Facilitated Diffusion Through Cell Membrane


  • Some molecules diffuse through the cell membrane utilising a protein carrier .
  • In this situation, the protein carrier has a special affinity to the molecule.
  • Once the molecule is bound to the protein, a conformational change (a change in shape) occurs in the protein carrier.
  • Through this change, the bound molecule is released into the cytoplasm.
    • This diffusion is called facilitated diffusion.
    • Though a protein carrier is utilised and a conformational change occurs, the diffusion is still a simple diffusion as it occurs down the concentration gradient without any energy use.
    • Examples of facilitated diffusion are the intracellular diffusion of glucose and amino acids.
    • Special characteristics of these carriers include:
    1. Specificity - The carrier molecules interact with specific molecules e.g. glucose carriers interact only with glucose, some amino acid carriers interact with one specific amino acid.
    2. Competition - When a carrier molecule can interact with more than one molecule (e.g. a carrier molecule that can carry two different amino acids) the rate of transport of the amino acids is less when both are present than when one is present. This is because the amino acid molecules compete with each other for the special sits on the protein.
    3. Saturation - As the concentration of the tranpsorted molecule is increased, so is its rate of diffusion but this up to a certain limit. Beyond this limit no further increase is observed. This transport maximum occurs when all the interaction sites are fully occupied.