Facilitated Diffusion

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•  Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport

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

facilitated diffusion

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•  Structure and function of sodium-potassium pumps for active transport and potassium channels for 

   facilitated diffusion in axons

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

potassium channel