4.2.4 Starch & Cellulose: Structure & Function

Starch

• 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

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

Amylopectin - the other polysaccharide present in starch

• 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

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 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