Why is the cell membrane impermeable to some ions and glucose, but is permeable to alcohol and urea?
Alcohol and Urea have non-polar and polar properties, meaning they can pass through the membrane, whereas glucose and ions are too big.
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.