3.1.2 Carbohydrates

• Monosaccharides are monomers from which larger carbohydrate are made
• Examples of monosaccharides are glucose, fructose and galactose
• The difference between the structure of alpha glucose and beta glucose is that OH group is below carbon 1 in alpha glucose but able carbon 1 in beta glucose
• Alpha and beta glucose are both isomers - same molecular formula, differently arranged atoms
• Disaccharides are 2 monosacchraides joined together with a glycosidic bond, formed by a condensation reaction whilst releasing a molecule of water
• 3 examples of disaccharides are maltose, sucrose and lactose
• Maltose = Glucose + glucose
• Sucrose = Glucose + fructose
• Lactose = Glucose + galactose
• Polysaccharides are many monosaccharides joined together with a glycosidic bond, formed by many condensation reactions whilst releasing a molecule of water
• Starch is an energy store in plant cells
• Starch is a polysaccharides of alpha glucose
• Starch is made up of amylose and amylopectin
• Amylose has a 1,4 glycosidic bond, making it unbranched
• Amylopectin has 1,4 and 1,6 glycosidic bonds, making it branched
• Glycogen is an energy store in animals cells
• Glycogen is a polysaccharide made of alpha glucose
• Glycogen has a 1,4 and 1,6 glycosidic bond making it branched
• Starch and amylose are both:

  • Helical -> Compact for storage in cell
  • Large, insoluble polysaccharide molecule -> can't cross the cell membrane
  • Insoluble in water -> Water potential of cell is not affected
• Glycogen, Starch and amylopectin are all:

  • Branched -> compact so it can fit more molecules in a small area. Also more ends for faster hydrolysis -> release glucose for respiration to make ATP for energy release
  • Large, insoluble polysaccharide molecule -> can't cross the cell membrane
  • Insoluble in water -> water potential of cell is not affected
• Cellulose provide strength and structural support to plant/ algal cell walls
• Cellulose is a polysaccharide of beta glucose
• Cellulose has 1,4 glycosidic bond, making it straight and unbranched chains
• In cellulose, every other beta glucose molecule is inverted in a long, straight, unbranched chain
• Cellulose has many hydrogen bonds which link parallel strands known as crosslinks to form microfibrils which are strong fibres
• Cellulose has hydrogen bonds which are strong in high numbers
• Cellulose also provides strength to plant cell walls
• Reducing sugars are monosaccharides, maltose and lactose
• The test for reducing sugars is:

  • Add benedicts soluction which blue to sample
  • Heat in a boiling water bath
  • Positive result = green/ yellow/ orange/ red precipitate
• Non-reducing sugars are sucrose
• The test for non-reducing sugars is:

  • Do Benedict's test and if it stays blue continue
  • Heat mixture in a boiling water bath with acid to hydrolyse into reducing sugars
  • Neutralise with alkalis such as sodium bicarbonate
  • Heat mixture in a boiling water bath with benedicts solution
  • Positive result would should green/ yellow/ orange/ red precipitate
• A method to measure the quantity of sugar in a solution is by carrying out the benedict's test, the filter and dry the precipitate and weigh it to find the mass
• Method to measure the quantity of sugar in a solution

  1. Make sugar solutions of known concentrations (e.g. dilution series)

  2. Heat a specific volume of each sample with a specific volume of Benedict's solution for the same time

  3. Use a colorimeter to measure the absorbance of light for each known concentration

  4. Plot the data, forming a calibration curve - concentration on the x axis and the absorbance on the y axis and then draw a line of best fit

  5. Repeat the Benedict's test with unknown sample and then measure the absorbance

  6. Read off the calibration curve to find the concentration associated with unknown sample's absorbance
• The biochemical test for starch is adding iodine dissolved in potassium iodide (orange/brown) and then shake, if starch is present then it'll turn blue-black