4.2 - Diffusion

• movement of particles

  constantly move randomly due to their kinetic energy so they constantly bounce off one another and other objects
• diffusion

  net movement of particles from an area of high concentration to an area of low concentration down their concentration gradient until they are evenly distributed
• what type of process is diffusion and why

  a passive process because no energy is needed, it comes from natural motion of particles
• simple diffusion

  when molecules diffuse directly through a cell membrane, through the gaps between phospholipids
• what 2 gases undergo simple diffusion and why

  oxygen and carbon dioxide

  they're small so can pass through spaces between phospholipids

  they're non-polar so are soluble in lipids so dissolve in hydrophobic bilayer
• factors affecting simple diffusion

  concentration gradient: the higher it is the faster diffusion is (lowers over time so diffusion slows over time)

  thickness of exchange surface: the thinner it is the shorter the distance particles have to travel so diffusion is faster

  surface area: the larger it is the faster diffusion happens
• microvilli

  projections formed by cell surface membrane

  folds up on itself so cell has a larger surface area so increases rate of diffusion
• facilitated diffusion

  process by which large polar molecules and charged ions diffuse through carrier proteins or channel proteins in the membrane down a concentration gradient
• what type of process is facilitated diffusion

  passive process as it doesn't require energy
• problem with large large molecules diffusing through phospholipids

  they're too big so diffuse very slowly through phospholipid bilayer
• problem with charged particles diffusing through phospholipids

  diffuse slowly because they're water soluble and the centre of the bilayer is hydrophobic
• proteins channels

  water filled hydrophilic channels across membrane that allows water soluble ions through
• function of protein channels

  controls entry and exit of ions by being selective: each opening opens in the presence of a specific ion
• how protein channels work

  ions bind to protein making it change shape in a way that closes one side of the membrane and opens the other
• what carrier proteins do

  they facilitate diffusion of large molecules across membranes down their concentration gradient

  different carrier proteins facilitate diffusion of different molecules
• how carrier proteins work

  complementary molecule binds to protein causing it to change shape in a way that releases the molecule on the opposite side of the membrane
• factors affecting facilitated diffusion

  concentration gradient: the higher it is the faster diffusion is (decreases over time so diffusion decreases over time too)

  number of channel or carrier proteins: the more there are the faster diffusion is
• aquaporins

  channel proteins that allow facilitated diffusion of water through cell membranes

  allow cells to reabsorb water that would have been excreted by the body
• other factors that affect diffusion

  temperature: high temperature gives molecules more kinetic energy so they move faster so rate of diffusion increases

  type of molecule or ion: large and polar molecules take longer to diffuse by simple diffusion, hence the facilitated diffusion