Biological Molecules
Starch, Glycogen and Cellulose
OxfordAQA International AS and A-level Biology
Starch and glycogen
- Starch and glycogen are polymers of α-glucose. Starch is in plants and glycogen is in animals.
- Starch is amylose and amylopectin.
| Feature | Why it suits storage |
|---|---|
| Insoluble | does not dissolve in the cell |
| Compact: coiled and branched | a large amount of glucose is held in a small space |
| Large molecule | glucose and energy storage |
| Branched | easily hydrolysed: more ends are available for enzymes, so quick release of glucose |
Cellulose
- Cellulose is made of long, straight, unbranched chains of β-glucose, joined by 1,4 glycosidic bonds with alternating orientation of monomers.
- The chains form fibrils.
- Many hydrogen bonds link the chains.
- Fibrils provide rigidity, strength and support to cell walls.
Only 1,4 bonds in cellulose
Cellulose has only 1,4 glycosidic bonds, so never write cellulose containing 1,6 glycosidic bonds. The 1,6 bonds are the branch points of starch and glycogen.
One function only
The function of cellulose is support in plant cell walls. Never add an extra wrong function for cellulose, such as partially permeable.
Comparing the polysaccharides
| Feature | Starch | Glycogen | Cellulose |
|---|---|---|---|
| Monomer | α-glucose | α-glucose | β-glucose |
| 1,4 glycosidic bonds | Yes | Yes | Yes |
| 1,6 glycosidic bonds | Yes | Yes | No |
| Branching | Branched | Branched | Unbranched |
| Shape | Coiled chain | Coiled chain | Straight chain |
| Hydrogen bonds between chains | No | No | Yes |
| Function | Glucose and energy storage in plants | Glucose and energy storage in animals | Support in plant cell walls |
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