Transport Across Membrane

• Simple Diffusion:

  • net movement of molecules from a region of high concentration to a region of low concentration down the concentration gradient
  • substances that can pass through the phospholipid bilateral by simple diffusion - non-polar soluble, uncharged substances, small molecules
• Factors affecting rate of simple diffusion:

  1. Steepness of concentration gradient - the greater the difference in concentration, the steeper the concentration gradient, the higher the rate of diffusion
  2. Temperature - the higher the temperature, the higher the kinetic energy of molecules, the higher the rate of diffusion
  3. Nature of molecules - Smaller, non-polar molecules diffuse faster
  4. Surface area to volume ratio - as the object size decreases, the SA:V increases, the shorter diffusion distance, the higher the rate of diffusion
• Facilitated diffusion:

  • diffusion through membrane using transport proteins from a region of high concentration to low concentration, down a concentration gradient
  • involves channel and carrier proteins
  • passive transport, no ATP is needed
  • substances that can pass through - large or water-soluble molecules, ions or polar molecules
• Factors affecting rate of facilitated diffusion:

  1. Steepness of concentration gradient
  2. Temperature
  3. Number of transport proteins available
  4. Surface area of the membrane - large surface area is able to fit more transport proteins
• Osmosis:

  • water potential is the tendency of water molecules to move from one area to another
  • osmosis is the diffusion of water from a region of high water potential to a region of low water potential, down the water potential gradient across a partially permeable membrane
• Active transport:

  • movement of molecules or ions through carrier proteins from a region of low concentration to a region of high concentration, against the concentration gradient, using energy in the form of ATP (needed for conformationial chamge of carrier protein)
• Na+ & K+ pump:

  • 3 Na+ out
  • 2 K+ in
  • ATP -> ADP + Pi
• Endocytosis & Exocytosis:

  • mechanism to transport large quantities of substances
  • requires energy in the form of ATP
  • endo - into cell (phagocytosis for solids and pinocytosis for liquids)
  • exo- out of cell