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 such as glucose, ions (Na+   , K+   , Cl-, Ca2+   ) and charged polar molecules instance for amino acids, ATP, glucose  6-phosphate. The initial group of molecules  can cross a biological membrane  unaided  and without  an input  of energy,  whereas  the latter group requires  the  presence  of  integral  membrane  transport  proteins  and,  in  many cases   an   input   of   energy   to   travel   by   the   otherwise   impermeable membrane  barrier.  Thus the plasma membrane and the membranes of internal organelles are selectively permeable barriers and maintaining a distinct internal environment.