transport across membranes

• Simple diffusion


  The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached
• Simple diffusion does not require ATP
• Diffusion


  • Occurs in liquids and gases, not solids
  • Molecules must be small and lipid-soluble to diffuse
• Facilitated diffusion


  Passive process that uses proteins embedded in the membrane to transport molecules from high to low concentration
• Facilitated diffusion


  • Enables transport of polar molecules and large molecules that cannot diffuse through the phospholipid bilayer
  • Uses protein channels or carrier proteins
• Protein channel facilitated diffusion


  1. Protein embedded in membrane forms a water-filled hollow tube

  2. Water-soluble ions or molecules can dissolve and pass through
• Carrier protein facilitated diffusion

  1. Molecule must be complementary in shape to bind to carrier protein

  2. Binding causes protein to change shape and release molecule on other side
• Osmosis


  The movement of water from an area of higher water potential (less negative) to an area of lower water potential (more negative) through a partially permeable membrane
• Water potential


  The pressure created by water molecules, measured in kilopascals (kPa)
• Pure water has a water potential of 0 kPa
• Isotonic


  When the water potential on both sides of a membrane are the same
• Hypotonic


  When the water potential of a solution is more positive (less negative) compared to the cell
• Hypertonic


  When the water potential of a solution is more negative compared to the cell
• Hypotonic solution and animal cells

  Water moves into the cell by osmosis, causing the cell to burst (lyse)
• Hypertonic solution and cells

  Water leaves the cell by osmosis, causing the cell to shrivel up
• Active transport


  The movement of molecules and ions from an area of lower concentration to an area of higher concentration, against the concentration gradient, requiring ATP and carrier proteins
• Active transport mechanism


  1. Carrier protein acts as a pump

  2. Specific molecules can bind to receptor site

  3. ATP binds to protein, providing energy to change shape and release molecules on other side