Membrane permeability

A pure  phospholipid  bilayer with  its  hydrophobic  interior,  is permeable  to water and gases such as O2, CO2, N2 and small uncharged  polar molecules such as urea, ethanol, but is impermeable  to large uncharged polar molecules like glucose, ions (Na⁺ , K⁺ , Cl^–, Ca²⁺ ) and charged polar molecules example for amino acids, ATP, glucose  6-phosphate. The first group of molecules  can cross a biological membrane  unaided  and without  an input  of energy,  while  the latter group needs  the  presence  of  integral  membrane  transport  proteins  and,  in  some cases   an   input   of   energy   to   travel   through   the   otherwise   impermeable membrane  barrier.  Therefore the plasma membrane and the membranes of internal organelles are selectively permeable barriers and maintaining a distinct internal environment.