This infl uences how cell membranes work because the phosphate heads can mix with
water and the lipid tails cannot. Therefore, the
lipid-based membrane prevents water-soluble
substances from fl owing freely between intracellular
and extracellular fluids. The phospholipid bilayer
forms a protective barrier around the cell and is
impermeable to all but lipid-soluble substances such
as fatty acids, fat-soluble vitamins, steroids, oxygen
and carbon dioxide (Tortora and Derrickson
2009a, Porth 2011).