Polysaccharides

• Polysaccharides

  polymers formed by combining many monosaccharide molecules joined by glycosidic bonds formed by condensation reactions. Polysaccharides are very large so they are insoluble making them suitable for storage.
• Test for starch:

  1. Place 2cm3 of sample being tested into a test tube
  2. Add two drops of iodine solution and shake or stir
  3. Presence of starch is indicated by yellow to blue black colour change.
• Starch stores energy in plants and is a mix of two polysaccharides:

  • Amylose = unbranched chain of glucose molecules joined by 1,4 glycosidic bonds meaning they are coiled so they're very compact storing a lot of energy.
  • Amylopectin = branched, made up of glucose molecules joined by 1,4 and 1,6 glycosidic bonds. Many side branches where each can be acted on simultaneously by enzymes, glucose monomers can be released more rapidly.
• Main role of starch is for storage and their structure especially suited to it:

  • Insoluble = does not affect water potential so water isn't drawn out via osmosis
  • Large and insoluble so wont diffuse out of cells
  • Compact = a lot of it can be stored in a small space.
  • When hydrolysed it forms alpha glucose which is both easily transported and readily used in respiration
  • Branched form has many ends which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly.
• Glycogen: structure suits role of storage because:

  • It is insoluble so doesn't draw water into cell via osmosis and also does not diffuse out of cell
  • Compact = a lot can be stored in a small space
  • More highly branched than starch so more ends that can be acted on simultaneously by enzymes, glucose monomers can be released more rapidly, important for animals as they have a higher metabolic and respiratory rate than plants as they are more active.
• Cellulose structure


  • Suited to its function of providing support and rigidity
• Cellulose


  Made up of monomers of beta glucose with straight unbranched chains running parallel to one another, allowing hydrogen bonds to form cross linkages between adjacent chains
• Cellulose


  • Overall number of hydrogen bonding contributes to strengthening cellulose making is a valuable structural material
• Cellulose


  A major component of plant cells providing rigidity
• Cellulose cell wall


  1. Prevents cell bursting when water enters by osmosis (osmotic pressure)

  2. Exerts an inwards pressure stopping any further influx of water
• Cellulose in Plant cells

  • Turgid and push against one another, making non woody parts of plant, semi rigid
  • Important in maintaining stems and leaves in a turgid state to maximise surface area for photosynthesis
• Cellulose molecules


  Grouped together to form microfibres then arranged in parallel groups called fibres