Diffusion

   Diffusion is a means of passive transport.  It results from the thermal, random movement of molecules. 

Rate of Diffusion    =            gradient x temp X surface area · solubility    
                                       distance  ·  sq. rt of the molecular mass

                                        Six things influence the rate of diffusion:
                                                                          Concentration gradient
                                                                          Size of molecule involved
                                                                          Distance the molecule has to travel
                                                                          Temperature
                                                                          Solubility of the molecule
                                                                          Surface area of the membrane over which the molecule can work

There are three main types of diffusion:  simple, channel, and facilitated diffusion. 

 

Simple diffusion is when a small,
non-polar molecule passes through a lipid bilayer. 
It is classified as a means of passive transport.

In simple diffusion, a hydrophobic molecule
can move into the hydrophobic region of
the membrane without getting rejected.

Simple diffusion does not involve a protein.

 

   An example of simple diffusion is osmosis.               

          diffusion.gif (1317 bytes)   

Hydrophilic molecules cannot participate in simple diffusion because they
would move into the hydrophobic region of the membrane and be rejected.

diffusion2.gif (917 bytes)

Saturation does not occur with simple diffusion or with channel diffusion. 
The bigger the gradient, the greater the transport of materials.

                diffusion3.gif (1688 bytes)

Channel diffusion is another type of passive transport.  

Channel diffusion involves channel proteins where
material moves through an open, aqueous pore. 

Channel diffusion can be regulated. 
Ions and charged particles can
    pass through the open pore.      

diffusion5.gif (3934 bytes)
Facilitated diffusion is a type of passive transport that is dependent on single transport protein carriers. 
These protein carriers operate on a bind, flip, release mechanism.  
Facilitated diffusion is non-diffusional because the molecule moves along with the carrier.

diffusion4.gif (1122 bytes)
Saturation occurs in facilitated diffusion because not enough
carriers may be available to handle all the free solute molecules.
The rate of movement may reach a maximum.

How can you tell the difference between a carrier and a channel protein?
Manipulate the concentrations of the molecules and see if saturation occurs. 
If saturation does occur, then you know that you are dealing with a carrier protein.

BACK to main

to passive versus active transport page