Why is the plasma membrane impermeable to most biological molecules?
Three items determine the permeability of a given species to cross the plasma membrane. 1) polarity – (Hydrophobic vs Hydrophylic) 2) charge – (charged vs uncharged) 3) size – (large vs small) Hydrophobic is easier to cross like gases such as O2 and N2. Reason: the interior of the plasma membrane is a fatty acid that is extremely hydrophobic. Uncharged is easier to cross and smaller is easier to cross. urea, CO2, O2 cross fairly readily based on concentration gradients. That is good as the cell does not have to expend energy to keep these molecules in a proper concentration. Anything polar or charged would have a hard time getting between adjoining phospholipids as it would disrupt the favorable energetics. That is good as the membrane is in an aqueous media and needs to hold up. The cell regulates the crossing of almost every other type of molecule through facilitated or active transport.
