AS Biology 9700: Starch, Glycogen and Cellulose Structure-Function

Starch, glycogen and cellulose are all polysaccharides built from glucose monomers, but each has a different structure that suits its biological role.

(a) Explain what is meant by a condensation reaction, using the formation of a glycosidic bond between two α-glucose molecules as your example. [2]

(b) Amylopectin (a branched component of starch) and glycogen are both highly branched polysaccharides made of α-glucose. Explain how this branched, coiled structure makes amylopectin and glycogen well suited to their function as storage carbohydrates. [3]

(c) Cellulose is made of β-glucose monomers linked into long, straight, unbranched chains, with hydrogen bonds forming between adjacent parallel chains. Explain how this structure makes cellulose an effective structural material in plant cell walls. [3]

Part (a): Condensation and the glycosidic bond

A condensation reaction occurs when the -OH group on carbon 1 of one α-glucose molecule reacts with the -OH group on carbon 4 of a second α-glucose molecule. A molecule of water is released, and a covalent 1,4-glycosidic bond forms between the two glucose units, joining them together. Repeating this reaction many times over builds up a long polysaccharide chain.

Part (b): Why branching suits a storage role

Amylopectin and glycogen are both branched: alongside the 1,4-glycosidic bonds running along each chain, occasional 1,6-glycosidic bonds create side branches. Two consequences follow from this:

Many free ends. Branching produces a large number of free (non-reducing) ends on the molecule, so many enzyme molecules can hydrolyse glycosidic bonds simultaneously, releasing glucose rapidly whenever the cell’s energy demand rises.
Compact and osmotically inactive. The branched, coiled shape packs a large number of glucose units into a small, relatively insoluble molecule, so a large mass of stored glucose has little effect on the water potential of the cell (unlike storing the same amount of glucose as free, soluble monosaccharides).

Glycogen is more extensively branched than amylopectin, which fits its role in liver and muscle cells, where glucose is often needed especially quickly and in large amounts.

Part (c): Why cellulose’s structure suits the cell wall

In cellulose, β-glucose monomers are joined by 1,4-glycosidic bonds, but because each glucose molecule is inverted (rotated 180°) relative to its neighbour, the resulting chains are long, straight and unbranched, rather than coiled. These straight, parallel chains lie alongside one another, and numerous hydrogen bonds form between the -OH groups of adjacent chains. Although each individual hydrogen bond is weak, the very large number of them acting together cross-links many parallel chains into bundles called microfibrils, which group further into fibres within the cell wall. This gives cellulose great tensile strength, allowing the wall to resist stretching and provide mechanical support to the plant cell, while remaining freely permeable to water and solutes.

Final answers

(a) Condensation between two α-glucose molecules releases water and forms a glycosidic bond.
(b) Branching gives many free ends for rapid enzyme hydrolysis, and a compact, insoluble shape that does not disturb water potential.
(c) Straight, unbranched chains cross-linked by many hydrogen bonds form strong microfibrils, giving cellulose high tensile strength.