Facilitated Diffusion Through Cell
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
- 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
- Special characteristics of these
- Specificity - The carrier
molecules interact with specific molecules e.g.
glucose carriers interact only with glucose, some
amino acid carriers interact with one specific
- 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.
- 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