molecule fits to allow molecule through
            • Large Molecules .E.G. glucose
          • Different Membranes can have different carrier/channel proteins so that they can have some control over what can get in/out
    • Active Transport
      • the net movement of a molecule from a low concentration to a high concentration using ATP
      • Some Carrier Proteins act as pumps. They have complmentry shapes and carry larger molecules or ions
        • they carry molecules ONE way across the membrane, using ATP tomove them against the conc. graidient
          • the changing shape ensures a one way flow - the molecule cannot re-enter the protein as it is a different shape
        • quicker than diffusion
        • .E.G. Ca Ions in muscles
      • Endo/Exocytosis - the bulk transport of materials into/out of a cell
        • Bulk Transport is possible because membranes are able to easily fuse, separate and pinch off
        • Requires enery - ATP, used to move the membranes / vesicles around
        • Endo -inwards Exo -outwards Phago -solids Pino -liquids


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