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Membrane Structure and Fluidity

  • Review of Membrane Structure: Continues from Chapter 5, focusing on how membranes work and what they accomplish, crucial for understanding subsequent chapters. The previously discussed figure of the membrane with various membrane-bound proteins is a foundational reference.

  • Fluid Mosaic Model: The membrane is not rigid; it is described by the fluid mosaic model.

    • Principle: The membrane holds together primarily by hydrophobicity of the fatty acid tails (hydrocarbons).

    • Analogy: Imagine a bathtub filled with water, with ping-pong balls representing phospholipid heads, packing the surface in a single layer. Sloshing the water creates waves, moving the ping-pong balls, illustrating fluidity. Adding a tennis ball (representing a membrane-bound protein) shows it also moves, demonstrating flexibility.

Phospholipids and Membrane Components

  • Phospholipid Structure:

    • Phosphate Head: Charged, hydrophilic, likes aqueous environments.

    • Fatty Acid Tails: Nonpolar, hydrophobic, likes nonpolar environments. They cluster due to hydrophobicity.

  • Fluidity and Tail Structure:

    • One fatty acid tail is typically straight (saturated), and the other is bent (unsaturated).

    • If both tails were straight, they would stack tighter, making the membrane less fluid.

    • If both tails were bent (unsaturated), they would be much more fluid.

    • The balance between saturated and unsaturated tails is crucial for maintaining proper membrane fluidity.

  • Bilayer Formation: When in water, individual phospholipids naturally cluster. If sufficient in number, they form bilayers, creating a boundary that partitions water. Water, being polar, is attracted to the phosphate heads on either side.

Modified Phospholipids

  • Glycolipids: Phospholipids modified by linking sugars to their heads.

  • Specialized Phospholipids: Some phospholipids can link to amino acids (e.g., phosphatidylserine). These modified phospholipids are often associated with specialized cellular responses, such as inflammation.

Cholesterol

  • Role in Membrane Rigidity: Cholesterol increases the rigidity of the membrane. More cholesterol means a more rigid membrane; less cholesterol means a less rigid membrane.

  • Lipid Rafts: Regions within the membrane with higher concentrations of cholesterol, forming stable