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.