Sugars, and glucose in particular, are important molecules for cells because they are the primary energy source. Sugars have the general chemical formula CH2O and can be joined together almost infinitely for storage. However, because they are hydrophilic, they allow water molecules to intercalate between them, and cannot pack as efficiently as fats, which are hydrophobic and thus exclude water. On the other hand, the sugars can be mobilized for use more quickly. Therefore, polysaccharides are usually short-term reservoirs of energy for an organism, while fats are used for longer-term storage.
The general chemical formula cannot fully define a particular sugar, because the same set of atoms, e.g. C6H12O6 can refer to glucose, fructose, mannose, or galactose, and that doesn’t even include the stereoisomers. Isomers are rearrangements of the same atoms, such as with glucose and fructose (fig. 5), while stereoisomers are much more similar: they are mirror-images of one another. Thus glucose can exist as L-glucose or D-glucose, depending on whether it is a “left-handed” or “right-handed isomer. This may seem like an esoteric distinction, but it becomes important in intermolecular interactions, because many are based on recognition of specific shapes, so an l-conformation molecule may not be recognized by an enzyme that recognizes its d-isomer.
Another important aspect of sugar chemistry is whether it is an aldose or a ketose, based on the type of carbonyl group it carries. This is easiest to understand looking at the position of the carbonyl group in the linear structure: put simply, an aldehyde is a terminal carbonyl group, while a ketone is an internal carbonyl group. Sugars in aqueous solution exist in an equilibrium between the linear form and the ring form, which is formed by intramolecular attack by a hydroxyl group on the carbonyl. Technically, the cyclic sugar is a pyranose (6-membered ring) or a furanose (5-membered ring), so that D-glucose cyclizes into D-glucopyranose. However, in most cell biology courses, the cyclic sugar will still be referred to as its non-cyclic alter ego. Note that due to the difference between the C6H12O6 aldose glucose, and the C6H12O6 ketose fructose, cyclization generates a pyranose in one case, and a furanose in the latter (fig. 5), although the number of carbons (and other atoms) are the same. These two molecules are therefore recognized differently by the enzymes of the cell, leading to different metabolic pathways.
Figure 5. Glucose is an aldose (terminal carbonyl) that cyclizes into a pyranose, fructose is a ketose (internal carbonyl) that cyclizes into a furanose.
Simple sugars can be joined together by condensation reactions to form glycosidic bonds. These reactions are called condensation reactions because they form water as a byproduct. The glycosidic bond is an —O— linkage between carbons of two sugars. The bond is usually named with the specific linkages: for example in cellulose, glucoses are linked by β(1,4) linkages, which means in a standard ring diagram, the upward-facing β-hydroxyl on the 1-carbon interacts with the —OH on the 4-carbon of a neighboring glucose (fig. 6B). [Technically, since only two glucoses are shown here, this is a molecule of cellobiose, not cellulose.] In contrast, the maltose shown in the same figure (fig. 6A), while also showing two glucoses linked together, is an α(1,4) linkage, with a downward-facing a-hydroxyl on the 1-carbon.
Figure 6. (A) the α(1,4) glycosidic bond of maltose, (B) the β(1,4) bond of cellobiose, and (C) the α(1,6) bond in branching glycogen.
Large polysaccharides generally have one of two functions: as a very strong structural component of a cell, and as a storage molecule for readily accessible energy. The two major structural polysaccharides made by cells are cellulose and chitin. Cellulose is primarily synthesized by plants, while chitin is mostly synthesized by invertebrates (think crab shells), though it is also made by many fungi and algae. As we just saw, cellulose is an array of parallel lengths of glucose monomers joined together by β(1,4) glycosidic bonds (fig. 7). These long glucans are stacked closely on one another so that many H-bonds can form along their lengths, which are virtually limitless, determined by the needs of the organism. Interestingly, chitin is also a homopolymer linked by β(1,4) glycosidic bonds, but instead of glucose, the monosaccharide used is N-acetylglucosamine (often abbreviated GlcNAc, see chapter 11). However, the macromolecular structure is very similar to cellulose, and like cellulose, it is very strong.
Figure 7. Cellulose is a very strong material due to the many hydrogen bonds (in red) possible when strands of β(1,4)-linked glucoses are aligned.
As with structural polysaccharides, there are also two primary energy-storage polysaccharides: starch, which is synthesized by plants, and glycogen, which is synthesized by animals. Starch is actually a mixture of two slightly different polysaccharides. One is a-amylose, which is a glucose homopolymer like cellulose, but connected by α(1,4) glycosidic linkages, which makes it completely different structurally. Unlike the linear and highly stackable cellulose polysaccharides, a-amylose takes on a twisting a-helical shape. The other starch polysaccharide is amylopectin, which is like a-amylose with the addition of branches formed from α(1,6) glycosidic bonds every 24-30 residues (fig. 6C). The storage polysaccharide for animals, glycogen, is essentially amylopectin with a higher frequency of branching, approximately every 8-14 residues. Whereas the tight packing of the structural polysaccharides renders them waterproof, this is certainly not the case for starch or glycogen, both of which can interact with many water molecules simultaneously, and swell up with the hydration, as any cook who has ever made a pudding (the thickening ingredient is starch from corn) can attest.