Sugars

• Monomer

  Small, basic molecular units that can form a polymer
• Polymer

  Large, complex molecules composed of long chains of monomers joined together
• Monomers

  • monosaccharides
  • amino acids
  • nucleotides
• Polymers

  • carbohydrates
  • proteins
  • nucleic acids
• Making polymers

  1. Monomers undergo condensation reactions

  2. Condensation reaction forms a chemical bond between monomers

  3. Releases a molecule of water
• Breaking down polymers

  1. Biological polymers can be broken down into monomers by hydrolysis reactions

  2. Hydrolysis reaction breaks the chemical bond between monomers using a water molecule

  3. It's basically the opposite of a condensation reaction
• Monosaccharides


  The monomers that carbohydrates are made from, e.g. glucose, fructose and galactose
• Glucose

  A hexose sugar-a monosaccharide with six carbon atoms in each molecule
• Glucose


  • There are two types: alpha (a) and beta (B) glucose-they're isomers (molecules with the same molecular formula as each other, but with the atoms connected in a different way)
• Monosaccharides


  • Glucose
  • Fructose
  • Galactose
• Disaccharide formation


  1. Two monosaccharides join together

  2. Condensation reaction occurs

  3. Glycosidic bond forms between the two monosaccharides

  4. A molecule of water is released
• Disaccharides

  • Maltose (formed from two a-glucose molecules)
  • Sucrose (formed from a glucose molecule and a fructose molecule)
  • Lactose (formed from a glucose molecule and a galactose molecule)
• The Benedict's test is used to test for sugars
• Reducing sugars


  All monosaccharides and some disaccharides, e.g. glucose and lactose
• Benedict's test for reducing sugars

  1. Add Benedict's reagent (which is blue)

  2. Heat in a water bath that's been brought to the boil

  3. Coloured precipitate forms (green-yellow-orange-brick red)
• Higher concentration of reducing sugar


  Further the colour changes
• Non-reducing sugars


  Sugars like sucrose that need to be broken down into monosaccharides before testing
• Testing for non-reducing sugars

  1. Get new sample

  2. Add dilute hydrochloric acid and heat in water bath

  3. Neutralise with sodium hydrogencarbonate

  4. Carry out Benedict's test
• A blue colour indicates no reducing sugar present, a coloured precipitate indicates reducing sugar present
• A blue colour after heating with dilute hydrochloric acid and neutralising indicates no non-reducing sugar present, a coloured precipitate indicates non-reducing sugar present
• Starch

  Cells get energy from glucose. Plants store excess glucose as starch (when a plant needs more glucose for energy, it breaks down starch to release the glucose). Starch is a mixture of two polysaccharides of alpha-glucose-amylase and amylopectin
• Amylose

  • A long, unbranched chain of a-glucose. The angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. This makes it compact, so it's really good for storage because you can fit more in to a small space
• Amylopectin

  • A long, branched chain of a-glucose. Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the glucose can be released quickly
• Starch is insoluble in water and doesn't affect water potential, so it doesn't cause water to enter cells by osmosis, which would make them swell. This makes it good for storage
• Glycogen

  Animal cells get energy from glucose too. But animals store excess glucose as glycogen-another polysaccharide of alpha-glucose. Its structure is very similar to amylopectin, except that it has loads more side branches coming off it. Loads of branches means that stored glucose can be released quickly, which is important for energy release in animals. It's also a very compact molecule, so it's good for storage
• Cellulose

  Cellulose is made of long, unbranched chains of beta-glucose. When beta-glucose molecules bond, they form straight cellulose chains. The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils. The strong fibres mean cellulose provides structural support for cells (e.g. in plant cell walls)
• Iodine test for starch


  1. Add iodine dissolved in potassium iodide solution to the test sample

  2. If starch is present, the sample changes from browny-orange to a dark, blue-black colour