Polysaccharides (Edexcel A Level Biology (A) SNAB): Revision Note

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

• 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

  • 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 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

• 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

Polysaccharides: Function

• Starch and glycogen are storage polysaccharides; they are adapted for this function by being

  • Compact

    • Large quantities can be stored

  • Insoluble

    • They will have no osmotic effect on cells, unlike glucose which can dissolve and raise the solute concentration of cell cytoplasm, causing water to move into cells by osmosis

Starch

• Starch is the storage polysaccharide of plants; it is stored as granules in plastids 

  • Plastids are membrane-bound organelles that can be found in plant cells

    • They have a specialised function, e.g. amyloplasts store starch grains and chloroplasts carry out photosynthesis

• The amylose in starch has a helical structure which makes it very compact, meaning that much can be stored in a small space

• The amylopectin in starch has branches that provide many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added for storage

Glycogen

• Glycogen is the storage polysaccharide of animals and fungi

• 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

• 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