Physio ex 9

For a given concentration gradient, a molecule's rate of diffusion is inversely proportional to its frictional coefficient, which depends on both size and shape. If you assume a constant shape (e.g. a sphere, for simplicity), then rate of diffusion is inversely proportional to the radius (or diameter) of the diffusing molecule. Assuming particle density doesn’t change, particle mass is proportional to the cube of particle radius.

So, given those assumptions, diffusion is inversely proportional to the cube root of molecular weight. In other words, an 80,000 MW spherical protein will diffuse half as fast as a 20,000 MW spherical protein (because the 80kDa protein has twice the diameter of the 20kDa protein).

If molecule shape is not constant, then diffusion will vary independent of size and molecular weight. For example, and long, rod-shaped protein of 20kDa will diffuse more slowly than a spherical protein of the same MW.

For starters, it's molecular mass, not weight. The larger mass atoms have a bigger physical appearance, hence the rate of diffusion decreased. You could try an experiment to prove my point. Take 2 balloons and blow one up with hydrogen and the other with helium. After a few days, the hydrogen one would shrink more than the helium-filled one. This is because hydrogen has an atomic mass of 1 but helium has like, 4. It is something like, water molecules, lighter and smaller in size and pass through a partially permeable membrane, but sand particles, heavier, coarser and larger, takes some time to pass through.

Thank you