Edexcel International A Level Biology

Revision Notes

4.3 Starch & Cellulose: Structure & Function

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Starch: structure

  • Starch is the storage polysaccharide of plants
  • It is stored as granules in plastids (e.g. chloroplasts) and amyloplasts (small, membrane-bound organelles containing starch granules)
  • Due to starch molecules being large polymers consisting of thousands of glucose monomers, starch takes longer to digest than glucose
  • Starch is constructed from two different polysaccharides:
    • Amylose (10 - 30% of starch)
      • Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules
      • The helix shape enables it to be more compact and thus it is more resistant to digestion

Starch and Glycogen_ Amylose

Amylose - one of the two polysaccharides present in starch

    • Amylopectin (70 - 90% of starch)
      • 1,4 glycosidic bonds between α-glucose molecules (as found in amylose) but also 1,6 glycosidic bonds form between glucose molecules creating a branched molecule
      • The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage

Starch and Glycogen_ Amylopectin

Amylopectin - the other polysaccharide present in starch

Starch: function

  • Starch is a storage polysaccharide because it is:
    • Compact (so large quantities can be stored)
    • Insoluble (so will have no osmotic effect, unlike glucose which would cause water to move into cells, meaning cells would then have to have thicker cell walls to withstand the increased internal water pressure)


Cellulose: structure

  • Cellulose is a polymer consisting of long chains of β-glucose joined together by 1,4 glycosidic bonds
  • As β-glucose is an isomer of α-glucose, consecutive β-glucose molecules must be rotated 180° to each other in order to form the 1,4 glycosidic bonds

2.2.1 Formation of Sugars 1

To form the 1,4 glycosidic bond between two β-glucose molecules, the glucose molecules must be rotated to 180° to each other

  • Due to the inversion of the β-glucose molecules many hydrogen bonds form between the long chains, giving cellulose it’s great strength

Cellulose_ hydrogen bond formation between chains

Cellulose is used as a structural component due to the strength it has from the many hydrogen bonds that form between the long chains of β-glucose molecules

Cellulose: function

  • Cellulose is the main structural component of cell walls due to its strength, which is a result of the many hydrogen bonds found between the parallel chains of microfibrils
  • The high tensile strength of cellulose allows it to be stretched without breaking which makes it possible for cell walls to withstand turgor pressure
  • The cellulose fibres and other molecules (e.g. lignin) found in the cell wall form a matrix which increases the strength of the cell walls
  • These strengthened cell walls provide support to plants

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.