10.3 Starch, Glycogen & Cellulose

Starch
1. Starch is a polysaccharide found in plants in the form of small grains.
  1. Starch is made up of chains of α-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions.
    1. The unbranched chain is wound into a tight coil that makes the molecule very compact.
2. The main role of starch is energy storage, something it is suited for because:
  1. ♦ Starch is insoluble so doesn't draw water into the cells by osmosis.
    1. ♦ being insoluble, it does not easily diffuse out of cells.
      1. ♦ It is compact, so a lot of it can be stored in a small space.
        ♦ when hydrolysed, starch forms α-glucose, which is easily transported and readily used in respiration.
Glycogen
1. Starch is never found in animal cells. Instead, a similar α-glucose polysaccharide, called glycogen, serves the same role.
  1. Animal cells get energy from glucose, but store it as glycogen.
    1. Glycogen has a very similar structure to starch but has shorter chains and more side branches; meaning that stored glucose can be released quickly which is important for energy release in animals.
2. Glycogen is the major carbohydrate storage product of animals and is stored as small granules in the muscles and liver.
  1. Because it is made up of smaller chains, it is more readily hydrolysed to α-glucose.
Cellulose
1. Cellulose is made of long, unbranched chains of β-glucose.
  1. In the β-glucose units, the positions of the -H group and the -OH group on a single carbon atom are reversed; the -OH group is above the ring.
    1. This means that to form glycosidic links, each β-glucose molecule must be rotated by 180° compared to its neighbour.
2. Cellulose is the major component of a cell wall in plants and is what makes the cell rigid.
  1. The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils.
    1. These strong fibres mean cellulose provides structural support for plant cell walls.
      1. The cellulose cell wall prevents the cell from bursting as water enters by osmosis by exerting an inward pressure that stops any further influx of water.
        As a result, living plant cells are turgid and push against each other, making herbaceous parts of the plant semi-rigid.
        This is important to maintain stems and leaves in a turgid state so they can provide the maximum surface area for photosynthesis.
3. Cellulose has straight chains parrallel to each other, allowing hydrogen bonds to form cross-linkages between adjacent chains.

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10.3 Starch, Glycogen & Cellulose

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