so that the bilayer is fluid. The microviscosity of the fatty acyl groups decreases pro­ gressively down the chain so that, at the hydrocarbon interior of the bilayer, the lipid phase has a viscosity approximating that of olive oil at room temperature. As a consequence of this membrane fluidity, a phospholipid molecule is very mobile within the plane of the membrane (moving a distance of about 1-2 fLm in 1 s) but the movement of a phospholipid molecule from one side of the membrane bilayer to the other (flip-flop) is very slow. The lipid bilayer is an essentially inert and rather impermeable struc­ ture, as shown by many studies with model systems. Proteins, of course, provide the catalytic components of the membranes. as well as playing a significant structural role.