Starch, Cellulose and Glycogen

Starch, Cellulose and Glycogen

1. STARCH

   • Polysaccharide
   • Major source of energy
   • Important component of food
   • Found in plants
   • In the form of small grains
   • Large amounts in seeds and storage organs
   • Made up of chains of alpha glucose monosaccharides
     • Linked by glycosidic bonds
       • Formed in a condensation reaction
     • Unbranched chains wound to form a coil
       • Makes it very compact
   • Why is it suited for energy storage?
     • When hydrolysed it forms alpha glucose which is easy to be transported and used for respiration
     • Compact so a lot can be stored in a small space
     • Insoluble
       • Doesn't diffuse easily out of cells
       • Does not draw water into the cell

2. CELLULOSE

   • Made up of chains of beta glucose monosaccharides
   • The positions of the -H group and the -OH group on a single carbon atom are reversed
     • In beta glucose the -OH group is above,rather than below the ring
       • This means that to form glycosidic links, each beta glucose molecule must be rotated 180 degrees compared to its neighbour
         • The result is that the -CH2OH group on each beta glucose molecule alternates between being above and below the chain
   • It forms straight, unbranched chains which run parallel to each other
     • This allows hydrogen bonds to form cross-linkages between adjacent chains
   • Each individual hydrogen bond is reasonably weak, however in large numbers they add considerable strength to cellulose
   • The cellulose molecules are grouped together to form microfibrils which are arranged in parallel groups called fibres
   • A major component of plant cell walls which provides rigidity
     • 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
       • As a result, living plant cells are turgid and push against one another, making herbaceous parts of the plant semi-rigid
         • This is important in maintaining stems and leaves in a turgid state so that they can provide maximum surface area for photosynthesis

3. GLYCOGEN

   • Very similar structure to starch
     • Shorter chains though, and more highly branched
   • Sometimes called 'animal starch' because it is the major carbohydrate storage product of animals
   • Stored in small granules mainly in muscles and the liver
   • Why is it suited for energy storage?
     • Compact so a lot can be stored in a small space
     • When hydrolysed it forms alpha glucose which is easy to be transported and used for respiration
       • Shorter chains and large number if free ends means its more readily hydrolysed with starch
     • Insoluble
       • Does not draw water into the cell
       • Doesn't diffuse easily out of cells