AS LEVEL- Biology- Plasma membranes (1)

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  • Plasma membranes (1)
    • Roles: *Partially permeable barriers between the cell + its environment, between organelles + the cytoplasm + within organelles *Site of chemical reactions *Site of cell signalling
    • Also called fluid- mosaic model
      • Phospholipids
        • *Forms a barrier to dissolved substances *Has a 'head' + 'tail' *Head = hydrophilic  (attracts water) *Tail = hydrophobic (repels water) *Membarne doesn't allow water-soluble substances eg. ions through it
      • Cholesterol
        • *Gives the membrane stability *Type of lipid *Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together- makes the membrane less fluid + more rigid
      • Proteins
        • *Control what enters + leaves the cell *Channel- allows small or charged particles through *Carrier- transporst molecules + ions across the membrane by active transport + facilitated diffusion
      • Glycolipids + Glycoproteins
        • *Act as receptors for messenger molecules *Stabilises the membrane by forming hydrogen bonds with surrounding water molecules *Also where drugs, hormones + antibodies bind *Also antigens
    • Cell signalling
      • Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to them
      • Drugs work by binding to receptors in cell membrane. They trigger a response in the cell
      • Hormones- Chemical messengers released by the cell
    • Factors affecting membrane structure + permeability
      • Temp
        • Below 0°C
          • 1) The phospholipids don't have much energy, so they can't move very much
          • 2) They're packed close together + membrane is rigid
          • 3) Channel + carrier proteins in the membrane deform, increasing the permeability of the membrane
          • 4) Ice crystals may form + pierce the membrane making it highly permeable when it thaws
        • Between 0-45°C
          • 1) The phospholipids can move around + aren't as tightly packed together- membrane is partially permeable
          • 2) As the temp increases, the  phospholipids move around more as they have more energy- increases permeability of the membrane
        • Above 45°C
          • 1) The  phospholipid bilayer starts to melt + the membrane becomes more permeable
          • 2) Water inside the cell expands, putting pressure on the membrane
          • 3) Channel + carrier proteins deform so they can't control what enters or leaves the cell- increases permeability of the membrane
      • Solvents-
        • 1) Surrounding cells in a solvent increases the permeability of their cell membranes
        • 2) This is because solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
        • 3) Some solvents increase cell permeability more than others eg. ethanol increases cell permeability than methanol
        • 4) Increasing the concentration of the solvent will also increase membrane  permeability


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