Transport across cell membranes

[Cheman]

Transport across cell membranes...

I was reading my textbook and it said, "Fat-soluble molecules, such as glycerol, can diffuse through the membrane easily. They dissolve in the phospholipid bilayer and pass through it in the direction of the concentration gradient, from a high concentration to a low concentration. Water, oxygen and carbon dioxide can also diffuse through the bilayer, passing easily through the temporary small spaces between the 'tails' of the phospholipids which result from its fluidity. However, ions of any size, most polar molecules and most large molecules cannot diffuse through the phospholipid bilayer. They have to go through the protein pores."
I can understand why large molecules would be unable to pass through, ( cause they are too big to go through the gaps) but why are ions of any size and polar molecules unable to?
Thanks in advance. :-)

 

[Monique]

The question would be: why does a lipid bilayer form.. because the phospholipids like each other or because they don't like the water? It is because they don't like water, it is energetically more favorable if they are not surrounded by water molecules. That is also why molecules that don't associate strongly with water are able to diffuse through the membrane, the energy barrier is lower.

Hydrophobic molecules such as O2 and CO2 readily dissolve in lipid bilayers and therefore diffuse rapidly across them, small uncharged polar molecules like H2O and glycerol are able to diffuse but much more slowly, large uncharged polar molecules like glucose and sucrose are able to diffuse but mostly don't, and ions are not able to diffuse down their concentration gradients because they are highly hydrated.

You mentioned water passes readily through the membrane, actually it doesn't. Only just recently scientists discovered special channels that allow water to pass over the membrane.