Diffusion

• Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration. Molecules will diffuse both ways, but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas
• A concentration gradient is the path from an area of higher concentration to an area of lower concentration - particles diffuse down a concentration gradient
• Diffusion is a passive process - no energy is needed for it to happen. Particles can diffuse across cell membranes, as long as they can move freely through the membrane
• what is simple diffusion?

  when molecules diffuse directly through a cell membrane e.g. oxygen and carbon dioxide - small so can pass through spaces between phospholipids, non-polar - makes them soluble in lipids - can dissolve int he hydrophobic layer
• Concentration gradient

  The higher it is, the faster the rate of diffusion
• Diffusion

  1. Difference in concentration between the two sides of the membrane decreases

  2. Reaches an equilibrium (the concentration on both times is equal)

  3. Diffusion slows down over time
• Factors affecting diffusion

  • Concentration gradient
  • Thickness of exchange surface
  • Surface area
• What is facilitated diffusion?

  some larger molecules (e.g. amino acids and glucose) would diffuse extremely slowly through the phospholipid bilayer because too big. Charged particles e.g. ions and polar molecules would also diffuse slowly - because they are water soluble and the centre of the bilayer is hydrophobic. Therefore to speed things up, large or charged particles diffuse through carrier or channel proteins in the cell membrane
• Like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher, to a lower concentration - also a passive process - doesn't use energy.
• Carrier proteins?

  move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules

  1. first a large molecule attaches to a carrier protein in the membrane
  2. the protein changes shape
  3. this releases the molecule on the opposite side of the membrane
• Channel proteins?

  Form pores in the membrane for charged particles to diffuse through, down their concentration gradient. Different channel proteins facilitate the diffusion of different charged particles
• Factors affecting facilitated diffusion?

  1. concentration gradient - higher the gradient the faster the rate - up to a point - as equilibrium reached, rate of FD will level out
  2. number of channel/carrier proteins - once all proteins in a membrane are in use, FD cannot happen any faster, even if increase concentration gradient
• How to calculate rate of diffusion from a graph?

  straight line graph - gradient of line

  curved graph - tangent - gradient of tangent