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BIO 111 Class 10: Cell Membranes

Cell Membrane Structure and Function

I. Fluid Mosaic Model of Membranes

  • Membrane Nature: Cell membranes are not rigid structures; they are fluid and flexible.
  • Composition: Primarily composed of phospholipids, each having:
    • Phosphate head: Charged, hydrophilic (water-loving), likes a polar environment.
    • Two fatty acid tails: Non-polar, hydrophobic (water-fearing), likes a non-polar environment.
  • Fluid Mosaic Model Concept: The membrane is held together primarily by hydrophobicity, where the non-polar fatty acid tails cluster together.
  • Analogy (Ping-pong balls and a tennis ball in a bathtub):
    • Imagine a bathtub filled with water, with a monolayer of ping-pong balls floating on the surface, representing phospholipid heads. When the water sloshes, the balls move fluidly, illustrating the membrane's fluidity.
    • Adding a tennis ball (representing a membrane-bound protein) into this system, it too will bob up and down and move around, demonstrating the flexibility and movement of components within the membrane.
  • Factors Affecting Fluidity:
    • Fatty Acid Tail Saturation: The structure of the fatty acid tails influences fluidity.
      • If both tails were straight (saturated), they would stack tighter, making the membrane less fluid.
      • If both tails were bent (unsaturated), they would be more fluid.
    • Balance: The balance of saturated and unsaturated fatty acid tails is crucial for maintaining appropriate membrane fluidity.
  • Phospholipid Clustering: In water, phospholipids naturally cluster due to hydrophobicity, forming bilayers. This bilayer acts as a boundary, partitioning polar water to either side, attracted to the phosphate heads.

II. Modified Phospholipids and Glycolipids

  • Glycolipids: Formed when sugars are linked to phospholipids.
  • Specialized Phospholipids: Some phospholipids can be modified by linking proteins or amino acids (e.g., phosphatidylserine, where serine is linked to a phospholipid). These specialized phospholipids, embedded in membranes, are often associated with unique cellular responses, such as inflammation.

III. Cholesterol

  • Nature: Cholesterol is another type of lipid found in membranes.
  • Effect on Membrane Rigidity: The amount of cholesterol directly impacts membrane rigidity.
    • More cholesterol: more rigid membrane.
    • Less cholesterol: less rigid membrane.
  • Lipid Rafts: Areas within the membrane that contain more cholesterol, forming more rigid platforms. These