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Ch. 7.2 Notes: The Plasma Membrane

1. Overview of the Plasma Membrane

  • Definition: The plasma membrane acts as a flexible boundary between the cell and its environment.

  • Functions:

    • Allows a steady influx of nutrients (glucose, amino acids, lipids) regardless of external conditions.

    • Eliminates excess substances that may become toxic.

    • Facilitates the removal of waste and other products from the cell.

    • Maintains homeostasis, ensuring a balanced environment within the cell.

2. Selective Permeability

  • Selective Permeability: The membrane permits certain molecules to pass while restricting others.

    • Water freely enters the cell, while ions (sodium & calcium) are only allowed in amounts defined by specific conditions and through designated channels.

3. Structure of the Plasma Membrane

  • Phospholipids:

    • Primary building blocks of the plasma membrane.

    • Composed of:

      • Glycerol Backbone

      • Fatty Acid Chains (2)

      • Phosphate Group

    • Polar Head: Attracted to water (hydrophilic).

    • Fatty Acid Tails: Repellent to water (hydrophobic).

4. Phospholipid Bilayer

  • The plasma membrane is structured as a phospholipid bilayer (two layers):

    • Interior formed by the nonpolar fatty acid tails.

    • Exterior faces the aqueous environments inside and outside the cell.

  • Polar phosphates interact with water, forming a water-soluble barrier.

  • Nonpolar fatty acids orient toward each other in the center, creating an impermeable barrier to water-soluble substances.

5. Fluid Mosaic Model

  • Description: Explains the dynamic structure of the plasma membrane:

    • Fluidity: Phospholipid molecules can move/flow within the layer.

    • Mosaic Pattern: Proteins embedded within phospholipids create a pattern resembling a mosaic.

6. Role of Cholesterol

  • Cholesterol molecules stabilize the phospholipid bilayer by preventing the fatty acid tails from sticking together, maintaining membrane fluidity.

7. Membrane Proteins

  • Transport Proteins: Act like doors, facilitating the movement of substances through the membrane.

    • Types:

      • Carrier Proteins: Transport molecules across the membrane.

      • Channel Proteins: Allow specific ions or water to pass through.

  • Functions in recognition of chemical signals and connection to the internal structural support of the cell.