2.3 Carbohydrates - disaccharides and polysaccharides

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  • carbohydrates - disaccahrides and polysaccahrides
    • when combined in pairs monosaccharides from disaccharides e.g. Glucose - glucose forms maltose, Glucose -fructose forms sucrose, Glucose to galactose forms lactose
      • condensation reaction- monosaccharides join, molecule of H2O removed forming glycosidic bond. When H2O added breaking glycosdic bond releases monosccharide - hydrolysis
    • Test for non-reducing sugars. Some disaccharidesare non-reducing suagrs, so  broken down by hydrolysis - monosaccharide components before using benedicts test
      • 1. if sample not liquid must be ground up with water, then 2cm3 and 2cm3 of benedicts into test tube
        • 2. place in water bath for 5 mins, non -reducing sugar = no colouration
          • 3. add another 2cm3 sample to 2cm hydrochloric acid in test tube. put in water bath for 5 mins. Hydrochloric acid hydrolyse disaccharide into monosaccharide
            • 4. slowly add sodium hydrogencarbonate solution to neautralise acid, test with pH paper to check alkaline
              • 5. re-test solution using benedicts test
    • polysaccharides - polymers formed from monosaccahrides joining in condensation reactions. very large -> insoluble.
      • When hydrolysed break down into di or monosaccharides
      • some polysaccharides - not used for storage but structural support e.g. cellulose
      • starch - common polysaccharide found in plants in form of small granules or grains, formed by linking a-glucose molecules by glycosdic bonds in condensation reactions
    • test for starch - easily detected by changing iodine from yellow - blue-black at room temp
      • 1. 2cm3 of sample into test tube
        • 2. add 2 drops of iodine and shake or stir
          • presence of starch indicated by blue-black coloration


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