II. Mechanism: Cell Membrane Chemical Transport

  1. Background
    1. Cellular membranes (phospholipid bilayer) are selectively permeable
    2. Lipid soluble substances are non-polar and may move across the cell membrane
    3. Charged substances (e.g. Electrolytes such as Calcium, Sodium, Potassium) move through water channels in the membrane
  2. Simple Diffusion
    1. Molecules transit the cell membrane based on osmotic gradients (moving from high concentration to lower)
    2. Molecules also move based on electrical charge (moving towards oppositely charged concentrations)
  3. Simple Facilitated Diffusion
    1. Transport proteins (channels, carrier proteins) allow charged particles to cross the cell membrane
      1. Unlike active transport, no energy is required, as the molecules are following an osmotic gradient
    2. Channel Proteins create a hydophilic tunnel through the hydrophobic core of the lipid bilayer
      1. Aquaporins allow water to rapidly cross cellular membranes
      2. Channels may be gated (only periodically allowing molecules to pass)
    3. Carrier Proteins
      1. Allow charged particles to be shielded from the hydophobic core of the cell membrane as they cross
      2. Unlike channel proteins which create a tunnel, carrier proteins change shape as molecules cross the membrane
  4. Active Transport
    1. Molecules move against their electrochemical gradient
    2. Primary Active Transport
      1. Movement against electrochemical gradient is fueled by ATP
      2. Examples: Sodium-Potassium ATPase pump
        1. sodiumPotassiumATPase.jpg
      3. Endocytosis is ATP-fueled invagination of material across the cell membrane
    3. Secondary Active Transport
      1. Movement against electrochemical gradient is fueled by another molecule moving WITH its electrochemical gradient
      2. In Cotransport, the 2 molecules move in the same direction across the cell membrane
      3. In Countertransport, the 2 molecules move in opposite direction across the cell membrane
  5. References
    1. Membrane Transport (Wikipedia)
      1. https://en.wikipedia.org/wiki/Membrane_transport

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