Starch & Glycogen: Structures & Functions
- Starch and glycogen are polysaccharides
- Polysaccharides are polymers formed from many monosaccharides joined by glycosidic bonds to form chains; these chains may be:
- Branched or unbranched
- Folded
- Straight or coiled
- Starch and glycogen are storage polysaccharides because they are:
- Compact
- Many molecules fit into a small space, so large volumes can be stored inside cells
- Insoluble
- They do not dissolve in the cell cytoplasm, so have no osmotic effect on cells; soluble molecules like glucose would lower the water potential of cell cytoplasm, drawing water into cells by osmosis
- Compact
Starch
- Starch is the storage polysaccharide of plants. It is stored as granules in plastids (e.g. chloroplasts)
- Due to the many monomers in a starch molecule, it takes longer to digest than glucose
- Starch is constructed from two different polysaccharides:
- Amylose (10 - 30% of starch)
- Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules
- The helix shape enables it to be more compact and thus it is more resistant to digestion
- Amylose (10 - 30% of starch)
Amylose structure diagram
Amylose is one of the two polysaccharides that forms starch, the storage polysaccharide in plants
-
- Amylopectin (70 - 90% of starch)
- 1,4 glycosidic bonds between α-glucose molecules but also 1,6 glycosidic bonds form between glucose molecules creating a branched molecule
- The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage
- Amylopectin (70 - 90% of starch)
Amylopectin structure diagram
Amylopectin is the other of the two polysaccharides that forms starch
Glycogen
- Glycogen is the storage polysaccharide of animals and fungi, it is highly branched and not coiled
- Liver and muscles cells have a high concentration of glycogen, present as visible granules, as the cellular respiration rate is high in these cells (due to animals being mobile)
- Glycogen is more branched than amylopectin making it more compact which helps animals store more
- The branching enables more free ends where glucose molecules can either be added or removed allowing for condensation and hydrolysis reactions to occur more rapidly – thus the storage or release of glucose can suit the demands of the cell
Glycogen structure diagram
Glycogen is a highly branched molecule used as a storage polysaccharide in animals and fungi
Summary of storage polysaccharides table
Feature
|
Starch | Glycogen | |
Amylose | Amylopectin | ||
Monomer | α-glucose | α-glucose | α-glucose |
Branched | No | Yes | Yes |
Helix shape | Yes | No | No |
Glycosidic Bond Present | 1,4 | 1,4 and 1,6 | 1,4 and 1,6 |
Source | Plants | Plants | Animals |
Examiner Tip
Be clear about the differences between starch (amylose and amylopectin) and glycogen.