Facilitated Diffusion

• Why can't all substances diffuse across the cell membrane?

  The center of the bilayer is hydrophobic.
• What substances can't simply diffuse across the cell membrane?

  Water-soluble ions and polar molecules.
• What substances can simply diffuse across the cell membrane?

  Hydrophobic substances.
• What polar molecule can diffuse across the membrane and why?

  Water molecules, as they are very small.
• What is facilitated diffusion?

  The movement of molecules from an area of higher concentration to an area of lower concentration across a cell membrane with the help of transport proteins.
• Why do cells use facilitated diffusion?


  To transport large or polar molecules across the cell membrane needed in the cell.
• What do polar molecules use to diffuse across the cell membrane?

  Proteins.
• What are the two proteins involved in facilitated diffusion?

  Carrier proteins and channel proteins.
• What do carrier proteins have that protein channels don't?

  A binding site for a specific molecule.
• How do carrier proteins work in facilitated diffusion?

  Molecules bind to specific carrier proteins which causes them to change shape and release the molecule across the other side of the membrane.
• What does a protein channel have that a carrier protein doesn't?

  A central pore.
• What are protein channel pores lined with?

  Hydrophilic amino acids.
• What transport protein in the cell membrane is selective?

  Protein channels.
• Why are protein channels selective?

  So the channel can have control over the entry and exit of ions.
• What process is facilitated diffusion?

  A passive process.