🍞Carbohydrates 🍞

• Carbohydrates are made of carbon, hydrogen and oxygen.
• carbohydrates are also known as saccaharides or sugars
• a single sugar unit is a monosaccharide, e.g. glucose, fructose, and ribose
• two monosaccharides joined together form disaccharides, e.g. sucrose = glucose + fructose
• many more than two monosaccharides linked together is a polysaccharide, e.g glycogen, cellulose, starch
• glucose is a monosaccharide composed of six carbons so it is a hexose monosaccharide
• in molecular structure diagrams carbons are numbered clockwise starting from the carbon to the right of the oxygen atom in the ring
• there are two structural variations of the glucose molecule: alpha glucose and beta glucose

  in alpha glucose, the hydroxyl group is positioned below the carbon ring

  in beta glucose, the hydroxyl group is positioned above the ring
• glucose molecules are polar and water soluble because of the hydrogen bonds that form between the hydroxyl groups and water molecules

  This is important because it means glucose can dissolve in the cytosol of the cell
• when two alpha glucose molecules are side by side the hydroxyl group at carbon 1 of one of the molecules interacts with the hydroxyl group at carbon 4 of the other molecule in a condensation reaction.

  This forms a water molecule from the two hydrogens and an oxygen of one of the monosaccharides

  This leaves an oxygen bridge to form between the two glucose molecules forming a 1,4 glycosidic bond

  the resulting disaccharide is maltose
• glycosidic bonds are types of covalent bonds
• a glycosidic bond between carbon 1 and carbon 6 would be a 1,6 glycosidic bond
• fructose, galactose and glucose are hexose monosaccharides
• sucrose = glucose + fructose
• lactose = glucose + galactose
• maltose = glucose + glucose
• fructose is sweeter than glucose and glucose is sweeter than galactose
• glucose made by photosynthesis in plants is stored as starch which is a chemical energy store
• starch is made of two polysaccharides: amylose and amylopectin
• amylose is made by alpha glucose molecules joined by only 1,4 glycosidic bonds

  The angle of the bonds means this long chain of glucose twists to form a helix further supported by hydrogen bonding within the molecule

  This makes amylose more compact and much less soluble than glucose
• amylopectin is formed by 1-4 glycosidic bonds between alpha glucose molecules and it also has some monosaccharides attached by 1,6 glycosidic bonds

  this causes amylopectin to have a branched structure with branching points occurring approximately every 25 glucose subunits
• animals and fungi store glucose as glycogen
• glycogen forms more branches than amylopectin so it is more compact; less space needed for it to be stored

  this is important a animals are mobile unlike plants
• branching and coiling of polysaccharides makes them compact (ideal for storage)

  branching also means there are lots of free ends where glucose molecules can be added or removed which speeds up the process of storing or releasing glucose molecules required by the cell
• to release glucose for respiration, starch/glycogen undergoes hydrolysis reactions which requires the addition of water molecules (reverse of condensation reaction)

  this is catalysed by enzymes
• beta glucose molecules are unable to join together in the same way that alpha glucose can because the hydroxyl groups on carbon 1 and carbon 4 are too far away form each other to react

  To overcome this and form a polymer, alternate beta glucose molecules have to flip 180 degrees (upside down)

  The polysaccharide formed is cellulose and because it was formed in this way, it can't coil or form branches- its a straight chain molecule
• cellulose molecules form hydrogen bonds with each other forming microfibrils which combine to form macrofibrils which combine to form fibres

  these fibres are strong and used to make cell walls
• cellulose is important in our diet as it forms the fibre/roughage necessary for a healthy digestive system because it it very hard to break down into its monomers
• all monosaccharides and some disaccharides are reducing sugars meaning they can reduce (donate electrons to) another molecule or chemical
• The chemical test for reducing sugars is the benedict's test

  Benedict's reagent is used which is an alkaline solution of copper (II) sulfate

  Reducing sugars will react with the copper ions in the benedict's reagent:

  Electrons are added to the blue Cu2+ ions reducing them to brick red Cu+ ions
• The steps to test for reducing sugars are:

  1. Place the sample being tested in a boiling tube (if its not liquid grind it up or blend in water)
  2. add an equal volume of benedict's reagent
  3. heat the mixture gently in a boiling water bath for 5 mins
  4. if a brick red precipitate forms, this indicates a positive result for reducing sugars
• Non-reducing sugars will produce a negative result in the Benedict's test (it will remain blue)

  However, if it is first boiled with dilute hydrochloric acid, it will give a positive result with the benedict's because it has been hydrolysed e.g. Sucrose can be hydrolysed to produce glucose and fructose which are both reducing sugars
• To test for starch, a few drops of iodine dissolved in potassium iodide solution are mixed with the sample.

  If it changes colour form yellow/brown to purple/black starch is present in the sample.

  If it doesn't change there is no starch
• A colorimiter is a device that is used to quantitively measure the absorbance or transmission of light by a coloured solution

  The more concentrated the solution, the more light it will absorb and the less light it will transmit

  This can be used to calculate the concentration of reducing sugar present in a sample
• The colorimiter works by placing a filter that is the complementary colour of the colour of the sample in it.
• The colorimiter is calibrated using distilled water.
• The calibration curve is used to determine the concentration of an unknown concentration of glucose.