Starch & Glycogen

Biology: IB Diploma Higher Level, topic 5.1.5

Starch

Starch is a polysaccharide and a common storage compound.

Functions of starch

Starch is the main energy storage material in plants.
Starch is stored in the seeds of plants.
Starch is broken down into glucose by plants when they need more energy.
Starch can act as a source of food for humans and animals.

Features of starch

Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose).
Plants are able to synthesize glucose, and any excess glucose is stored as starch in different plant parts, including roots and seeds.
The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose.
The cells can then absorb the glucose.

Glycosidic bonds

Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds.
The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond.
Amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages).
Amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).
It should be noted amylopectin also has α 1-4 linkages between glucose monomers.,

Amylose

Amylopectin

Glycogen

Glycogen is formed by condensation reactions of alpha-glucose molecules and is the main energy storage material in animals. It can be thought of as the animal equivalent of starch.

Glycogen

Glycogen is a highly branched molecule, similar to amylopectin (a form of starch).
When animals need to release energy, glycogen's highly branched structure means that glucose can quickly be released.
This means glycogen has α 1,4 glycosidic linkages between glucose monomers and α 1,6 glycosidic branch points.
Glycogen is more branched than cellulose.

Glycogen in the human body

Glycogen is a good storage molecule because it is so compact.
When blood glucose levels decrease, glycogen is broken down to release glucose.
- This is called glycogenolysis.

Test yourself on Starch & Glycogen

Which of the following is true of starch?
Where is starch primarily stored in plants?
Which of the following is true of glycogen?
When blood glucose levels decrease, glycogen is broken down to release glucose. What is this process called?
What makes the structure of glycogen good for its function as an energy storage molecule in humans?

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