Organic Substances Assignment

Background

Organic substances are carbon-rich and energy-rich molecules that make up cells. They are classified into four major types. Carbohydrates consist of repeated units of H-C-OH and include monosaccharides such as glucose sugar and polysaccharides such as starch. Proteins are a diverse group of large molecules consisting of chains of amino acids. They include enzymes such as catalase, carrier molecules such as hemoglobin and hormones such as insulin. Proteins are abundant in all cells, especially muscles. Lipids dissolve in organic solvents but are insoluble in water. They include fats and steroids. Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are organic substances made up of chains of nucleotides.

When we eat, the food that we consume consists of organic substances made by other organisms. It nourishes our body because these organic substances provide our cells energy (mainly from carbohydrates and fats) and appropriate “raw materials” (mainly from proteins) required to make specific biomolecules our body needs. 

To review biomolecules, watch this video:Biomolecules by Amoeba sisters (Links to an external site.)  (8 min 12 sec)

Hypothesis and prediction: If a food sample contains organic substances, then a test for the presence of those substances using specific reagents will be positive.

Purpose

• Test for the presence of two types of carbohydrates (glucose, starch) and lipids in food samples.  

Organic Substances Assignment Part 1: Test for Carbohydrates and Protein

Materials

The following materials will be used in the demo videos:                                     

• Benedict’s solution
• Iodine solution (diluted) 
• Ninhydrin reagent (1%)                                 
• Starch solution                                      
• Glucose sugar solution
• Protein solution
• Beakers 
• Test tube
• Test tube racks
• Stirring rods
• Disposable droppers
• Food samples (diced)                             

Procedure

Testing the reagents (demonstration by the instructor)  

A specific reagent is used to test for the presence of a specific organic substance such as glucose, starch or protein. To determine which reagent can test for the presence of a specific organic substance, we have to show the reagent reacting in a characteristic way with that organic substance (e.g., specific color change) and not reacting in the same way with other substances. 

            To demonstrate this process, the instructor will test three reagents: iodine, Benedict’s solution and ninhydrin. From these tests, you will discover what organic substance (i.e. glucose sugar, starch and protein) each reagent tests for.

For a demonstration of the full lab experiment, watchOrganic substances in food lab (Links to an external site.)(watch to mark 6:08 for the first demonstration on testing reagents). Observe color changes and record in appropriate data tables. Follow procedure from the lab exercise. Organic Substances Assignment

Record the original color of the reagents below:

Benedict’s  ______________   Iodine _______________   Ninhydrin   _______________  

The instructor will then add the reagents, one at a time, to sugar, starch and protein solutions to show whether a chemical reaction occurs.

Record the final color of the mixture in Table 1.

Table 1. Final Results of Reagents with Organic Substances
Reagents/ Organics	Benedict’s Solution	Iodine	Ninhydrin
Glucose sugar
Starch
Protein

Organic Substances Assignment Interpret the previous table, determine the appropriate reagent to identify each organic substance and fill in the table below.

Table 2. Reagent Test for Organic Substances
Organic substance	Reagent	Expected color change
Glucose sugar
Starch
Protein

Testing for the presence of organic substances (carbohydrates and protein) in food samples

            Now that you know which reagent can test for the presence of glucose sugar, starch and protein, you can determine which of those organic substances are present in food.

1. Prepare a water bath (beaker half-full with tap water) on a hot plate.
2. Take three samples of one type of food about the size of a small pea.
3. Grind the food into fine pieces then place the ground food into two separate test tubes and one sample into a petri dish. Mark one of the tubes “B” for Benedict’s and the other tube “N” for ninhydrin. Label the petri dish “I” for iodine.
4. Add Benedict’s reagent to the food sample in tube “B” and ninhydrin to the food sample in tube “N”.
5. Heat tube “B” for 5 min. and tube “N” for 7 min. in the water bath. Record the color in the table below.
6. Add iodine to the food sample in the petri dish. Wait 5 min. and record color in the table below.
7. Choose another food sample and repeat steps 1-6.
8. Pool class results. The instructor will record the results from each group on the board so each student can record data for all food samples tested in class.
9. Dispose food samples in a designated tub then wash the test tubes and petri dish.

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