Storing carbohydrate in plants
Plants store carbohydrate in the form of starch. Starch contains many α-glucose units joined by glycosidic bonds and may be stored as granules inside plastids such as chloroplasts.
Its compact structure allows substantial quantities to be stored in a small volume. Because starch is insoluble, storing it has little osmotic effect on the cell compared with storing large numbers of free glucose molecules.
Starch contains two different polysaccharides: amylose and amylopectin.
Amylose and amylopectin
- Amylose forms roughly 10–30% of starch. It is an unbranched chain of α-glucose with α-1,4 glycosidic bonds.
- The amylose chain coils into a helix, making it compact and relatively resistant to digestion.
- Amylopectin forms roughly 70–90% of starch.
- Its chains contain α-1,4 glycosidic bonds, with α-1,6 glycosidic bonds producing branch points.
Amylopectin's branches provide numerous ends at which glucose units can be added for storage or removed by hydrolysis when carbohydrate is required for respiration.
As starch contains many glucose units joined together, its complete digestion involves considerably more bond-breaking than the use of a single glucose molecule.
Exam Tip: Link each storage adaptation to its consequence: insoluble → little osmotic effect, compact → much can be stored in a small space, and amylopectin branches → many sites from which glucose can be released.