diffusion

• Diffusion


  Net Movement of particles from area of high concentration to low concentration
• Diffusion


  • Passive process - doesn't require metabolic energy (energy released by respiration)
  • Uses own kinetic energy within molecules
• Factors that affect rate of diffusion

  • Concentration gradient (greater, greater diffusion rate)
  • Particle charge/size
  • Membrane partially permeable only to small molecules
  • Charged/polar molecules (hydrophilic) can't diffuse across due to hydrophobic tails
  • Temperature (greater, greater diffusion)
  • Surface area (larger, greater diffusion)
  • Distance (greater, slower diffusion)
  • Protein channels/carriers
• Cells need substances that can only diffuse across via protein channels/carriers, so cells use Facilitated diffusion
• Facilitated diffusion


  Hydrophilic substances diffuse across via protein molecules (without touching hydrophobic centre of phospholipid bilayer)
• Carrier proteins


  Have binding site for specific chemicals, when it binds to protein, shape changes so that chemical is brought across membrane and released
• Protein channels are lined with hydrophilic amino acids and contain water, they are selective, only certain chemicals can pass through each channel
• Some channels only open in response to a trigger (chemical binding, change in voltage across membrane)