Carbohydrates

• Glucose is a hexose sugar
• There are 2 types of glucose: alpha-glucose and beta-glucose
• A disaccharide is formed when two monosaccharides join via condensation reactions. A glycosidic bond is formed and one molecule of water is released.
• glucose + glucose = maltose
• glucose + fructose = sucrose
• glucose + galactose = lactose
• Test for reducing sugars

  • add Benedict’s reagent
  • heat in water bath
  • positive test = coloured precipitate
• test for non-reducing sugars

  • add dilute HCl and heat in water bath
  • add sodium hydrogencarbonate to neutralise
  • carry out Benedict’s test as usual
• Reducing sugars include all monosaccharides and some disaccharides eg. maltose and lactose
• Starch is a mixture of two polysaccharides of alpha glucose: amylose and amylopectin
• amylose is a long unbranched chain of alpha glucose
• starch is a good storage molecule as it is

  • insoluble - doesn’t affect water potential
  • compact
  • branched structure of amylopectin allows enzymes to access glycosidic bonds easily
• Animals store excess glucose as glycogen
• plants store excess glucose as starch
• Glycogen is a highly branched chain of alpha glucose
• cellulose is made of long unbranched chains of beta glucose
• Cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils
• Test for starch

  Add iodine dissolved in potassium iodide solution to test sample

  positive result = browny-orange to blue-black
• Alpha glucose

  
• Beta glucose

  
• Monosaccharides are the monomers from which larger carbohydrates are made
• Glucose, galactose and fructose are common monosaccharides