Diffusion: An overview

1. Molecules in a liquid, aqueous solution or a gas would have kinetic energy and would be moving in random directions.
2. If there are more of a specific type of molecule in one region and less in another region, you will find that the distribution of these molecules will even out after some time.
3. This NET (overall) movement of molecules from the more concentrated region to the less concentrated region is known as DIFFUSION.
4. The stage where the molecules are evenly distributed is known as DYNAMIC EQUILIBRIUM. (DYNAMIC: Molecules are still moving in random directions, EQUILIBRIUM: Molecules are evenly distributed)
5. This movement occurs spontaneously and does not require energy: PASSIVE TRANSPORT.
6. A similar kind of movement occurs when water molecules are involved. When this occurs across a partially permeable membrane, we call this process OSMOSIS.
7. Both osmosis and diffusion occur DOWN a water potential/concentration GRADIENT.
8. Water always enters and leaves the cells and bloodstream through osmosis.
9. For other solutes, eg. glucose, cells usually cannot just rely on diffusion to take in glucose (as it only enables cells to take up glucose until equilibrium is reached). Hence usually cells will sometimes pump in solutes using energy: ACTIVE TRANSPORT. (Not in your syllabus)
10. Main factors affecting diffusion/osmosis:
• Temperature: Increasing temp makes the molecules gain KE. More KE, faster movement, faster rate of diffusion.
• Surface area of membrane across which transport of molecules occurs: Increasing surface area means more molecules can cross the membrane at any one time, this increases rate of diffusion.
• Gradient: The steeper the gradient (ie, the greater the difference in conc between the 2 regions), the faster the rate of diffusion.