10.3 Starch, Glycogen and cellulose

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  • Polysacchride.
  • Found in parts of a plant in the form of small grains
  • Especially in seeds and storage organs.
  • Important component of food.
  • Major energy source in most diets.

Starch is made up of, alpha glucose, which form together to form a chain of monosacchrides. Linked together by glycosidic bonds which are formed by condensation reactions. 

The unbranched chain is wounded into a tight coil that amkes the molecule very compact. This arrangement means that:

  1. Insouble--> does not effect osmosis.
  2. Doesn't easily diffuse out of cells.
  3. Compact--> a lot can be stored in a small space.
  4. when hydrolysed--> forms an alpha glucose--> easily transported and used in respiration.

Starch is NEVER found in animal cells!

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  • Similar structure to starch, BUT has shorter chains and is highly branched.
  • Sometimes called--> "animal starch"--> major carbonhydate storage product of animals.
  • Storaged as small granules mainly in muscles and the liver.
  • Structure suits it for storage, (same reasons as starch)--> but made up of smaller chains--> more readily hydolysed to alpha glucose.

It is found in animal cells NEVER plant cells.

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Cellulose is different to starch and glycogen for 1 major reason--> it is made by beta glucose.


  • The -OH is above rather than below.
  • The -H is below rather than above.
  • The beta glucose has to flip 180 degrees--> the -CH(2)OH group alternates between each beta glucose.

Cellulose forms straight unbranched chains, these run parrallel to each other, which allows hydrogen bonds to form cross-links between lines. These strengthen cellulose making it the valuable structural matieral.

Cellulose molecules group together to form microfibrils, which arrange into parallel groups called fibres. 

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Celulose use in the cell wall

Cellulose is used in the cell wall:

  • provides rigidity to the plant cell.
  • prevents the cell from bursting while water enters by osmosis--> by exerting an inward pressure-->living plant cells are turgid and push against each other--> making parts of the plant semi-rigid --> this maintains stems and leaves in a turgid state so they can provide maximum surface area for photosynthesis.
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