Glucose is soluble because its small molecules form hydrogen bonds with water and disperse throughout it. Starch also contains groups that can hydrogen-bond, but its very large polymer molecules interact strongly with one another, making starch relatively insoluble, especially in cold water.
The Reasoning
Water is polar. Each glucose molecule has several polar hydroxyl groups (), which form hydrogen bonds with water molecules. Water surrounds individual glucose molecules, separates them and keeps them dispersed in solution.
Starch is a polysaccharide produced by condensation of many alpha-glucose monomers. Although it also has hydroxyl groups, its molecules are much larger and form many hydrogen bonds within and between polymer chains. These interactions help starch chains associate into compact granules that water cannot easily separate into individual molecules.
| Feature | Glucose | Starch |
|---|---|---|
| Size | Small monosaccharide | Large polysaccharide |
| Water interaction | Water readily surrounds molecules | Chains remain associated in granules |
| Role | Transportable respiratory substrate | Compact storage carbohydrate |
This difference supports biological function. Glucose dissolves in cytoplasm and blood plasma, so it is readily transported and available for cell respiration. Starch's relative insolubility allows plants to store carbohydrate without it diffusing away or greatly increasing cellular solute concentration.
A common misconception is that starch is insoluble because it has no polar groups. It has many hydroxyl groups; its low solubility mainly results from molecular size, compact structure and extensive interactions between chains. Heating starch in water causes granules to swell and gelatinize rather than forming a simple molecular solution.
Exam Technique
For IB Biology B1.1 at both SL and HL, link structure to function: glucose is small, polar and transportable, whereas starch is large, compact and relatively insoluble, making it suitable for energy storage. Avoid stating only that starch “does not dissolve”; explain hydrogen bonding, polymer size and biological significance.
