Lecture Notes: Biology B2.1 Membranes and Transport

Fluid Mosaic Model

• Fluidity: Describes how easily components of the cell membrane move around; impacts membrane permeability.
• Factors Influencing Fluidity:
  • Saturated vs Unsaturated Fatty Acids:
    • Saturated fatty acids result in less membrane fluidity than unsaturated ones.
    • More unsaturated fatty acids are found in colder habitats to maintain fluidity.
  • Cholesterol:
    • Found in animal cells; stabilizes membranes at high temperatures and prevents stiffness at low temperatures.

Vesicles and Transport

• Vesicles: Small pouches made of lipid bilayers, essential for transporting materials.
  • Endocytosis: Process of bringing materials into the cell.
    • Cell membrane forms an indentation, pinches off, forming a vesicle.
    • Active process requiring ATP.
  • Intracellular Transport:
    • Vesicles carry proteins synthesized in the rough ER to the Golgi apparatus.
    • Golgi modifies and repackages proteins for exocytosis or other functions.
  • Exocytosis: Process of expelling materials from the cell.
    • Vesicles fuse with the cell membrane to release contents outside.
    • Requires ATP and is crucial for waste expulsion and secretion.
  • Cell Growth: Vesicles contribute to cell growth by integrating into the cell membrane.

Membrane Proteins and Ion Channels

• Channel Proteins: Facilitate movement of polar or charged particles across the membrane.
• Voltage-Gated Ion Channels:
  • Open or close in response to changes in voltage across the membrane.
  • Important for neural signaling (to be covered in more detail elsewhere).
• Nicotinic Acetylcholine Receptor:
  • Integral protein that acts as both receptor and ion channel.
  • Binds acetylcholine, changes shape, allowing ions to pass and initiate nerve signals.

Active Transport

• Sodium-Potassium Pump:
  • Pumps sodium ions out and potassium ions into the cell using ATP.
  • Establishes concentration gradients crucial for nerve transmission.
  • Functions as an exchange transporter.
• Indirect Active Transport:
  • Sodium-Dependent Glucose Co-Transporters:
    • Use established sodium gradients to transport glucose against its concentration gradient without direct ATP usage.
  • Example: Glucose reabsorption in kidneys.

Cell Adhesion Molecules (CAMs)

• Function: Ensure tight cell junctions to control molecular movement.
• Importance:
  • Prevents unwanted movement of molecules between cells.
  • Critical for tissues, such as intestinal cells, to direct nutrients efficiently into the bloodstream.