There are several mechanisms allowing material to cross the cell membrane. Simple diffusion is thought sufficient for small uncharged molecules- CO2, urea, ammonia; channels and trasporter proteins are used for ions and larger molecules like glucose. There is a water channel as well (aquaporin).
Even larger things are transported in via endocytosis.
Thanks for the reply,
My question is about diffusion across membrane. Steroid hormones are able to diffuse while peptide hormone can't. If the membrane is amphipathic (hydrophobic and hydrophilic), why does it selectively allow hydrophobic stuff to pass thru? and not hydrophilic. Thanks
Well water can diffuse through a membrane and it's pretty hydrophillic.
What you're really looking for though is unfavorable energetics. Polar molecules are far less favored to diffuse through a membrane than a lipid molecule.
And that stems from the fact that they are in solution. Polar and charged ions will be favored to stay in their current solvent or stick to the surface of the bi-layer since, in order to pass through, they would need energy to shed their solvent and their interactions with the charged phosphates on the surface so they could pass through.
Lipid molecules interact much more weakly even with like-minded molecules ... just through van der waal's. Therefore, they're not covered in a solvent shell, and it's not that much more unfavorable for them to pass through the charged phosphate layer than it was for them to pass through the water they were just in on the other side especially if it's down their concentration gradient. The van der waals forces that would "stick" them into the middle of the bilayer are also weak, so if there is a concentration gradient providing energy for diffusion all the way through, they'll diffuse.