Starch, glycogen, and cellulose are all polysaccharides made from glucose, but they differ in glucose orientation, structure, properties, and function. Starch and glycogen store energy, whereas cellulose provides structural support in plant cell walls.
This is SL and HL content in IB Biology subtopic B1.1, Carbohydrates and lipids.
| Polysaccharide | Structure | Main function |
|---|---|---|
| Starch | Polymer of alpha-glucose; consists of coiled amylose and branched amylopectin | Energy storage in plants |
| Glycogen | Polymer of alpha-glucose; more highly branched than amylopectin | Energy storage in animals and fungi |
| Cellulose | Polymer of beta-glucose; alternating monomers form straight, unbranched chains linked by hydrogen bonds | Structural support in plant cell walls |
All three form through condensation reactions, in which glucose monomers are joined by glycosidic bonds and water is released. They can be broken down by hydrolysis, which uses water to split these bonds.
Starch and glycogen are suitable for energy storage because they are compact and relatively insoluble, so they do not strongly affect the osmotic concentration of cells. Their branched regions provide many ends where glucose can be added or removed, allowing stored glucose to be mobilized.
In cellulose, every second beta-glucose monomer has an alternating orientation. This produces straight chains that align in parallel, with hydrogen bonds forming between them. Bundles of these chains provide high tensile strength to plant cell walls.
A common misconception is that cellulose is simply “unbranched starch.” It is not: starch contains alpha-glucose, whereas cellulose contains beta-glucose, producing fundamentally different three-dimensional structures and functions.
Exam technique: For a “distinguish” question, compare both substances directly. State the glucose form or branching pattern and link structure to function; saying only that one occurs in plants and another in animals is usually insufficient.
