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• The plasma membrane is composed of a lipid bilayer that serves as a highly impermeable barrier to most "charged (polar)" and "non-lipid soluble" substances

  
• Integral proteins in the plasma membrane

  • Act as "pores, channels" or "carriers" to allow substances to cross the membrane

  
• Water molecules


  Penetrate the membrane by diffusion through the lipid bilayer or through aquaporins (transmembrane proteins) that function as water channels
• Osmosis


  The movement of water from a low solute concentration to a high solute concentration across a semi-permeable membrane

  
• Water is the solvent for all solutes and is present at a very high concentration: 56 molar
• Osmotic pressure


  The "drawing power" of a solution to encourage water to move towards it
• Water always moves to the solution with the highest osmotic pressure (highest solute concentration)

  
• Osmosis


  • The net movement of water through a selectively permeable membrane
  • Occurs only when the membrane is permeable to water but not to certain solutes

  
• Osmotic pressure


  Proportional to the number of "osmotically-active particles" in solution

  
• Tonicity


  A measure of a solution's ability to change the volume of cells by altering their water content (amount/volume)
• Isotonic solution


  • No net movement of water, cells maintain their normal shape
• Hypotonic solution


  • Cells gain water, in danger of swelling and bursting
• Hypertonic solution


  • Cells lose water, in danger of shrinking and becoming dehydrated
• There are important medical uses of isotonic, hypotonic, and hypertonic solutions
• Tonicity demonstrated with red blood cells

  • In an isotonic solution, red blood cells maintain their normal shape
  • In a hypotonic solution, red blood cells undergo haemolysis
  • In a hypertonic solution, red blood cells undergo crenation
• Osmotic flow across a cell membrane can result in cells maintaining normal shape, undergoing haemolysis, or undergoing crenation depending on the tonicity of the solution