In general, small and/or hydrophobic molecules, such as water itself, lipids and gas molecules, diffuse across the cell membrane. ... Large and/or hydrophilic molecules, such as proteins and carbohydrates, have increasing difficulty crossing the cell membrane or do not cross at all. read more
The membrane is composed of a phospholipid bilayer (which has a fluid consistency) in which proteins are wholly or partially imbedded (in a mosaic pattern). The membrane forms a solubility barrier which separates the contents of the enclosed volume from its surroundings. read more