2.2.6—Molecular structure of polysaccharides
- Syllabus
- 9700–2028–2029
- Objective
- 2.2.6
- Level
- AS
Polysaccharide structure sets storage or structural function
Starch, glycogen and cellulose are polysaccharides made from glucose, but their glucose linkage and chain architecture differ. Those differences change how compact the molecule is, how accessible its ends are and whether it is suited to storage or structural support.
| Polysaccharide | Monomer and main links | Architecture | Function linked to structure |
|---|---|---|---|
| Amylose (starch) | α-glucose; α-1,4 glycosidic bonds | Unbranched, coiled/helix-forming chain | Plant glucose storage in a compact, insoluble form |
| Amylopectin (starch) | α-glucose; α-1,4 backbone with α-1,6 branch links | Branched chain | Plant storage with more accessible ends than amylose |
| Glycogen | α-glucose; α-1,4 backbone with α-1,6 branches | More highly branched than amylopectin | Animal and fungal storage; many ends support rapid glucose release |
| Cellulose | β-glucose; β-1,4 glycosidic bonds | Straight, unbranched chains aligned in parallel; hydrogen bonds form strong fibres | Plant cell-wall support because fibres provide tensile strength |
The α- or β-glucose form determines the geometry of the glycosidic chain. Branching creates more chain ends for enzymes to access, while cellulose’s β-linked straight chains align and hydrogen-bond into fibres. Insolubility allows storage polysaccharides to hold many glucose units without producing the same osmotic effect as free glucose.
Do not treat starch and glycogen as interchangeable: both store glucose, but glycogen is more highly branched. Do not explain cellulose strength using glycosidic bonds alone; the alignment of chains and hydrogen bonds between them are also essential. Branching changes accessibility and packing, not the energy content of each glucose unit.
ConceptA-Level CAIE Biology AS