BIO 111: Cell Membranes - Structure, Function, and Transport

Membrane Structure: The Fluid Mosaic Model

• Membranes are not rigid structures. They are dynamic and flexible.

• Fluid Mosaic Model: This model describes the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.

  • Phospholipids: Composed of a phosphate head (hydrophilic/polar) and two fatty acid tails (hydrophobic/non-polar).

  • Hydrophobicity: The fatty acid tails are hydrocarbons that cluster together due to their hydrophobic nature, forming the core of the membrane.

  • Analogy (Ping-pong balls and a bathtub): Imagine a bathtub filled with water with a monolayer of ping-pong balls (phospholipid heads) floating on the surface. When you create a wave, the ping-pong balls move around fluidly but remain on the surface. Adding tennis balls (membrane-bound proteins) shows how they also move within this fluid environment.

• Factors influencing fluidity:

  • Fatty acid tails:

    • Saturated (straight) tails: Stack tighter, making the membrane less fluid.

    • Unsaturated (bent/kinked) tails: Create more space, making the membrane more fluid.

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

• Clustering of Phospholipids: When placed in water, phospholipids naturally cluster due to hydrophobicity. With a sufficient number, they form bilayers, creating a boundary that partitions water, as water is attracted to the polar phosphate heads on either side of the non-polar tails.

Modified Phospholipids and Glycolipids

• Modifications: Phospholipids can be modified by linking other molecules.

• Glycolipids: Formed when sugars are linked to a phospholipid.

• Specialized Phospholipids: Linking amino acids (e.g., serine to form phosphatidylserine) creates unique phospholipids. These are often associated with specialized responses, such as inflammation.

Cholesterol

• Impact on Membrane Rigidity:

  • More cholesterol: membrane becomes more rigid.

  • Less cholesterol: membrane becomes less rigid.

• Lipid Rafts: Regions within the membrane with higher concentrations of cholesterol, providing more rigid