1.2 - carbohydrates

• Monosaccharides - The monomers from which larger carbohydrates are made
• 1. Disaccharides - Formed by the condensation of 2 monosaccharides
• 1. Polysaccharides - Formed by the condensation of many monosaccharides
• Monosaccharides and disaccharides are simple carbohydrates (sugars).
• examples of monosaccharides - glucose, fructose and galactose.
• examples of disaccharides - maltose, sucrose, lactose
• Glucose+glucose = maltose
• Glucose+fructose = sucrose
• Glucose+galactose = lactose
• A condensation reaction between 2 monosaccharides forms a glycosidic bond.
•  Isomer: same molecular formula but differently arranged atom
•  Difference in structures:

  •  OH group is below C1 on alpha -glucose but above C1 in beta -glucose 
• Examples of polysaccharides glycogen, starch and cellulose.
• • Function of glycogen : energy store in animal cells 
• • Structure of glycogen : polysaccharide of alpha-glucose with C1-C4 and C1-C6 glycosidic bonds so branched
• Structure of glycogen related to its function: 

  • Branched; can be rapidly hydrolysed to release glucose for respiration to provide energy 
  • Large polysaccharide molecule; can’t leave cell 
  • Insoluble in water; water potential of cell not affected i.e. no osmotic effect
• Function of starch: energy store in plant cells
• • Structure of starch : polysaccharide of alpha-glucose. Mixture of amylose and amylopectin; amylose has C1-C4 glycosidic bonds so is unbranched, while amylopectin has C1-C4 and C1-C6 glycosidic bonds so is branched.
• Structure of starch related to its function (amylose): 

  • Helical; compact for storage in cell 
  • Large polysaccharide molecule; can’t leave cell 
  • Insoluble in water; water potential of cell not affected i.e. no osmotic effect
• • Function of cellulose: provides strength and structural support to plant cell walls
• Structure of cellulose related to function: 

  • Every other beta-glucose molecule is inverted in a long, straight, unbranched chain 
  • Many hydrogen bonds link parallel strands (crosslinks) to form microfibrils (strong fibres) 
  • H bonds are strong in high numbers 
  • Provides strength and structural support to plant cell walls
• Reducing sugars

  • All monosaccharides e.g. glucose

  • Some disaccharides e.g. maltose/ lactose
• Non-reducing sugars

  • No monosaccharides

  • Some disaccharides e.g. sucrose
• Benedict’s test for reducing sugars:

  1. Add benedict’s reagent (blue) to sample 

  2. Heat in a boiling water bath 

  3. Positive = green / yellow / orange / red precipitate (reducing sugar present)
• Benedict’s test for non-reducing sugars:

  1. Add a few drops of dilute hydrochloric acid (hydrolyse sugar into its constituent reducing sugars) 

  2. Heat in a boiling water bath 

  3. Neutralise with sodium bicarbonate 

  4. Add Benedict’s reagent and heat again 

  5. Non-reducing sugar present = green / yellow / orange / red precipitate
• iodine test for starch:

  1. Add iodine dissolved in potassium iodide to solution and shake/stir

  2. Blue-black colour = starch present