What Is the Structure of Starch, Glycogen and Cellulose?
Compare starch, glycogen and cellulose for AQA A-level Biology: glucose types, glycosidic bonds, branching, and how each structure supports its function.
Starch, glycogen and cellulose are all polysaccharides: large molecules made by joining many glucose molecules with glycosidic bonds. A glycosidic bond forms in a condensation reaction, which releases water; hydrolysis uses water to break the bond. The key difference is that starch and glycogen are made from α\alphaα-glucose, whereas cellulose is made from β\betaβ-glucose. This changes the shape of their chains and what they can do.
Starch
Starch is a plant storage carbohydrate made of two polysaccharides:
- Amylose has long, unbranched chains of α\alphaα-glucose joined by 1,41,41,4-glycosidic bonds. The chains coil into a compact helix.
- Amylopectin has α\alphaα-glucose chains with 1,41,41,4-glycosidic bonds and branches formed by 1,61,61,6-glycosidic bonds. Its branches provide ends from which glucose can be released.
Starch is insoluble, so storing it does not substantially lower a cell’s water potential. Its compact structure also allows a large amount to be stored in a small space.
Glycogen
Glycogen is the main storage carbohydrate in animals. Like amylopectin, it contains α\alphaα-glucose joined by 1,41,41,4-glycosidic bonds, with 1,61,61,6-glycosidic bonds at branch points. Glycogen is more highly branched than amylopectin, so it has more ends from which glucose can be released rapidly for respiration. Glycogen is also compact and insoluble, making it suitable for storage in cells.
Cellulose
Cellulose is a structural carbohydrate in plant cell walls. Its β\betaβ-glucose molecules are joined by 1,41,41,4-glycosidic bonds, with alternate glucose molecules inverted. This produces long, straight, unbranched chains. Hydrogen bonds form between neighbouring chains, holding them together in strong microfibrils that help the cell wall resist stretching. Unlike starch and glycogen, cellulose is built for support rather than glucose storage.
For example, if a diagram shows a polysaccharide with frequent 1,61,61,6-glycosidic branch points, glycogen is a better match than cellulose. Exam mistake to avoid: do not say all three are simply ‘chains of glucose’. Name the glucose isomer, describe any branching, and link the structure to storage or strength.
