Brent Cornell

    
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 (e.g. large, polar molecules and ions) 
  
• This process is mediated by two distinct types of transport proteins – channel proteins and 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
• Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
• Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)

Channel Proteins

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

Channel Proteins versus Carrier Proteins

    
The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

• At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
• When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

Potassium Channels

• Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
• The channel is comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions
• Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage

Voltage-Gated Ion Channels