carbohydrates

• carbohydrates - organic compounds containing hydrogen and carbon and oxygen

  a basic unit is called a monosaccharide, 2 monosaccharides is a disaccharide and multiple is a polysaccharide
• monosaccharides -

  • general formula (CH2O)n
  • name depends on how many carbon atoms there are
  • triose = 3 pentose = 5 hexose = 6
• hexose sugars -

  • all share the formula C6H12O6 but they differ in molecular structure
  • the carbon atoms make a ring when the sugar is dissolved in water and they can all alter their binding to make straight chains
  • glucose is a hexose sugar, it has two isomers, alpha and beta
• alpha glucose

  
• beta glucose

  
• glycogen - stored form of glucose found in animals, made up of chains of alpha glucose units held together by 1-4 links and 1-6 links which create branch points - more branched than amylopectin because 1-4 links are shorter
• functions of monosaccharides -

  • source of energy in respiration - bonds are broken to release energy (carbon-carbon and carbon-hydrogen)
  • building blocks for larger molecules (glucose makes glycogen and cellulose)
  • intermediates in reactions - trioses are intermediates in respiration and photosynthesis
  • constituents of nucleotides - deoxyribose in DNA and ribose in ATP RNA and ADP
• disaccharides -

  • bonded together by a glycosidic bond formed in a condensation reaction

  
• hydrolysis of a glycosidic bond

  
• maltose = glucose + glucose - germinates seeds

  sucrose = glucose + fructose - transport in phloem of flowering plants

  lactose = glucose + galactose - in mammalian milk
• testing for the presence of sugars -

  benedict's test is for reducing sugars (glucose) - equal volumes of it and the solution being tested heated to at least 70 degrees

  if reducing sugar is present solution will turn from blue through green, yellow, and orange to brick red.

  some disaccharides are non reducing sugars (sucrose) and have to first be broken down into its constituent monosaccharides - by heating with hydrochloric acid - but benedict's reagent needs alkaline conditions to work so alkali is added.

  • using a biosensor is much more accurate so is used to monitor medical conditions.
• adding sucrase to test for sucrose -

  sucrase is an enzyme that hydrolyses sucrose into glucose and fructose

  so benedict's test will then give a positive result
• polysaccharides -

  • large complex polymers linked by glycosidic bonds
• glucose is converted into starch because

  1. starch is insoluble so has no osmotic effect
  2. it cannot diffuse out of the cell
  3. compact molecule and can be stored in a small space
  4. carries a lot of energy in its C-C and C-H bonds
• starch -

  • has two polymers - amylose and amylopectin both made of alpha glucose
• amylose is a linear, unbranched molecule with alpha 1-4 glycosidic bonds. the chain coils into a helix.
• amylopectin has chains of alpha glucose linked by 1-4 glycosidic bonds, but they are cross linked with alpha 1-6 glycosidic bonds which form branches
• amylose

  
• labelled carbon atom

  
• amylopectin, 1-6 bonds

  
• testing for the presence of starch -

  iodine solution colour changes from brown to blue-black

  accurate concentration cannot be gained.
• cellulose - structural polysaccharide present in cell walls to provide strength

  consists of many long, parallel chains of beta glucose joined by 1-4 glycosidic bonds - the adjacent molecule rotates 180 degrees and hydrogen bonds form between adjacent parallel chains which add to the structural stability

  between 60 and 70 molecules form bundles called microfibrils and these form bundles called fibres.

  a cell wall has layers of fibres running parallel but at an angle to adjacent layers - laminated structure.
• structure of cellulose

  
• chitin - a polysaccharide that is present in the exoskeleton of insects and fungal cell walls

  it has long chains of beta 1-4 links and has groups derived from amino acids added to form a heteropolysaccharide

  it is strong waterproof and lightweight

  monomers are rotated 180 degrees to their neighbour monomers and long parallel chains are cross linked to each other by hydrogen bonds forming microfibrils
• structure of chitin