AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 2: carbohydrates

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Disaccharide = macromolecule consisting of 2 sugar molecules joined by a glycosidic bond
Polysaccharide = macromolecule consisting of more than 2 sugars - often a long chain polymer - joined together by glycosidic bonds
Two sugar molecules join together by a glycosidic bond (shown below):
Glycosidic Bond
  • Repeats between thousands of sugar monomers to form a polysaccharide
    • A polysaccharide is not a sugar
  • If just two sugar monomers bond, they form a specific disaccharide
  • Form by a condensation reaction
  • Broken by hydrolysis
  • Generally 1,4-glycosidic bonds
Beaning the bond forms between Carbon atom 1 and Carbon atom 4
EXAM TIP
You should be aware of the different combinations of sugar that form different disaccharides:
  • Glucose + Fructose = Sucrose (found in sugar cane)
  • Glucose + Galactose = Lactose (found in milk)
  • a-Glucose + a-Glucose = Maltose (found in germinating seeds)
There are two types of glucose molecule. They have the same chemical formula, but their structures are slightly different. The two versions are called alpha-glucose and beta-glucose, with reference to the first Carbon atom in the hexose ring. The difference is highlighted in the diagram, and involves simply the different direction of the H and OH groups attached to the Carbon atom.
Starch
  • A mixture of amylose and amylopectin.
    • Both amylose and amylopectin are long-chain a-Glucose polymers
  • The main form of carbohydrate used for energy storage in plants.
  • Starch grains are stored in chloroplasts and are insoluble in water so they don’t affect water potential up and down the xylem - important for water and nutrient transport in the plant.
Amylose
Value
  • Long, unbranching chains
  • 1,4-glycosidic bonds
  • Chains coil into helical shape
  • Shorter chains than amylose
  • 1,4-glycosidic bonds
  • Branches out (Carbon 1 bonds to Carbon 6)
Glycogen
  • Multibranched a-Glucose polymer.
  • Main form of carbohydrate used for energy storage in animals.
  • Glycogen is stored in the muscles and in the liver.
  • In times of high energy usage, these organs hydrolyse the glycogen stored and break it down into glucose molecules which can be used in respiration.
  • Soluble in water.
  • Compact shape makes it optimal for an energy storage molecule.
Cellulose
  • Highly abundant in cell walls, making it most common molecule on the planet.
  • Polymer of about 10,000 β-glucose molecules in a long unbranched chain called a microfibril.
  • High stability due to structure - several polysaccharide chains running parallel to each other with cross links between them.
  • The chain cross links are hydrogen bonds and contribute to the strength of the molecule.
  • Stability renders plants strength and resistance to wind and rain.
Benedict’s test for reducing and non-reducing sugars
All monosaccharides are reducing sugars, while disaccharides can be either reducing or non-reducing.
  1. Benedict’s reagent is composed of copper-II sulphate in alkaline solution
  2. Add the reagent to the sample and heat in a water bath (80℃) for 3 minutes
  3. In the presence of a reducing sugar, Cu2+ ions are reduced to Cu+ and the sugar is oxidised
  4. Colour changes to red
  5. If no colour change present, this could indicate either:
    1. Absence of any sugar
    2. Presence of non-reducing sugar

To check whether a non-reducing sugar is present, we first need to break down the sugar into its reducing sub-parts. For instance, sucrose needs to be broken down into glucose and fructose before it will react with Benedict’s reagent. We can achieve this by:
  1. Heat solution with acid to hydrolyse any glycosidic bonds present
  2. Neutralise solution with sodium hydroxide
  3. Add benedict’s reagent and heat in water bath for 3 minutes
  4. If colour changes to orange/red then a non-reducing sugar was originally present

Iodine test for starch
  1. Use potassium iodide solution as source of iodine
  2. In the presence of starch, the solution will turn a dark blue-black colour
  3. This is due to iodine becoming trapped inside the coils of starch