15.7 Lipids

235

The study of the glycome is very much more complicated than of the genome,

proteome, or lipidome, due to its enormous structural diversity, and global, high-

throughput methods are still lacking, although carbohydrate microarrays have been

developed.¹⁹ Some of the different classes requiring different experimental

approaches include glycoproteins, glycolipids, N-glycans, O-glycans, neutral gly-

cans, and sulfated (negatively charged) glycans. Mass spectrometry is the technique

of choice for analysing glycan structure once they have been isolated.

Problem. Examine whether polysaccharides could be used as the primary informa-

tion carrier in a cell.

15.7

Lipids

Lipids are not polymers, but in water they spontaneously assemble to form large

supramolecular structures (planar bilayer membranes and closed bilayer shells,

called vesicles). Lipids are amphiphiles; that is, they consist of a polar moiety (the

“head”) attached to an apolar one (the “tail”, typically an alkane chain). The struc-

tures formed when lipids are added to water depend on the relative sizes of the polar

and apolar moieties. If the tail is thinner than the head, as with many detergents,

micelles, compact spherical aggregates with all the heads facing outward, may form.

Natural lipid molecules are typically roughly cylindrical—the head has about the

same diameter as the tail—and readily form planar or slightly curved membranes

(Fig. 15.7). Obconical shapes (head larger than tail) favour convex structures of small

radius, such as endosomes or the borders of large (hydrophilic) pores in planar bilayer

membranes. Conical shapes (such as phosphatidylethanolamine, which has a very

small head) oppose this tendency.

Fig. 15.7 A bilayer lipid

membrane formed by two

apposed sheets of molecules

19 Feizi et al. (2003).