4. Facilitated Diffusion and Active Transport


The large size of charged nature of some molecules prevents them from being able to move directly through a membrane. So facilitaed diffusion and active transport play a role...

Facilitated Diffusion:

  • some larger molecules would take too long to diffuse through because they are so BIG.
  • charged particles (ions) also diffuse slowly because they're water soluable, and the bilayer is hydrophobic so it would repel it.
  • so to speed it up they diffuse through carrier proteins or channel proteins 
  • like diffusion, facilitated diffusion moves down a concentration gradient, from a high to low concentration
  • passive- no energy required 
  • Carrier Proteins: move large molecules into or out of the cell, down the conc. gradient 
    • here's how they work:
      • a large molecule attaches to a carrier protein in


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