General ---------Biology----------------------------------------------------- ------------------Online--------------------------------------- Ch. 8 Membrane Structure and Function

Diffusion is the movement of atoms or molecules from an area of greater concentration to an area of lesser concentration, due to the random motion generated by their kinetic energy. 

Example:  Perfume will diffuse out of the bottle once it is opened.  The warmer the molecules the faster they will move and diffuse.  At first the molecules have a greater concentration inside the bottle, a lesser concentration outside the bottle, and their random molecular motion causes them to diffuse from an area of greater concentration to an area of lesser concentration.

Concentration gradient:  Assume you are standing in the room close to the perfume bottle, and your friend is sitting across the room.  You smell the perfume molecules first, then, after  a few moments, your friend does.  As your friend walks toward the bottle the smell gets stronger because the concentration of molecules is greater closer to the bottle.  Your friend could be said to have walked up the concentration gradient.  Diffusion is said to move down the concentration gradient.

Equilibrium:  If you leave the top off the perfume bottle and seal up the room, in several hours most of the molecules will have diffused out of the bottle and the smell will be evenly distributed through out the room.  At this point the molecules have reached a dynamic equilibrium.  It is dynamic because the molecules still have kinetic energy and are still moving.  However, the random motion maintains a uniform concentration.  When equilibrium is reached, diffusion is said to have stopped, although molecular motion continues.

What role does diffusion play in active transport?
Active transport would be analagous to transporting the moleucles of perfume back into the bottle.  This would require energy.  You could run around and catch each molecule with a pair of micro tweezers, open the bottle and quickly stuff the molecule inside.  Another way, analogous to what a cell does, is to drill micro holes in the perfume bottle.  Then insert proteins in the holes like little rubber stoppers.  The proteins would have their active sites on their outside surfaces.  When a perfume molecule hit the active site of the protein, the protein would change shape, using ATP as the energy source, and pump the molecule inside.  It would take a while but over time the concentration in the room would drop as the concentration in the bottle increased.