2.2.3

Examples of Carbohydrates

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Starch

Starch is a polysaccharide formed by condensation reactions of alpha-glucose molecules.

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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.
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Features of starch

  • Starch does NOT change the water potential in a cell because it's insoluble in water.
  • Starch is made up of amylopectin and amylose, which are both alpha-glucose polysaccharides.
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Amylopectin

  • Amylopectin is a highly branched chain of alpha-glucose monomers.
    • The branched structure means that enzymes can easily access the glycosidic bonds and so the glucose molecules can be quickly released when needed.
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Amylose

  • Amylose is a linear chain of alpha-glucose monomers.
    • Because of the way the monomers are joined, the glucose chains have a helical structure.
    • This means that the amylose strands can pack closely together, making this form of starch particularly good for storage.
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Iodine test

  • A common test for the presence of starch, particularly in food testing, is to add iodine.
  • Iodine is used commonly as a name for a solution of potassium iodide.
    • Step 1: Place a small sample into the dimple of a spotting tile or to a boiling tube.
    • Step 2: Add a few drops of iodine and observe any colour change.
  • If starch is present, the solution will change colour from orange → blue-black.

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.

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Function of 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 be released fast.
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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.

Cellulose

Cellulose is the major component of cell walls in plants.

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Structure of cellulose

  • Cellulose is a long chain of beta-glucose.
  • Beta-glucose molecules are linked by glycosidic bonds to form linear cellulose chains that are unbranched.
  • Microfibrils are strong fibres that are made of many cellulose chains that are held together by hydrogen bonds.
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Function of cellulose

  • Cellulose is the most abundant natural polymer.
  • The cell walls of plant cells are mostly made of cellulose.
    • These cell walls offer structural support because of the strength of the microfibril fibres that they are made of.
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Digestion of cellulose

  • Cellulose cannot be broken down by human digestive enzymes.
  • Herbivores, such as cows and horses, are able to digest plant material rich in cellulose.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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