Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein

• It is utilised by molecules that are unable to freely cross the phospholipid bilayer (large, polar molecules and ions) 
  

• This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins
  

Channel Proteins

• Integral lipoproteins which contain a hydrophilic pore via which ions may cross from one side of a membrane to the other
  

• Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
  

• Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
  

• Channel proteins have a much faster rate of transport than carrier proteins
  

Carrier Proteins

• Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane
  

• Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
  

• When a carrier protein moves material against the gradient (using ATP hydrolysis) it is called a protein pump
  

• Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)
  

Facilitated Diffusion