CHEM 153L

• Carbohydrates


  polyhydroxy aldehydes or ketones or substances that yield them upon hydrolysis
• Carbohydrates


  • Constitute the major material by weight in our diet and serve as a source of energy
• Monosaccharides


  Carbohydrates not decomposable to simple sugars by hydrolysis
• Monosaccharides


  • All free monosaccharides are “reducing” carbohydrates
• Monosaccharides used in the experiment

  • Arabinose (aldopentose)
  • Glucose (aldohexose)
  • Fructose (ketohexose)
  • Galactose (aldohexose)
• Monosaccharides classified by number of carbon atoms

  • Triose, n=3, C3H6C3
  • Tetrose, n=4, C4H8C4
  • Pentose, n=5, C5H10C8
  • Hexose, n=6, C6H12C6
  • Heptose, n=7, C7H14C7
  • Octose, n=8, C8H16C8
• Monosaccharides in equilibrium


  They are reversible
• Arabinose


  • A component of complex plant polysaccharides called gums & found in the free state in the wood of coniferous trees
• Glucose


  • Also called grape sugar and a dextrose, and referred as blood sugar
• Fructose


  • Fruit sugar; sweetest-tasting of all the sugars
• Galactose


  • Also called cerebrose or brain sugar; found in cerebrosides
• Disaccharides


  Contains two simple sugars linked by glycosidic bond
• Disaccharide: Maltose


  • Glucose + Glucose; malt sugar; a-d-glucose + a
• Disaccharide: Lactose


  • Glucose + Galactose; milk sugar; a-d-glucose + a-d-g
• Disaccharide: Sucrose


  • Glucose + Fructose; table sugar; found in sugar cane, sugar maple and sugar beets. a-D-glucose+
• Oligosaccharides


  Contains two to ten simple sugars linked by glycosidic bond
• Polysaccharides


  Carbohydrates having nine or more monosaccharides linked together by glycosidic bonds
• Polysaccharides used in the experiment

  • Starch
  • Agar-agar
  • Gum-arabic
  • Dextrin
  • Cellulose (cotton)
• Starch


  • Found in seeds, tubers, and roots of plants such as potatoes, corn, and cassava
• Starch composition


  Comprising glucose monomers joined in 𝑎(1 → 4) and 𝑎(1 → 6) linkages
• Starch components


  • Amylose – not branched but helical in configuration with a hollow core
  • Amylopectin – highly branched
• Dextrins


  • Partial hydrolytic products of starch; used in various food products and as adhesives and binders
• Glycogen


  • Used as a storehouse in the liver and muscles of animals
• Cellulose (cotton)


  • Most abundant organic compound on earth; linear polymer made up of 𝛽(1 → 4) glycosidic bonds
• Agar-agar


  • A jelly-like substance obtained from the cell walls of some species of red algae
• Cellulose


  • Linear polymer made up of 𝛽(1 → 4) glycosidic bonds
• Components of agar-agar


  • Linear polysaccharide agarose
  • Heterogenous mixture of smaller molecules called agaropectin
• Uses of agar-agar


  • Commonly used in Asian cuisines
  • Flavorless vegan substitute for gelatin
  • Helps gel, stabilize, texturize, and thicken various food products
  • Used as a microbiological medium for bacteria culture
  • Component of cosmetic and pharmaceutical preparations
• Complete hydrolysis of agar-agar yields galactose
• Properties of gum-arabic


  • Dried water-soluble exudate from the stems of Acacia senegal or related species
  • Carbohydrate polymer, complex and highly branched with arabinose as the central core
  • Used in pharmaceutical preparations and as an adhesive and thickening agent
• Molisch Test


  1. Test distinguishes carbohydrates from non-carbohydrates

  2. Carbohydrate undergoes dehydration in the presence of concentrated sulfuric acid, resulting in the formation of an aldehyde which condenses with phenol-type molecules to form a purple or reddish-purple colored complex
• Anthrone Test


  1. Qualitative and quantitative estimation of polysaccharides and monosaccharides

  2. Sulfuric acid hydrolyzes bound sugars and reacts with sugars to yield furfural or furfural derivatives, producing a blue or green-colored compound
• Iodine Test for Carbohydrates


  1. Used to differentiate helical polysaccharides from non-helical polysaccharides

  2. Starch interacts with iodine to become bluish black, distinguishing it from other sugars and polysaccharides
• Iodine interaction with coiled molecules

  Forms bluish black color
• Anthrone Reagent


  • Positive results give solution with colors varying from blue, purple, wine red, red, red-brown to brown
• Iodine forms colored complexes with polysaccharides

  Color of the complex depends on the 3-D structure of the polysaccharide
• The darker the color

  Presence of starch
• Qualitative tests for sugars

  • 5% solutions of glucose, fructose, arabinose, maltose, galactose, lactose, sucrose, and water (as blank)
• Benedict’s Test


  General test for detecting presence of reducing sugars
• Components of Benedict’s reagent


  • Cu2+ ions, alkaline solution, sodium citrate, sodium carbonate