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Biological Molecules

Starch, Glycogen and Cellulose

AQA A-level Biology

Comparing the polysaccharides

FeatureStarchGlycogenCellulose
Monomerα-glucoseα-glucoseβ-glucose
Glycosidic bonds1,4 and 1,6 glycosidic bonds1,4 and 1,6 glycosidic bondsonly 1,4 glycosidic bonds
BranchingBranchedBranchedUnbranched
ShapeCoiledCoiledLong, straight chains
Chains linkedNoNoHydrogen bonds form microfibrils
FunctionEnergy store in plant cellsEnergy store in animal cellsProvides strength to the cell wall
  • Similarities: both are polysaccharides, both are made of glucose monomers, both contain glycosidic bonds.
  • Starch is branched and cellulose is unbranched.
OOOOOOOOOOOOO1,6 glycosidic bond1,4 glycosidic bondStarch and glycogen: α-glucose, branchedOOOOOOOOOCellulose: β-glucose, straight chain
Starch and glycogen are branched chains of α-glucose; cellulose is a straight chain of β-glucose with alternate monomers inverted.

Starch and glycogen: storage

FeatureWhy it suits storage
Insolubleso does not affect water potential
Coiled and branchedso compact
Polymer of α-glucoseso provides glucose for respiration
Branchedmore ends for faster hydrolysis, so glucose is readily released
Large moleculeso cannot leave the cell

Write that insoluble starch does not affect water potential. Never write that starch doesn't affect water leaving cells.

Cellulose: strength

  1. Cellulose is made of long, straight, unbranched chains of β-glucose.
  2. Alternate glucose monomers are rotated 180°, joined by 1,4 glycosidic bonds. Never write 1,6 bonds in cellulose.
  3. Parallel chains are linked together by many hydrogen bonds.
  4. The linked chains form microfibrils.
  5. Microfibrils provide strength and rigidity to the cell wall.

Write many hydrogen bonds between cellulose chains. Never write strong hydrogen bonds between cellulose chains: each hydrogen bond is weak, and the strength comes from their number.

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