Key Questions

  • Any time you witness one substance gradually spreading, that's diffusion.

    Sometimes this is a gas diffusing in another gas. If yoyou leave the lid off a bottle of perfume, soon the whole room will smell - because of diffusion. If your nostrils have ever been the unfortunate recipients of flatulence that has spread through a room, you've also experienced it.

    In terms of liquids, make yourself a cup of tea. Literally. Then watch it brew. The "tea" (or, more correctly, the tannins in the tea) will start out strongest nearest the tea leaves, and then spread out from there.

    Regardless of the type of diffusion, it always occurs randomly, and will continue until dynamic equilibrium is reached.

  • Answer:

    Any large or polar molecule usually requires facilitated diffusion and cannot cross the lipid bilayer via simple diffusion.


    Since the outer and inner parts of the lipid bilayer are non-polar, polar molecules cannot be diffused across. For example, in order for a potassium ion to enter the cell, it must pass through a specialized transmembrane protein. These ions and molecules move down the concentration gradient and therefore requires no energy. Glucose is a large molecule that relies on transport proteins and the movement of sodium ions to enter the cell, instead of ATP.

    Here is a discussion of how transport proteins such as channel proteins are involved in moving materials in or out of cells by the process of facilitated diffusion.

    Video from: Noel Pauller

    Hope this helps!

  • Molecules diffuse from areas of high concentration, to areas of low concentration, down a concentration gradient.

    The concentration gradient therefore represents the concept that, just as a ball rolls down a slope, during diffusion molecules move down the concentration gradient.

    Higher concentration gradients will result in higher rates of diffusion. As the molecules move the gradient evens out until equilibrium is reached.