Chemical Detection of Organic Molecules

Introduction: A monosaccharide is a simple sugar and forms a disaccharide (a double sugar) when condensation occurs. Hydrolysis breaks a disaccharide apart to form two monosaccharides by splitting water molecules. Many monosaccharides can combine to form a polysaccharide, an example being starch. Carbohydrates (sugars) are stored temporarily as glucose and for longer periods as starch. Amino acids have an amino group and a carboxyl group attached to the central group. Amino acids join in a variety of patterns and lengths to form proteins. Proteins are important as enzymes that control our metabolism and are the primary structural components of living cells. Two amino acids bonded together form a dipeptide and the addition of more amino acids form a polypeptide (protein). Lipids (fat, oil, wax) are insoluble in water, can store energy, build cell membranes, and serve as cushions and insulators. Fatty acids contain a carboxyl group bonded to a long carbon chain. Fats consist of three fatty acids and a glycerol molecule. Double bonds between carbons in fatty acids make the chain unsaturated while single bonds between carbons make it saturated.

Purpose: To test for the presence of specific organic molecules

Procedure: Part 1- Chemical Detection of Monosaccharides and Disaccharides
1. Macerate small piece of potato in mortar. Form thick juice. Repeat with piece of onion.
2. Label test tubes #1, #2, and #3. Mark 1cm and 3cm from bottom.
3. Tube #1, add water to 1cm mark and Benedict’s reagent to 3cm mark
4. Tube #2, add potato juice to 1st mark and Benedict’s reagent to 2nd mark.
5. Tube #3, add onion juice to 1st mark and Benedict’s reagent to 2nd mark.
6. Place all 3 tubes in beaker of boiling water at the same time. When one tube turns yellow, orange or red, stop the experiment.
7. Read all tubes immediately and record results.

Part 2- Chemical Detection of Starch
1. Place thin slice of onion on microscope slide, add drop of water, cover with cover slip and observe under microscope. Add drop of iodine and observe.
2. Place thin slice of potato on slide, add drop of water, cover with cover slip and observe. Add drop of iodine and observe.

Part 3- Chemical Detection of Amino Acids
1. Touching piece of filter paper around edges only, write words “water”, “protein”, “thumb”, and “amino acid” 2-3 inches apart.
2. Near word place drop of water, protein or amino acid and press thumb near “thumb”.
3. Allow drops to dry then spray with ninhydrin reagent.
4. Dry in incubator at 37°C and fill in chart.

Part 4- Chemical Detection of Protein
1. Label test tubes #1, #2, and #3. Mark each 1cm and 2cm from bottom.
2. Fill each to first mark with Biuret reagent
3. Fill #1 to 2nd mark with distilled water, #2 with protein, and #3 with starch. Record results.

Part 5- Chemical Detection of Lipids
A. Sudan-Black Test
1. Mark test tube with 2 lines (1st 2cm from bottom, 2nd 3cm from bottom). Add tap water to 1st mark and oil to second. Shake and observe.
2. Add a few drops of bile salts to tube and shake again.
3. Place drop of contents of tube on slide and add drop of Sudan-Black. Cover with cover slip. Use microscope to determine which material is stained by Sudan-black.
B. Paper Test for Lipids
1. Place drop of water on piece of brown paper and add drop of oil 2-3 inches away. What is immediate effect? What do you observe after 5 minutes?

Discussion and Conclusion:
Part1 – Benedict’s reagent tests for monosaccharides and disaccharides. A polysaccharide has no color change when combined with Benedict’s reagent and heat. A disaccharide turns green when combined with Benedict’s reagent and heat. A monosaccharide turns yellow, orange, or red when combined with Benedict’s reagent and heat. The first test tube will have no color change when heated because this is the control group with no carbohydrate present. Immediately after termination of the experiment, tube #2 was green, however the color change was only slight if not at all. The sugars that are present could therefore be either starch and/or maltose. Tube #3 turned yellow, showing that glucose is the sugar present.

Part 2 – The mixture of iodine and starch causes the color to become blue, purple, or black depending on the concentrations of the two materials. According to Benedict’s and iodine tests an onion stores carbohydrates in the form of glucose and a potato stores carbohydrates in the form of starch.

Part 3 – A positive test for amino acids will be a purple or violet color. According to the chart, water has no amino acid present, proteins have some amino acids and skin could possibly have some amino acids but contamination from the other chemicals is also a possibility.

Part 4 – Biuret reagent will turn pink in presence of a small polypeptide and violet in the presence of a large polypeptide. Water, as discovered in part 3, has no amino acids and would therefore have no color change. Starch is a polysaccharide and therefore has no amino acids present. Proteins are complex molecules that have one or more polypeptides and therefore the violet color proves this conclusion.

Part 5 – Emulsification breaks down large fat globules into little fat globules. Bile released from the gall bladder does this action in the small intestine, allowing the digestion of lipids like oil. Oil congeals while water is more fluid and therefore easier to digest. The bile aids the body in digesting fats. The oil is stained by the Sudan-black since fat molecules are long strains of fatty acids. This explains the spider-web formation displayed on the microscope.

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