5.1.5

Starch & Glycogen

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Starch

Starch is a polysaccharide and a common storage compound.

Functions of starch

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

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

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.,

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

  • 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 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.
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Practice questions on Starch & Glycogen

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which of the following is true of starch?True / false
  2. 2
    Where is starch primarily stored in plants?Multiple choice
  3. 3
    Which of the following is true of glycogen?True / false
  4. 4
    When blood glucose levels decrease, glycogen is broken down to release glucose. What is this process called? Multiple choice
  5. 5
    What makes the structure of glycogen good for its function as an energy storage molecule in humans?Multiple choice
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Cellulose

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