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9.9 Forensic Chemistry 2. Carbohydrates distinguish plant and animal material

Syllabus reference (October 2002 version)
2. Analysis of organic material can distinguish plant and animal material	Students learn to: identify that carbohydrates are composed of carbon, hydrogen and oxygen according to the formula: Cx (H2O)y identify glucose as a monomer and describe the condensation reactions which produce: sucrose as an example of a dissacharide polysaccharides including glycogen, starch and cellulose describe the chemical difference between reducing and non-reducing sugars distinguish between plant and animal carbohydrates’ composition in terms of the presence of: cellulose starch glycogen	Students: choose equipment, plan and perform a first-hand investigation to carry out a series of distinguishing tests for the carbohydrates: reducing and non-reducing sugars starch use available evidence and perform first-hand investigations using molecular model kits, computer simulations or other multimedia resources to compare the structures of organic compounds including: monosaccharides starch

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 7Sep06]

Prior learning: HSC module 9.2.1, 9.2.2

choose equipment, plan and perform a first-hand investigation to carry out a series of distinguishing tests for the carbohydrates:

• reducing and non-reducing sugars
• starch

• Use a range of resources e.g. biology, chemistry textbooks to plan the steps required to perform the tests outlined in the table below.

• The information gathered from the first-hand investigation should be qualitative. To ensure the data is qualitative your plan should include the identification of controls and variables. Controls that should be considered are; the same amount of reagent added, similar carbon compound structures (same number of carbon atoms).

• From your plan, choose equipment appropriate to perform a first-hand investigation to carry out the distinguishing tests.

• In your choice of equipment include risk assessments using the “Chemical safety in schools” package for all reagents and the experimental procedure to be used in the distinguishing tests.

  Organic compound	Distinguishing test
  reducing sugar	Tollen’s reagent: Black precipitate or shiny mirror when solution of sample is warmed with Tollen’s reagent Benedict’s or Fehling’s solution: Orange brown precipitate when solution of sample is warmed with Benedict’s or Fehling’s solution
  starch	dark blue colour when iodine is added to an aqueous solution or suspension of the sample

identify that carbohydrates are composed of carbon, hydrogen and oxygen according to the formula C_x(H₂O)_y

• Carbohydrates are compounds that contain carbon, hydrogen and oxygen only.

• They may be represented by the formula C_x(H₂O)_y

• Carbohydrates are classified into three main groups:

  • monosaccharides (these are the basic building blocks of more complex carbohydrates). Examples include glucose and fructose. Deoxyribose, an important part of DNA (deoxyribonucleic acid) is also a monosaccharide.
     
  • disaccharides (molecules of these contain two monosaccharide units linked together). Sucrose (cane sugar or table sugar) is an example.
     
  • polysaccharides (molecules of these are polymers made up of long chains of monosaccharide units). Examples include celluloses, starches and glycogen.
     

use available evidence and perform first-hand investigations using molecular model kits, computer simulations or other multimedia resources to compare the structures of organic compounds including:

• monosaccharides
• starch

• Use available evidence, for example diagrams of monosaccharides and starch, to perform a first-hand investigation using molecular model kits to form 3D models of the carbohydrates listed. You may not be able to form complete chains of starch, (they are hundreds to thousands of glucose units long). Work as a class group to form as many glucose units as possible and correctly join them by condensation reaction (elimination of water).

identify glucose as a monomer and describe the condensation reactions, which produce:

• sucrose as an example of a dissacharide
• polysaccharides including glycogen, starch and cellulose

• Hexose sugars (C₆H₁₂O₆) are the most common monosaccharides and include glucose and fructose.

• When two monosaccharides (monomer units) combine a disaccharide is formed. The reaction is called a condensation reaction, resulting in the elimination of a water molecule.

• Sucrose is a dissacharide, which is formed by linking glucose and fructose together, eliminating water.

  Sucrose, glucose, fructose from The Concord Consortium, USA.

• Polysaccharides are formed when many monosaccharides are linked together in a condensation reaction. Cellulose, starch and glycogen are all polysaccharides formed from glucose monomer units. Different arrangements of the glucose units produce different polymers – cellulose molecules are straight and hydrogen bond together to form water insoluble fibres, starch molecules are coiled while glycogen is highly branched.

  Arrangement of glucose units in starch, cellulose and glycogen, Bioweb, Western Kentucky University, Bowling Green, Kentucky, USA.

describe the chemical difference between reducing and non-reducing sugars

• Reducing sugars (monosaccharides, e.g. glucose and some disaccharides, e.g. lactose and maltose) act as reductants when heated with a weakly alkaline solution of copper (II) sulfate (e.g. Benedict’s solution) to form an orange-brown precipitate of copper (I) oxide.

• The Cu²⁺ is reduced to Cu⁺, which reacts with OH^- to give Cu₂O.

  2Cu²⁺(aq) + 2e^- + H₂O(l) → Cu₂O(s) + 2H⁺(aq)

  Aldehyde (-CHO) group in a reducing sugar can be the source of electrons that reduces copper (II) to copper (I)
  

  R-CHO (aq) + H₂O(l) → R-COOH(aq) + 2H⁺(aq) + 2e^-

  Benedict test of glucose and sucrose, University of Manitoba, Winnipeg, Manitoba, Canada.

• Fehling’s solution (strongly alkaline copper sulfate) and Tollen’s reagent (silver nitrate) can also be used.

  Negative (1) and positive(2) Fehling test , Sciences de la vie et de la terre, Didier,(Sciences of life and the earth) Paris, France.

• The reaction only occurs if the carbohydrate exists in equilibrium between a ring and open-chain form. The open-chain form contains –CHO and –CO-CH₂OH functional groups, which are easily oxidised to carboxylic acids R-COOH and R-CHOH-COOH respectively. Since the carbohydrates are oxidised the reagent must have been reduced, therefore the carbohydrates are reducing agents. Thus the sugars that cause a reaction are called reducing sugars.

distinguish between plant and animal carbohydrates’ composition in terms of the presence of:

• cellulose
• starch
• glycogen

Carbohydrate	Animal or Plant	Found in
cellulose	plant	matrix structure in plant cell walls
starch	plant	stored in cytoplasm of cells
glycogen	animal	stored in muscle and liver cells

• Cellulose, starch and glycogen are all polymers containing linked glucose units. Each has a different structure.

• The structure of starch and glycogen are similar. Both are formed by long glucose unit chains. Glycogen however has a longer chain and is more highly branched.

  

• Cellulose differs by the linkage of the glucose units. The orientation of the units result in a linear chain, which allows other chains to closely pack by hydrogen bonding. This increases the strength of cellulose and accounts for its low solubility and importance in maintaining plant cell structure.