Testing for Macromolecules

Essay by firenbellHigh School, 11th gradeA+, October 2009

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PURPOSEThe purpose of the lab is to learn how to determine the presence of glucose, starch, lipid, and protein in various samples through standard tests.

MODIFICATION1. Protein solution, vegetable oil, glucose solution, sucrose solution, starch solution and distilled water were added separately into six marked test tubes. The volume added was about one finger thick in depth of a test tube.

2. 5 drops of Iodine solution was added to each test tube.

3. The sample's reaction with iodine was recorded.

4. The samples connected by another group with 5 drops of Benedict's solution added were heated all together in hot water bath.

5. The test tubes containing samples were taken out of the hot water bath with test tube holders, the reaction was carefully examined and the observations were recorded.

6. The test of the Sudan IV solution was changed to Brown paper bag testing, which was done by the teacher before the class.

7. The brown paper bags were dried off by the teacher before the class. The remainders and reactions on the dried brown paper bags were examined and observations were recorded.

8. The samples reacted with chemicals were disposed in the special disposal Erlenmeyer flask provided by the teacher. Samples were recollected as stated in the first procedure.

9. 5 drops of NaOH and 5 drops of CuSO4 were added to each test tube. The Biuret reagent was made by the mixing of two chemicals.

10. The reaction with Biuret reagent was observed and recorded.

11. The solutions were disposed as stated in the 8th procedure; the lab equipment and chemicals were returned to the teacher.

CONCLUSIONSThrough this lab, we learned how to determine protein, lipids, glucose and starch with their indicators from unknown samples.

During the lab, I have noticed that the protein solution reacted most dramatically with the Biuret reagent. When adding drops of Biuret reagent to other samples, they all turned blue, some with particles and yellow or green layers. However, with the protein solution, the solution turned purple, which was most noticeable in all the blue colours. So I have concluded that the Biuret reagent is the indicator of the protein solution. It went the same with samples containing lipids, glucose and starch. The vegetable oil had the most obvious reaction with the brown paper bag test. As among all the samples, only vegetable oil made the brown paper bag turned translucent. The other samples did not make any change in colour or clarity of the brown paper bag while reacting. So I have concluded that the Brown paper bag test is the method to indicate the lipids. The glucose solution reacted most dramatically with Benedict's solution after it got heat up in hot water bath. Other samples all turned blue and stayed transparent or translucent after reacting with the Benedict's solution after heating up, but the glucose solution turned opaque and its colour changed to orange. So I have concluded that we can use Benedict's solution and then heat up the sample to indicate glucose. The starch solution reacted most dramatically with iodine solution. Other samples changed their colour to orange after dropping the iodine solution in, only the starch solution turned purple and opaque. So it leads to the conclusion that the iodine solution is the indicator for starch.

To answer the purpose, when we want to identify protein, lipids, glucose and starch, we use their indicators to react with the sample we want to examine separately. If the sample we want to examine turned orange after adding Benedict's solution and heating up, we know the sample contains glucose. If the sample turned purple after dropping iodine solution in, we know the sample contains starch. If after the brown paper bag dried off, the paper bag stilled stayed translucent, we know the sample contains lipids. If the sample turned purple after we drop Biuret reagent in, we know the sample contains protein. We use the chemicals as indicators because they have an obvious and dramatic reaction with specific macromolecules and made it easy for us to identify the macromolecules contained in unknown samples.

POST LAB QUESTIONS1.Describe a positive test for starch. Explain how you know.

A positive test for starch would be adding iodine solution to the solution for test. As in the experiment, the starch solution had the most dramatic reaction with the iodine solution. It turned to an opaque bluish-purple solution while other samples all turned to a colour related to orange and yellow.

2.Describe a positive test for glucose. Explain how you know.

A positive test for glucose would be adding Benedict's solution to the solution for test and then heat it. I know because in the experiment, the glucose solution had the most dramatic reaction with the Benedict's solution after it got heat up in the hot water bath. It turned to an opaque yellowish orange solution with dark-orange precipitate at the bottom while other samples all turned to a colour related to blue and green.

3.Describe a positive test for lipids. Explain how you know.

A positive test for lipids would be Brown paper bag test in which you pour some solution for test onto the brown paper bag and see the reaction after it dried off. I know this as in the lab, the vegetable oil had the most dramatic reaction with brown paper bag. The brown paper bag turned translucent which can be observed quite obviously while other samples did not have any obvious reaction with the brown paper bag. Except for oil, the colour of the brown paper bag did not change with other samples.

4.Describe a positive test for protein. Explain how you know.

A positive test for protein would be adding Biuret reagent to the solution for test. I know this because in the experiment, the protein solution had the most dramatic reaction with Biuret reagent. The solution turned to a colour of purple while other samples all had a colour related to blue and yellow.

5.What was the purpose of testing distilled water for each part of the investigation?The purpose of testing distilled water for each part of the investigation is because distilled water is a pure substance and we know it doesn't have any thing mixed in it. So the water's reaction with the indicators can represent the other solutions which didn't have the four macromolecules dissolved in them. Also it was to show the most natural reaction and colour change with the indicators without any other chemicals affecting the results.

6.Suppose you have a sample of breakfast cereal that may contain one, two, three, or all four of the macromolecules you tested for in this investigation. Write a procedure describing how you would test the sample to determine which macromolecules it contains.

Chop out small amount of breakfast cereal into four test tubes separately; the size is approximately 1/5 of one breakfast cereal sample. Add 1 drop of Benedict's solution to one test tube and heat it in hot water bath, 1 drop of iodine solution to one test tube, and 1 drop of Biuret reagent (or 1 drop of NaOH and 1 drop of CuSO4 to mix Biuret reagent) to one test tube. Also, pour the cereal in last test tube on brown paper and squeeze it.

If the heated cereal sample turned orange, then the sample contains glucose.

If the cereal reacted with iodine solution and turned purple, then the sample contains starch.

If the cereal reacted with Biuret reagent and the colour turned purple, then the sample contains protein.

If after the cereal was cleaned out of the brown paper bag, the brown paper bag turned transparent, then the cereal sample contains lipids.

There's no citing for this lab report because it is based on what we did in the lab