Question

Why is the cell membrane impermeable to some ions and glucose, but is permeable to alcohol and urea?

Answer 1

Alcohol and Urea have non-polar and polar properties, meaning they can pass through the membrane, whereas glucose and ions are too big.

Explanation:

Alcohols and urea both have hydrogen bonded with oxygen and nitrogen respectively.

This means that they can exhibit both polar and non-polar characteristics, and so can pass through the bi-layer, just like other polar molecules (water) and non-polar molecules.

However, ions and glucose on the other hand are simply too big, and meaning that they are physically too big to go through the bi-layer, hence they require channel proteins.