Notes on Cell Membrane Structure and Function

Introduction

• Importance of cell membrane in maintaining cell integrity
• Prevents cells from filling with water or leaking substances

Cell Membrane Basics

• Definition: The outer layer of a cell
• Function: Protects cells and regulates entry and exit of substances
• Property: Semi-permeability
  • Some substances can enter/leave, while others cannot

Structure of Cell Membrane

• Main Building Block: Phospholipids
  • Other substances present, but phospholipids are key
• Components of Phospholipids:

  1. Phosphate head group (hydrophilic)

  2. Glycerol backbone

  3. Two fatty acid tails (hydrophobic)

  • Visualization:
    • Phosphate head = circle (loves water)
    • Fatty acid tails = strings (fear water)

Amphipathic Nature of Phospholipids

• Definition: Molecules that have both hydrophobic and hydrophilic parts
• Behavior in Water:
  • Heads cluster toward water; tails shield away from water
  • Forms structures like phospholipid bilayer

Phospholipid Bilayer

• Structure: Consists of two layers of phospholipids
  • Heads facing out towards water (extracellular and intracellular)
  • Tails facing inward, creating a hydrophobic core

Semi-Permeability of Cell Membrane

• Molecule Passage:
  • Small Nonpolar Molecules: Pass easily (e.g., O2, CO2)
  • Small Polar Molecules: Pass slowly (e.g., water, ethanol)
  • Large Nonpolar Molecules: Can pass slowly (e.g., benzene)
  • Large Polar Molecules: Cannot pass (e.g., glucose)
  • Charged Molecules: Cannot pass (e.g., ions, amino acids)

Summary

• Cell membrane: Protects cells and regulates substance movement
• Composed mainly of phospholipids
• Semi-permeable, preferring small and nonpolar molecules for passage
• Hydrophobic core influences molecular permeability