Lecture Notes: Cell Membranes & Their Properties

Introduction

• Importance of cell membrane in preventing cells from filling with water or leaking substances.
• Cell membrane: outer protective layer of a cell.
• Property: Semipermeability – allows selective entry and exit of substances.

Structure of the Cell Membrane

• Main Component: Phospholipids
  • Phosphate Head Group: Hydrophilic (water-loving).
  • Glycerol Backbone: Connects the head and tails.
  • Fatty Acid Tails: Hydrophobic (water-fearing).
• Amphipathic Molecule: Possesses both hydrophilic and hydrophobic sections.

Phospholipids in Water

• Formation of phospholipid bilayer:
  • Phosphate heads face outward towards water.
  • Fatty acid tails face inward, away from water.
• Bilayer structure essential for cell membrane.

Shape and Arrangement

• Real-cell structure forms a circle.
• Internal and external sides always have hydrophilic heads facing water, and hydrophobic tails protected inside.

Semi-Permeability of the Cell Membrane

• Small Non-Polar Molecules: (e.g., O₂, CO₂) pass through easily via passive diffusion.
• Small Polar Molecules: (e.g., water, ethanol) pass through slowly due to small size.
• Large Non-Polar Molecules: (e.g., benzene) pass through very slowly; tend to align with hydrophobic region.
• Large Polar Molecules: (e.g., glucose) cannot pass through; require different absorption methods.
• Charged Molecules/Ions: (e.g., chloride ion, sodium ion, amino acids) cannot pass through.

Summary

• Cell membrane allows survival by regulating entry and exit of substances.
• Made of phospholipids which prefer small, non-polar molecules.
• Molecules pass through the membrane based on size and polarity.