starch and cellulose

• starch

  storage polysaccharide of plants

  stored as granules in plastids (e.g chloroplasts) and amyloplasts
• starch takes longer to digest than glucose


  due to starch molecules being large polymers consisting of thousands of glucose monomers
• amylose (10-30% of starch)

  unbranched helix-shaped chain with 1,4 glycosidic bonds between alpha glucose molecules

  helix shape enables it to be more compact and so it is more resistant to digestion
• amylopectin (70-90% of starch)

  1,4 glycosidic bonds between alpha glucose molecules and 1,6 glycosidic bonds form between gucose molecules creating a branched molecule

  branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage
• starch is a storage polysaccharide because it is

  • compact (large quantities can be stored)
  • insoluble (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: polymer consisting of long chains of beta glucose joined together by 1,4 glycosidic bonds

  as beta glucose is an isomer of alpha glucose, consecutive beta glucose molecules must be rotated 180 degrees to eachother in order to form the 1,4 glycosidic bonds
• due to inversion of the beta glucose molecules many hydrogen bonds form between the long chains, giving cellulose it's great strength
• cellulose

  function: main structural component of cell walls due to its strength, which is a result of many hydrogen bonds found between the parallel chains of microfibrils

  • high tensile strength of cellulose allows it to be stretched without breaking which makes it possible for cell walls to withstand turgor pressure
  • cellulose fibres and other molecules (e.g lignin) found in cell wall form a matrix which increases the strength of the cell walls
  • strengthened cell walls provide support to plants