Polysaccharides: Structure
- Starch, glycogen, and cellulose are examples of polysaccharides
- Polysaccharides are carbohydrate polymers; repeated chains of many monosaccharides joined by glycosidic bonds in a condensation reaction
- Polysaccharides may be
- Branched or unbranched
- Being branched increases the rate at which a polysaccharide can be broken down
- Straight or coiled
- Being straight makes the molecules suitable for constructing cellular structures e.g. cellulose
- Being coiled makes a molecule more compact and suitable for storage e.g. amylose in starch
- Branched or unbranched
- Starch and glycogen are useful as storage polysaccharides because they are
- Compact; large quantities can be stored
- Insoluble; they will have no osmotic effect, unlike glucose which would increase the solute concentration of a cell and causing water to move in by osmosis
Starch
- Starch is the storage polysaccharide of plants
- It is stored as granules inside plant cells
- Plants make glucose during photosynthesis and the molecules of glucose are joined to make the polysaccharide starch
- Starch is constructed from two different polysaccharides
- Amylose
- Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules
- A helix is a spiral shape
- The helix shape enables it to be more compact and thus more can be stored
- Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules
- Amylopectin
- A branched molecule containing 1,4 glycosidic bonds between α-glucose molecules and 1,6 glycosidic bonds
- The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage
- Amylose
Amylose is a helix-shaped polysaccharide found in starch, the storage polysaccharide in plants
Amylopectin is a branched polysaccharide found in starch
Glycogen
- Glycogen is the storage polysaccharide of animals and fungi
- It is highly branched and not coiled
- It contains both 1,4 and 1,6 glycosidic bonds
- Glycogen is more branched than amylopectin
- The branching provides more terminal glucose molecules which can either be added to or removed by hydrolysis; this allows the quick storage or release of glucose to suit the demands of the cell
- This is essential in animal cells as animals are very metabolically active
- The branching provides more terminal glucose molecules which can either be added to or removed by hydrolysis; this allows the quick storage or release of glucose to suit the demands of the cell
- Glycogen is compact which means that much can be stored in a small space
- Liver and muscles cells have a high concentration of glycogen, present as visible granules; this enables a high cellular respiration rate
Glycogen is a highly branched molecule used as a storage polysaccharide in animals and fungi
Summary of Storage Polysaccharides Table
Types of Carbohydrate Summary Table
Examiner Tip
Although cellulose is an important polysaccharide you do not need to know about it in this topic
Make sure you are clear on the differences between starch and glycogen.