Describe the relationship between molecular size and rate of diffusion. Explain.

Answer 1

The relationship between molecular size and the rate of diffusion is an inverse relationship; more simply put, the smaller a molecule, the faster it diffuses. It's important to know that charge of the molecule matters as well, and can change this diffusion rate, but to stay focused on your original question, the reason smaller molecules can diffuse faster is because they can physically fit through or in between parts of a cell's structure to move down their concentration gradient passively. If they are too big, they have to be actively assisted in the diffusion process, or will just move a lot more slowly through the barrier in their way.

If you want to see this effect in person you could fill a kitchen colander with granulated sugar (smaller "molecule") and see how long it took for the sugar to move through the pores of the colander, and then compare it to how long it takes dry rice to move through that same colander.