AS LEVEL- Biology- Plasma membranes (3)

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  • Plasma membranes (3)
    • Facilitated Diffusion (Passive- No energy is required)
      • Passive-Energy for movement comes from the natural/kinetic energy of molecules
      • Facilitated diffusion- Particles move from a region of high to low concentration
      • 1) Some large molecules eg. amino acids + polar molecules don't diffuse directly through the phospholipid bilayer of the cell membrane
      • 2) Instead they diffuse through carrier or channel proteins in the cell membrane
        • Carrier- Aqueous channel for ions + large polar molecules
        • Channel- *A conformational change in the shape of the protein has to occur to allow the molecules/ ions to pass through *This protein generally transports
      • Factors that affect the rate of diffusion
        • Concentration gradient
        • Thickness of membrane
        • SA
        • Temp
        • Number of channel + carrier
    • Active Transport (Active - Requires energy)
      • Active- Requires metabolic energy in the form of ATP
      • Active transport- Movement of particles from an area of low to high concentration using metabolic energy in the form of ATP
      • Particles move against a concentration gradient
      • 1) The molecule/ion to be transported binds tio receptors in the carrier protein
      • 2) ATP binds to the protein on the other side of the membrane + is hydrolysed into ADP and inorganic phosphate
      • 3) Binding of the phosphate molecule to the carrier protein causes a change in the structure of the protein meaning that the inside of the carrier protein opens
      • 4) The molecule/ion is then released from the carrier protein
      • 5) The phosphate molecule is released by the carrier protein + recombines with ADP to ATP
      • 6) The carrier protein reverts back to its original shape
    • Bulk Transport (Active - Requires energy)
      • Active- Requires metabolic energy in the form of ATP
        • ATP is needed as the vesicles + cell surface membrane need to fuse, vesicles are moving along the cytoskeleton + cell surface membrane changes the shape of the cell
      • Endocytosis- (Enter = in)
        • 1) Cell surface membrane invaginates when it comes into contact with the material
        • 2) Membrane enfolds until the membrane fuses, forming a vesicle
        • 3) Vesicle pinches off + movies into cytoplasm + is transferred to sites of further processing within the cell
      • Exocytosis (Exit = out)
        • Some substances produced by the cell eg. hormones need to be released from the cell
        • 2) The vesicle + cell surface membrane fuses which leads to the contents releasing
        • 1) Vesicles move to the cell surface membrane using the cytoskeleton
    • Osmosis  (Passive- No energy is required)
      • Passive-Energy for movement comes from the natural/kinetic energy of molecules
      • Osmosis- Movement of water across a PPM from an area of high to low water potential
      • 1) Water potential- pressure exerted by the water molecules as they collide with a membrane
        • Units = Pascal (Pa) or Kilopascal (kPa)
      • 2) Pure water has the highest water potential as it has a high concentration of water molecules and therefore a lower concentration of solute
      • 3) *Very concentrated = low water potential (more negative)   *Not very concentrated = high water potential (not so negative)
      • How water potential affects animals cells
        • Solution has the same water potential as the cell (Isotonic)
          • *Water molecules pass into + out of the cell in equal amounts *The cell stays the same
        • Solution has a higher water potential than the cell (Hypotonic)
          • *Net movement of water molecules is into the cell *Cell bursts
        • Solution has a lower water potential than the cell (Hypertonic)
          • *Net movement of water molecules is out of the cell *Cell shrinks
      • How water potential affects plant cells
        • Solution has a higher water potential than the cell (Hypotonic)
          • *Net movement of water molecules is into the cell  *The vacoule swells *The vacuole + cytoplasm push against the cell wall *The cell becomes turgid (swollen)
        • Solution has a lower water potential than the cell (Hypertonic)
          • *Net movement of water molecules is out of the cell *The cell becomes flaccid (limp) *The cytoplasm +  membrane pull away from the cell wall- plasmolysis

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