Cell Membrane and Semi-Permeability

Introduction

• Importance of the cell membrane in preventing cells from filling with water (e.g., when swimming or showering).
• The cell membrane protects the cell and determines what can enter and leave.

Structure of the Cell Membrane

• Main Building Block: Phospholipids
  • **Components of Phospholipids:
  • Phosphate head group (polar, hydrophilic)
  • Glycerol backbone (holds fatty acid tails to the head)
  • Two fatty acid tails (non-polar, hydrophobic)
• Phospholipid Bilayer: Hydrophobic fatty acid tails cluster together to avoid water, while hydrophilic head groups face water.
• Amphipathic Molecule: Molecule containing both hydrophilic and hydrophobic sections.
• The bilayer forms a circle with hydrophilic heads facing water inside and outside the cell, creating a hydrophobic interior.

Semi-Permeability of the Cell Membrane

• Extracellular (outside environment) vs Intracellular (inside the cell)
• Passive Diffusion: Movement of molecules through the cell membrane.
  • Small nonpolar molecules (e.g., gases like O2 and CO2) can pass through easily and quickly.
  • Small polar molecules (e.g., water and ethanol) pass through slowly.
  • Large nonpolar molecules (e.g., benzene) also pass through slowly.
  • Large polar molecules (e.g., glucose) cannot pass through.
  • Charged molecules (e.g., ions like Cl-, Na+, and amino acids) cannot pass through.

Summary

• The cell membrane protects cells and determines what enters and leaves, a property known as semi-permeability.
• The structure is based on a phospholipid bilayer with hydrophilic heads and hydrophobic tails.
• The membrane prefers small and nonpolar molecules for passage due to its hydrophobic interior and tightly packed phospholipids.