Methods of transport across a membrane

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  • Methods of transport across a membrane
    • Passive
      • Diffusion
        • Movement of ions from a region of higher concentration to a region of lower concentration
        • Down a diffusion gradient
        • Rate of diffusion depends on surface area, thickness of the membrane, strength of diffusion gradient and temperature
        • Small, non-polar molecules can diffuse through a membrane: oxygen, carbon dioxide, steroids, fat soluble vitamins
      • Osmosis
        • Specialised form of diffusion
        • Diffusion of water molecules from an area of high water potential to an area of lower water potential
        • Down a water potential gradient
        • Movement is through a partially permeable membrane
        • Cells are affected by water potential outside the cell
      • Facilitated diffusion
        • Faster than normal diffusion
        • Used to transport large, polar molecules such as glucose, fructose, non-fat soluble vitamins, urea and ions
        • Three steps involved:
          • 1. Molecule binds with carrier protein molecules on the cell surface
          • 2. The protein changes shape (conformational change). In doing so, the molecule is transported to the inside of the cell
          • 3. The molecule detaches from the protein and it returns to its original shape
        • Small polar molecules can also diffuse through channel proteins
        • Rate of facilitated diffusion is proportional to the concentration gradient and the number of channel or transporter proteins available
    • Active
      • Active transport
        • Moves substances against a concentration gradient
        • Uses carrier proteins such as sodium or potassium ion pumps which span the membrane and use ATP to transport molecules
        • Carrier protein goes through conformational change upon binding that pushes molecule in or out of the cell/organelle
      • Endocytosis
        • Bulk transport of substances into a cell
        • Cell membrane dimples to engulf substances
        • Membrane surrounds molecules, forming a vesicle inside the cell that contains them
        • Endocytosis of liquids is called pinocytosis
      • Exocytosis
        • Bulk transport of substances out of the cell
        • A membrane bound vesicle moves to the plasma membrane
        • The membranes fuse together, releasing the contents of the vesicle to the extracellular environment


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