Starch, Cellulose and Glycogen

Nota:

• Polysaccharide

1. STARCH
   1. Polysaccharide
   2. Major source of energy
   3. Important component of food
   4. Found in plants
   5. In the form of small grains
   6. Large amounts in seeds and storage organs
   7. Made up of chains of alpha glucose monosaccharides
      1. Linked by glycosidic bonds
         1. Formed in a condensation reaction
      2. Unbranched chains wound to form a coil
         1. Makes it very compact
   8. Why is it suited for energy storage?
      1. When hydrolysed it forms alpha glucose which is easy to be transported and used for respiration
      2. Compact so a lot can be stored in a small space
      3. Insoluble
         1. Doesn't diffuse easily out of cells
         2. Does not draw water into the cell
2. CELLULOSE
   1. Made up of chains of beta glucose monosaccharides
   2. The positions of the -H group and the -OH group on a single carbon atom are reversed
      1. In beta glucose the -OH group is above,rather than below the ring
         1. This means that to form glycosidic links, each beta glucose molecule must be rotated 180 degrees compared to its neighbour
            1. The result is that the -CH2OH group on each beta glucose molecule alternates between being above and below the chain
   3. It forms straight, unbranched chains which run parallel to each other
      1. This allows hydrogen bonds to form cross-linkages between adjacent chains
   4. Each individual hydrogen bond is reasonably weak, however in large numbers they add considerable strength to cellulose
   5. The cellulose molecules are grouped together to form microfibrils which are arranged in parallel groups called fibres
   6. A major component of plant cell walls which provides rigidity
      1. The cellulose cell wall also prevents the cell from bursting as water enters it via osmosis, by exerting an inward pressure that stops any further influx of water
         1. As a result, living plant cells are turgid and push against one another, making herbaceous parts of the plant semi-rigid
            1. This is important in maintaining stems and leaves in a turgid state so that they can provide maximum surface area for photosynthesis
3. GLYCOGEN
   1. Very similar structure to starch
      1. Shorter chains though, and more highly branched
   2. Sometimes called 'animal starch' because it is the major carbohydrate storage product of animals
   3. Stored in small granules mainly in muscles and the liver
   4. Why is it suited for energy storage?
      1. Compact so a lot can be stored in a small space
      2. When hydrolysed it forms alpha glucose which is easy to be transported and used for respiration
         1. Shorter chains and large number if free ends means its more readily hydrolysed with starch
      3. Insoluble
         1. Does not draw water into the cell
         2. Doesn't diffuse easily out of cells