Membrane permeability. Illustration of a phospholipid bilayer (cell membrane), which has varying permeability to different molecules and ions. Permeability is high for small, uncharged molecules such as carbon dioxide (CO2), nitrogen (N2), oxygen (O2) and ethanol. Permeability is moderate for water and urea. Permeability is low for polar organic molecules like glucose. Permeability is very low for ions and large macromolecules like sodium, potassium, magnesium, calcium, chloride, carbonate, and phosphate ions, and amino acids, proteins, polysaccharides and nucleic acids (DNA and RNA). For this artwork with labels, see image C023/8768.