LAB 2

ORGANIC MOLECULES

 

 

In this laboratory you will perform simple chemical and physical tests to evaluate the presence or absence of some organic molecules in several common foods.  You will employ the scientific method in your evaluation.  The scientific method is a method of problem solving.  This standard practice in scientific investigation provides a careful means for obtaining reproducible results.  Science is based on observation. To use the scientific method, you must have a standard against which make comparisons.  This is routinely accomplished by establishing two groups:  a control group and an experimental group. The control will provide the basis or 'standard' for comparison with the experimental group.

 

 

In these experiments, you will prepare a set of controls using known solutions for a positive and negative test for an organic molecule.  The variable is each food to be tested.  As you perform these experiments, think about the following:

 

    1.      Purpose:        the reason for performing the experiment

    2.      Procedure:     what you did in performing the experiment

    3.      Results:          what happened when you performed the experiment                                               

    4.     Conclusion:    what you learned from performing the experiment

  

LAB OBJECTIVES:

 

                        To practice the scientific method in testing common foods to determine the presence or absence of organic molecules.       

 

    To perform tests and interpret results which identify carbohydrates (glucose and starch), proteins and lipids present in common foods when given        

    reagents and  directions for their use.

 

I.        Carbohydrates

 

As you know, sugars are carbohydrates.  They are short-chain carbon compounds (commonly 3 - 7 carbons).  These short-chain molecules may be linked together to form dissaccharides (2 sugar units), trisaccharides (3 sugar units), or polysaccharides (many sugar units).  A single sugar unit unlinked to any other is called a monosaccharide.  The most common monosaccharide is glucose.  The most common disaccharide is sucrose (table sugar). Common polysaccharides are cellulose, starch, and glycogen.

 Glucose and Fructose are isomers. They both have a

chemical formula C6H12O6 but a different configuration.

 

Glycogen, cellulose and starch are all polysaccharides made up of glucose units linked together.  Single glucose units are linked together when water is    

          removed.  That's why the reaction is called condensation!  Because water is removed.  This basic chemical reaction is what builds all kinds of complex        

          molecules from simple units.  In nature, there is a great economy in materials and actions to make them.  Its a though once Mother Nature finds something that

          works and works well, the molecule or process is repeated -- sometimes in very surprising ways!

 

                

 

 

Question l:        What is a carbohydrate?

 

 

Question 2:       How are polysaccharides made?  Give three examples of common polysaccharides.

 

 

 

 

 A.      Test For Glucose:

 

The test for glucose involves a reaction between the aldehyde group of the glucose molecule

  H     and a solution known as Benedict's Solution.

   |

- C = O

 

 

Prepare a control:  Place a few drops of glucose solution in a clean test tube and add a few drops of Benedict's Solution.  Heat the test tube by placing the test tube in a hot water bath.  A  yellow to brick-red precipitate is the positive reaction for glucose.  Heat a small amount of Benedict's Solution in distilled water in a test tube.  No color change or precipitate will result. This is a negative reaction for glucose.  Keep your positive and negative results, your controls,  for this glucose test for later comparisons.

 

Question 3:       What reagent tests for glucose?

 

 

                B.      Test For Starch:

 

                             The test for starch involves an unstable union between starch molecules and a solution of iodine. 

 

                              Prepare a control: Place a few drops of starch solution in a clean test tube.  (Shake the bottle of starch solution before pouring.)  Add several drops of

                              iodine solution.  A blue-black color will appear.  This is a positive test for starch.  Place several drops of iodine in some distilled water. No change will

                              occur since water does not contain starch.  This is your negative result.   Hold on to your positive and negative controls for the starch for later

                              comparison.   

 

Question 4:       What reagent tests for starch?

 

       II.         Protein

 

Proteins are composed of individual units called amino acids.  There are 20 naturally occurring amino acids.  Adults can synthesize only 13 and infants only 12.  This why one-third of the amino acids necessary for health are described as 'essential' amino acids. Without these essential amino acids our bodies can not successfully synthesize necessary polypeptides and proteins required for good health and homeostasis.  A balanced diet is necessary to provide these essentials.    

 

Amino acids have the general formula:     H2N - CH - COOH

                                                                                          R

These 20 amino acids differ from one another by the presence of various  R groups bonded to the central carbon in the general formula.  The simplest amino acid is glycine which has an H in place of the R.  Another simple amino acid is alanine. Amino acids are linked together 'head to tail' by a special peptide bond to form proteins.

 

        

Test For Protein:  The test for protein involves the reaction between a substance called Biuret Reagent (an alkaline copper solution) and the peptide bonds of protein.

 

Prepare a control: In a clean test tube, place approximately 2 mL of protein solution (Knox gelatin solution) and add Biuret Reagent, drop by drop until a dark blue-violet color appears.  This is your positive control.  Compare this to a test tube containing 2 mL of water and several drops of Biuret Reagent.  Hold on to your controls for later comparison. 

 

Question 5:       What is an amino acid?  

 

Question 6:       What is a peptide bond?  

 

 

Question 7:       What reagent tests for proteins? 

 

     

 

III.         Lipids

 

            Lipids are compounds that are insoluble in water.  Lipids include oils, waxes, phospholipids, steroids, and cholesterol.  A typical fat is composed of

            one molecule of glycerol and three molecules of fatty acids and are often referred to as triglycerides..

 

What happens when you leave a potato chip on a paper towel?  The greasy spot left on the paper towel results from the absorption of the lipid by the paper.     In a similar way, a food solution containing lipid will create a translucent spot on brown paper.  This very characteristic was exploited by pioneers traveling west by covered wagon. Unable to transport panes of glass across the rugged terrain, the settlers rubbed brown paper with animal fat to create a translucent covering for the window openings of their sod dwellings. Who knew?

Test for lipid:  Obtain a sheet of brown paper.  Place one drop of vegetable oil (positive control) and one drop of distilled water (negative result) on the paper.  When the drop of water has dried completely, hold the paper up to the overhead light.  The oily spot allows light to be transmitted through the paper.  

Test for solubility: Mark a test tube at 2 cm and 3 cm (using a cm ruler).  Add distilled water to the first mark and add vegetable oil to the second mark.  Cover the tube with your thumb and shake the tube vigorously.  Observe for the dispersal of oil followed by the rapid separation back into two layers.  

Test for emulsification: Add a few drops of Tween or bile salts to the tube containing water and oil.  Shake vigorously.  Do the liquids separate into layers as before?  Describe your observations. 

Emulsifier + fat --> emulsion

 

Question 8:       Is vegetable oil water soluble?   What is emulsification?

 

 

IV.               Procedure for Testing Foods:

 

Several types of foods will be made available for you to test for the presence of glucose, starch, protein, and lipid in the sample.  Where necessary, prepare a solution of the food by grinding up several pieces of the food using the 'recipe' below.  Divide the solution into three test tubes and place drop onto brown paper for the lipid test.. 

 

Food preparation: 

 

   l.        Obtain a mortar and pestle, distilled water, sheet of brown paper, test tube rack and test tubes.

  2.       Place a little water in the mortar.

  3.       Add a few pieces of the food to be tested.

  4.       Grind thoroughly adding water as necessary to made a solution.

  5.       Pour off your solution into 3 clean test tubes.

  6.       On one test tube perform the glucose test, on another the starch test, on the third the protein test.

  7.       The test for lipid is a translucence test on brown paper. Using your pencil, divide the brown paper into a grid on which a drop of each food 

            solution is placed. 

            When completely dry, hold the brown paper to the light.  Those solutions which leave a translucent spot through which light passes indicate

            foods containing lipids.    

  8.       Compare the result of each test with the control for each organic molecule and record your results in the table below.

  9.        Repeat the process of each food provided.

10.       Clean mortar, pestle and test tubes thoroughly in soapy water as directed by your instructor.  Allow the tubes to drain in their rack

            over paper towels ready for the next class.

 

The table provided shows the organic compounds and the foods tested.  Record your reactions as either positive and negative in this table. 

 

 

    FOODS TESTED

 

          GLUCOSE

 

            STARCH

 

           PROTEIN

           LIPID

 

Banana

 

 

 

 

 

 

  

 

Apple

 

 

 

 

 

 

  

 

Egg White

 

 

 

 

 

 

  

 

Egg Yolk

 

 

 

 

 

 

  

 

Potato

 

 

 

 

 

 

  

 

Cottage Cheese

 

 

 

 

 

 

  

 

Onion

 

 

 

 

 

 

  
         
         
         
         

 

 

MAKE SURE ALL YOUR APPARATUS ARE THOROUGHLY CLEANED. MAKE CERTAIN YOUR DESK IS CLEAN AND DRY. PUSH YOUR CHAIR BENEATH THE LAB TABLE, PLACE ALL WASTE PAPER IN THE RECYCLING CONTAINER AND ALL TRASH IN THE WASTEBASKET.  LOOK AROUND BEFORE LEAVING THE ROOM.  IS THE SPACE IN ORDER READY FOR THE NEXT CLASS?   IF NOT, MAKE IT SO!