Polysaccharides

• Polysaccharides are two major groups: the storage polysaccharides and the structural polysaccharides
• The storage polysaccharide of which starch is the most abundant in plants and glycogen in animals, are usually deposited in the form of granules in the cytoplasm of the cells.
• Starch occurs in two forms, the a-amylase and amylopectin
• The major components of starch can be enzymatically hydrolyzed in two different ways.
• Amylose can be hydrolyzed by a-amylase which is abundant in the saliva and pancreatic juice.
• Amylose can be hydrolyzed also by b-amylase
• glycogen is the main storage polysaccharide of animal cells, the counterpart of starch in plant cells.
• glycogen is essentially abundant in the liver, where it may attain up to 10% of the wet weight. it is also present about 1% to 2% in skeletal muscle.
• glycogen gives a red violet color with iodine solution
• polysaccharide is a carbohydrate molecule that contains MANY monosaccharides linked by glycosidic linkage.
• polysaccharide is not sweet
• polysaccharide tests negative on benedict's test
• polysaccharides are less soluble in water
• isolation of starch from potato

  1. wash, peel, and grate 4 potatoes
  2. transfer the grating and adhering liquids to a clean cheese cloth which has been folded into bags then move the bag up and down through 100ml of water in a beaker. do this for 5 mins and notice the substance which settle out then filter the solution.
  3. mix 5g of the residue with 20ml of warm water in a small beaker and label it as solution b
• molisch test

  • to the test tubes containing the liquids, add 2-3 drops of molisch reagent then mix.
  • incline the test tube and carefully pour down the side of the tube, 3ml of concentrated sulfuric acid such that it forms a layer at the bottom.
  • note the color
• iodine test

  • place a drop of iodine solution in an evaporating dish and then add to it a drop of filtrate a obtained from part 1
  • note color change
• starch

  it is also positive on molisch test since it showed a purple color.

  it is positive on iodine test since it showed a blue-black color upon contact with the iodine.
• When iodine (I₂) is added to a starch solution, the iodine molecules are able to fit into the helical structure of the amylose. The iodine molecules slip into the spiral of the amylose chains, forming a complex with the starch.

  This complex is very distinctive because it causes the starch to absorb and scatter light in a way that results in the blue-black color.
• The Molisch test is a general test for carbohydrates, and it is positive with starch because starch is a carbohydrate, and the test detects carbohydrates in general. The reaction forms a purple ring due to the breakdown of starch into simpler sugars (like glucose) and their subsequent reaction with α-naphthol.
• materials used

  • test tube
  • chopping board
  • knife
  • beaker
  • iron plate
  • cheesecloth

  reagents

  • potato
  • iodine solution
  • molisch reagent
  • concentrated sulfuric acid