3.2 Movement of Substances across plasma membrane

• Polar molecule


  A molecule with an even distribution of charges, having positive and negative poles
• Selectively permeable membrane


  A membrane that only allows some substances or molecules to move across it freely, but prevents other substances from doing so
• Plasma membrane


  • Hydrophobic regions
  • Hydrophilic regions
• Movement of substances across plasma membrane


  1. Through phospholipid bilayer

  2. Through channel proteins

  3. Through carrier proteins
• Simple diffusion


  The movement of molecules through the phospholipid bilayer down their concentration gradient, without the use of energy
• Facilitated diffusion


  The movement of molecules across the plasma membrane with the assistance of transport proteins (channel proteins or carrier proteins), down their concentration gradient, without the use of energy
• Lipid soluble substances that can move across plasma membrane

  • Glycerol
  • Fatty acids
  • Vitamins A, D, E, K
  • Steroids
  • Alcohols
• Lipid insoluble substances that can move across plasma membrane


  • Small nonpolar molecules (e.g. oxygen, carbon dioxide)
  • Small polar molecules (e.g. water)
  • Small charged ions (e.g. potassium, sodium, calcium, chloride)
  • Large polar molecules (e.g. glucose, amino acids)
• Lipid soluble substances


  Can dissolve in and move through the hydrophobic regions of the phospholipid bilayer
• Small nonpolar molecules


  Can move through the phospholipid bilayer by simple diffusion
• Small polar molecules like water


  Can move through the phospholipid bilayer by simple diffusion due to their small size
• Small charged ions


  Cannot move through the hydrophobic regions of the phospholipid bilayer, so they use channel proteins for facilitated diffusion
• Large polar molecules like glucose and amino acids


  Cannot move through the phospholipid bilayer, so they use carrier proteins for facilitated diffusion
• Osmosis is the diffusion of water through a selectively permeable membrane