What is a reason for failure of some molecules to diffuse through a tube?

Answer 1

One important principal in Biology and chemistry is that like dissolves like. This applies most particularly to substances that are known as either polar or nonpolar. A polar substance, like water, has an uneven distribution of charge throughout the molecule, whereas charge distribution is uniform (and often neutral) throughout a nonpolar molecule. If a membrane is polar, then polar molecules will be able to diffuse through it more easily than nonpolar ones, and vice-versa.

However, in nature it is not usually that simple, and there are usually other factors involved, such as the size of the molecule trying to pass through the membrane. In biology, the membranes that are most frequently studied are what are known as lipid bilayers, and they form many important barriers around and inside of cells. Lipid bilayers contain both polar and nonpolar regions, so polarity definitely has an effect, and nonpolar molecules pass through the membrane much more easily than polar molecules do. However, although polar, water has no trouble passively diffusing across a lipid bilayer because of its small size. Gases also pass through with relative ease.

In addition to this, the passage of molecules is also affected by something called gradients. If there is a large amount of a molecule on one side of the membrane and a small amount on the other, the molecules will tend to diffuse toward the side with the lower amount. Molecules always want to move down their concentration gradients, so as to make everything neutral and balanced.

Hope that helped!