Membrane Structure, Permeability, and Transport

Membrane Structure and Transport

Plasma Membrane Structure

• Components of the Plasma Membrane (PM):

  • Phospholipid Bilayer: Forms the basic structure.

  • Cholesterol:

    • Inserted into the membrane with its four rings positioned next to the fatty acid chains.

    • Provides mechanical strength to the membrane.

    • Maintains membrane fluidity.

    • Found only in animal cells.

    • Has a polar head and a nonpolar body.

  • Proteins:

    • Integral Membrane Proteins:

      • Embedded within the phospholipid bilayer of the PM.

      • Can span the entire membrane.

    • Peripheral Membrane Proteins:

      • Loosely attached to the surface of the PM.

    • Glycoproteins:

      • Integral membrane proteins with sugars (carbohydrates) covalently attached.

      • The part of the protein with the sugars is located on the outer (extracellular) surface of the PM.

  • Glycolipids: Lipids with sugars attached, found on the extracellular leaflet.

  • Carbohydrates: Often found on the extracellular surface, associated with glycoproteins and glycolipids.

• Structure Overview:

  • Extracellular leaflet: The outer layer of the phospholipid bilayer, facing the extracellular environment.

  • Cytosolic leaflet: The inner layer of the phospholipid bilayer, facing the cytosol.

Function of Plasma Membrane

• Compartmentation: Separates the living cell from the external environment, defining cellular boundaries.

• Communication: Mediates cell-to-cell signaling and interactions.

• Transport: Regulates the movement of substances into and out of the cell.

Membrane Permeability: Differentially Permeable (Semipermeable)

• Definition: Only certain molecules can pass through the membrane.

• Barrier Properties:

  • The lipid bilayer acts as a good barrier because it is hydrophobic.

  • Most salts and biological molecules are hydrophilic and cannot pass directly through the hydrophobic lipid layer.

• **Substances that *CAN* pass directly through the lipid bilayer (without a transport protein) via Simple Diffusion:**

  • These substances must be soluble in the hydrophobic part of the lipid bilayer.

  • Small, uncharged molecules (e.g., $ext{O}<em>2, ext{CO}</em>2$).

  • Water (despite being polar, its small size allows some passage).

  • Hydrophobic molecules (e.g., steroids).

• **Substances that *CANNOT* pass directly through the lipid bilayer:**

  • Charged ions.

  • Charged molecules.

  • Large molecules (e.g., large macromolecules).

  • Polar organic molecules.

Movement of Solutes in a Solution

Diffusion

• Definition: The net movement of a substance from a region of higher concentration to a region of lower concentration.

• Mechanism: Movement is