facilitated diffusion

• • Some larger molecules (e.g. amino acids, glucose) would diffuse extremely slowly through the phospholipid bilayer because they're so big.
  • Charged particles, e.g. ions and polar molecules, would also diffuse slowly - that's because they're water soluble, and the centre of the bilayer is hydrophobic.
  • So to speed things up, large or charged particles diffuse through carrier proteins or channel proteins in the cell membrane instead - this is called facilitated diffusion.
• facilitated diffusion 


  large or charged particles diffuse through carrier proteins or channel proteins in the cell membrane
• Like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration. It's also a passive process - it doesn't use energy.
• Carrier proteins move large molecules (including polar molecules and ions) into or out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.
• Here's how carrier proteins work:

  • First, a large molecule attaches to a carrier protein in the membrane.
  • Then, the protein changes shape.
  • This releases the molecule on the opposite side of the membrane
• Channel proteins form pores in the membrane for smaller ions and polar molecules to diffuse through, down their concentration gradient. Different channel proteins facilitate the diffusion of different charged particles.