1.3 - Carbohydrates - Disaccharides and Polysaccharides

• disaccharides

  2 monosaccharides joined together (in a condensation reaction) by a glycosidic bond

  are non-reducing sugars

  monomer + monomer = disaccharide

  hydrolysis breaks polysaccharides into monosaccharides
• examples of disaccharides and what monomers form them

  glucose + glucose = maltose

  glucose + fructose = sucrose

  glucose + galactose = lactose
• test for non-reducing sugars

  . carry out test for reducing sugars, if it doesn't change colour there aren't any reducing sugars

  . add 2cm^3 of sample to 2cm^3 of HCl in a test tube and place in a hot water bath for 5 mins to heat it

  . HCl will hydrolyse disaccharide (non-reducing sugar) into monosaccharide (reducing sugar)

  . add sodium hydrogencarbonate to test tube to neutralise HCl and test with pH paper to check it's alkaline

  . carry out test for reducing sugars on solution - it will now be positive
• why sodium hydrogencarbon is used in test for non-reducing sugars

  to neutralise HCl in the sample as Benedict's reagent won't work in acidic conditions
• polysaccharides

  insoluble (because they're large) polymers formed by combining many monosaccharides using through condensation reactions, forming glysosidic bonds
• uses of polysaccharides

  structure: large so can't leave cell; insoluble si aren't affected by osmosis

  storage: by tightly coiling into a helical shape
• starch

  polysaccharide in plants in the form of small granules/grains

  formed by joining alpha glucose molecules together by glycosidic bonds in condensation reactions
• test for starch

  . place 2cm^3 of sample in test tube

  . add 2 drops of iodine solution and shake/stir

  . if starch is present solution will go from orange/brown to blue-black

  test done at room temperature
• glycosidic bond

  strong covalent bond formed when 2 hydroxyl- (OH) groups (on different saccharides) interact

  oxygen link that holds the 2 molecules together