Lecithin

The structure of lecithin is illustrated here. Glycerol, which contains three carbon atoms, serves as the backbone of the lecithin molecule. The two fatty acids are linked to glycerol at carbon atoms 1 and 2 and the phosphate group is linked to carbon atom 3. Choline is, in turn, linked to the phosphate group. Typically, though not always, the fatty acid attached to carbon 1 of glycerol is saturated, while that attached to carbon 2 is unsaturated.

To get a crude idea of the structure of the lecithin molecule, imagine a balloon with two long paper streamers attached to it. The balloon or "head" region corresponds to the polar portion of the molecule, the negatively-charged phosphate group, and the positivelycharged choline, which readily dissolve in water. The streamers or "tails" represent the nonpolar part, the long chain of 12 to 18 carbon atoms in the two fatty acids, which are insoluble in water. As a result of the nature of its head and tail groups, lecithin molecules tend to disperse themselves in water with their nonpolar tails back-to-back to form bilayers, or double layers, in which the polar heads project outward into the water. This arrangement sequesters, or conceals, the nonpolar tails away from the water, forming a structural barrier to the passage of polar and ionic molecules as shown.