Cell Membranes (biological membranes

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  • Created by: Jasmine
  • Created on: 19-02-16 13:43
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  • Biological Membranes
    • Membrane Function
      • Plasma membranes-barrier between the cell and its environment
        • They are partially permeable
        • Substances can move across through osmosis, diffusion or active transport
        • Also allow recognition by other cells (immune system for example)
        • Can be used for cell signalling
      • Membranes within cells
        • Membranes around the cells divide cells into different compartments-barrier between cytoplasm and organelles
        • Can form vesicles
        • Membranes within cells can also be partially permeable
        • Membranes within organelles which act as barriers to keep membrane contents separate
        • Membranes can be the site of chemical reactions- can be folded to increase surface area for more efficient chemical reactions
    • Membrane Structure
      • Composed of lipids (phospholipids), proteins and carbohydrates
      • Fluid Mosaic Model-
        • Bilayer is the fluid (phospholipid always moving
          • Cholesterol molecules present
        • Proteins molecules scattered throughout
          • Some proteins have a carbohydrate attached (glycoproteins)
          • Some lipids have carbohydrates attached called glycolipids
    • Membrane Components
      • Phospholipids
        • Form a barrier to dissolved substances
        • Head and tail
          • Head is hydrophilic (attracts water)
          • Tail is hydrophobic (repels the water molecules)
        • Arrange into a bilayer where the head faces out towards the water on each side of the membrane
          • Centre of the bilayer is hydrophobic so the membrane prevents water molecules from diffusing through
          • fat soluble substances dissolve in the bilayer and pass straight through the membrane
      • Cholesterol
        • Membrane stability
        • Fit between the phospholipids
          • Bind to the hydrophobic tails
            • Less fluid- more ridgid
        • Has hydrophobic regions-create further barrier to polar substances moving through the membrane
      • Proteins
        • Controls what enters and leaves the cells
          • Forms channels for charged particles (active transport and facilitated diffusion)
        • Act as receptors for molecules in cell signalling
          • When molecule binds then a chemical reaction is triggered
      • Glycolipids and glycopoteins
        • form hydrogen bonds with surrounding water molecules
        • Act as receptors for drugs, antibodies and hormones to bind
          • They're also antigens- self recognition and immune response
    • Factors Affecting Membrane Permeability
      • Solvents
        • Some solvents can dissolve the lipids in the cell membrane eg ethano- membrane then loses structure
        • Increasing the concentration will increase the membrane permeability
      • Temperature
        • Below 0 degrees
          • Phospholipids do not have as much energy, membrane is rigid. Channel proteins and carrier denature. Increases the membrane permeability.
        • Temperatures 0-45 degrees
          • Phospholipids can move around- membrane is partially permeable. Increase in temp means increase in energy( increase in membrane permeabilty
        • Above 45 degrees
          • bilayer starts to melt- more permeable membrane. water inside causes the cell to expand. Channel and carrier proteins begin to denature (cannot control what enters or leaves the cell anymore) increases the permeability of the membrane


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