Testing for Macromolecules

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Table 3: Biuret test for protein

Test tube number

Before colour change

Positive/negative reaction

After colour change

1

15ml of 1% glucose solution (clear)

Negative

Blue

2

15ml of 0.3% glucose-1-phosphate (clear)

Negative

Blue

3

15ml of 1% maltose solution (clear)

Negative

Blue

4

15ml of honey solution (yellow)

Negative

Light Blue

5

15ml of 1% sucrose solution (clear)

Negative

Blue

6

15ml of 1% lactose solution (clear)

Negative

Blue

7

15ml of 1% glycogen solution (clear)

Negative

Blue

8

15ml of 1% starch solution (clear)

Negative

Blue

9

15ml of protein (clear)

Positive

Purple

10

15ml of beer (clear)

Negative

Blue

11

15ml of distilled water (clear)

Negative

Blue

12

15ml of unknown #72 solution (clear)

Negative

Blue

This table shows the results of the Biuret test which was used to test for proteins present in the test tube solutions. Since amino acids are used to build proteins that are linked together into polypeptides by peptide bonds (Alberts et al, 2014, p.56). The peptide bonds react with the Biuret solution and form a violet colour. Most of the test tubes had negative results except for test tube #9 which had a positive result indicating protein is present within this solution.

Discussion

The results obtained from the Iodine test indicated a lack of starch and glycogen in most test tube solutions. Most test tube solutions turned a pale yellow colour after iodine solution was added while test tube #7, #8, and #12 changed colours. Solutions that turned blue-black indicated the presence of starch while the colour reddish-brown of solution indicated the presence of glycogen. The positive results occurred because test tube #7 contained 15mL of 1% glycogen solution and test tube #8 contained 15mL of 1% starch solution. Therefore, it was known that these solutions would change colour to indicate the correct macromolecule present. Test tube #12 which contains the unknown solution turned a blue-black colour tested positive for starch. The chemistry behind this experiment is that starch contains two polymers, amylose and amylopectin. Amylose molecules are made up of monomer glucose molecules therefore, when iodine is added to the test tube solutions, when reacting with the amylose molecule it forms a blue-black colour (Bates et al, 1943, p.143). Similarly, glycogen has a structure similar to the amylopectin molecule which is branched, therefore when reacting with iodine, it forms a reddish-brown colour (Bates et al, 1943, p.143).

The results obtained from the Benedict’s test showed that the test tubes that changed colours ranging from yellow to green or red to brown indicated the presence of reducing sugars. Test tubes #1, #3, #4, #6, and #10 has positive results in this experiment. This occurred because each test tube that had positive results contained 15mL of a sugar solution such as 1% glucose solution or maltose solution. Glucose is a sugar molecule used to storage energy in cells (Alberts et al, 2014, p. 52). Maltose is a disaccharide that is formed by combining two glucose molecules in a condensation reaction therefore it contains reducing sugar (Alberts et al, 2014, p.53). The unknown solution in this case, tested negative. According to the iodine test, the unknown solution tested positive for starch therefore starch does not contain reducing sugars hence the negative result in this experiment.

The results obtained from the Biuret test showed that the test tubes that changed colour from blue to violet indicated the presence of protein. Test tube #9 tested positive for protein which was expected since the test tube solution contained 15 mL of protein. The chemistry behind this reaction is that amino acids are used to build proteins that are linked together by peptide bonds (Alberts et al, 2014, p.56). The peptide bonds react with the biuret solution which contains copper ions that form the violet colour (Department of Biology, 2016). The unknown solution tested negative for protein. Referring back to the results of the iodine, the unknown solution is a starch therefore it does not contain any proteins.

References

Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., . . . Walter, P. (2014). Essential cell biology (4th ed.). USA: Garland Science, Taylor & Francis Group.

Andreoni, J. (1995). Warm-glow versus cold-prickle: The effects of positive and negative framing on cooperation in experiments. Oxford University Press, 110(1), 1-21.

Bates, F. L.,