Cell membranes and transport

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  • Created by: GAdams
  • Created on: 28-03-16 10:21
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  • Cell membranes
    • Structure
      • Phospholipids: Hydrophobic non-polar fatty acid tails and hydrophilic phosphate head. A bilayer is formed in which fatty acid tails point inwards while phosphate heads point outwards.
        • Lipids can move past each other within the membrane. Shorter, saturated fatty acid tails speed up movement.
      • Glycolipds: These occur on the external surface of the cell membrane and the carbohydrate section extends into intercellular space (called 'glycoalyc'). These are used for cell-to-cell recognition.
      • Cholesterol: Adds structural support to animal membranes, but is rarely found in plant membranes.
      • Intrinsic proteins span the whole membrane. Functions: Channels for facilitated diffusion; transporter proteins that attach to certain substances; receptors; and enzymes.
    • Transport
      • Simple diffusion: Lipid-soluble materials can follow their concentration gradients into cells through the phospholipid bilayer.
        • Oxygen, carbon dioxide, ammonia, alcohols and fat-soluble vitamins can diffuse across the phospholipid bilayer.
      • Osmosis: The diffusion of water molecules from a region of high concentration to that of low concentration across a semi-permeable membrane.
      • Facilitated diffusion: Faster than normal diffusion. Used to transport sugars and ions.
        • Glucose binds to the outside of a transport protein. The protein changes shape, forcing glucose to the other side. The glucose detaches and the protein reverts to its original shape. Glucose is phosphorylated to maintain the concentration gradient.
        • Ions may pass through a polar channel protein.
      • Na+ ions can bind to the protein. The phosphate group is removed, which changes the shape of the protein. This process can regulate the amount of water inside the cell.
        • Na+/K+ pump: Inside the cell, Na+ ions bind to the membrane. ATP breaks down into ADP and the energy released phosphorylates the protein which forms the pump.
      • Active transport moves ions against their concentration gradients.
        • Phosphorylation -- the addition of a phosphate group.


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