Lecture Notes: Chapter 4 - Cell Membrane and Transport

Revision from Chapter 1

• Eukaryotic Cells:
  • Have a cell surface membrane.
  • Contains internal membranes forming membrane-bound organelles.
  • Examples include nuclear membrane, Golgi apparatus membrane, vesicle membrane, tonoplast (vacuole membrane in plants), mitochondria, chloroplasts, and endoplasmic reticulum.
• Importance of Membranes:
  • Allow separation of functions.
  • Example: Lysosomes contain hydrolytic enzymes that are isolated by membranes to prevent damage to other cell structures.

Chapter 2 Recap: Phospholipids

• Phospholipid Structure:
  • Made of a polar phosphate head (hydrophilic) and two nonpolar fatty acid tails (hydrophobic).
  • Polar heads interact with water; nonpolar tails do not.
• Water Solubility:
  • Only the polar phosphate head is water-soluble.
  • Nonpolar fatty acid tails are not soluble in water.

Importance of Phospholipids in Cell Membranes

• Membrane Formation:
  • Phospholipids are the main building blocks of cell membranes.
  • In water, phospholipid molecules arrange themselves so that:
    • Hydrophilic heads face water.
    • Hydrophobic tails face each other.
  • This arrangement is due to hydrophobic interaction.

Phospholipid Bilayer

• Structure:
  • Forms a spherical structure in a watery environment.
  • Heads face both external (watery) environment and internal cell environment (cytoplasm).
  • Tails face each other forming a bilayer.
• Functionality:
  • The phospholipid bilayer is the fundamental structure of cell membranes.
  • Provides a barrier and a matrix for membrane proteins.

Exam Tips

• Describing Phospholipid Membrane Formation:
  • Hydrophilic heads interact with water inside and outside the cell.
  • Hydrophobic tails interact with each other.
  • Results in the formation of the phospholipid bilayer (two layers of phospholipids).

Visualization

• Size and Visualization:
  • Phospholipid bilayer is about 7-8 nanometers thick.
  • Can only be seen using an electron microscope (light microscope resolution is >200 nm).
• 3D Theoretical Representation:
  • Cross-sectional view of the membrane would show hydrophilic heads facing outward and hydrophobic tails facing inward.

Summary

• Key Points:
  • Phospholipids are essential for forming cell membranes.
  • They organize themselves into a bilayer due to their hydrophilic and hydrophobic properties.
  • The bilayer is crucial for cellular function, protecting internal components and mediating transport processes.