The Iodine Test sor Starch

ntroduction The purpose of this experiment was to use Iodine, Benedict and Biuret to test the reaction of the following 12 samples: 1% glucose, 0.3% glucose-1-phosphate, 1% maltose, honey, 1% sucrose, 1%lactose, 1% glycogen, 1% starch, protein, beer, distilled water and an unknown solution (test tube: 300). The iodine test for starch was to test how would starch reacted if we put iodine in it. The color of starch before the test was clear. The color of the iodine was brown. When you added iodine into starch, the result was the starch solution turned dark blue. Starch had a positive result is because of the complex of iodine stuck inside the amylase coil which produced a characteristic purple - black color (Elservier Science Publishers, 1988). Also, starch was composed of polymers of glucose and long linear chains were amylase (Alberts Bray, Hopkin Johnson, Lewis Raff, Roberts Walter, 2009). The Benedict’s test for reducing sugars was used to determine the presence of reducing sugars. The color of the samples before was clear. The color of the Benedict was blue. When you added Benedict into the samples and heated it up in the hot water bath for around 5 minutes. The aldehyde functional group was the reducing agent in reducing sugars and they all had a positive result was because of their ability to act as a reducing agent during the Benedict’s test. A reducing agent donated electrons during a redox reaction and was itself oxidized. Reducing sugars had either an aldehyde functional group or have ketone group – in an open chain form – which can be converted into aldehyde (Sur, B. K., R. K. Shukla, and V.S Agashe, 1972). The Biuret test for protein was used to determine the presence of peptide bonds in proteins. The color of samples before was clear. Both sodium hydroxide and copper sulfate were clear color as well. When you added sodium hydroxide and copper sulfate into the samples, protein would turn into purple. The structure of proteins was form by the peptide bond, which was found between the carboxyl and amino group of two adjacent amino acid residues (Alberts Bray, Hopkin Johnson, Lewis Raff, Roberts Walter, 2009). When peptide bonds were presented in an alkaline solution, the copper (II) ions would form a coordinated with four nitrogen atoms involved in peptide bonds. Copper sulfate solution was a blue color, but when the copper (II) ions were coordinated with the nitrogen atoms of these peptide binds, the color of the solution changes from blue to violet. The color changed was dependent on the number of peptide binds in the solution, so the more protein, the more intense the changed would be (American Chemical Society, 2000).

Materials and Methods The experiment was carried out as stated in the Biol 130L Lab manual. (Department of Biology, 2103) Without deviation.

Results

Table 1 – Iodine Test for Starch
Samples
Observations
Positive / Negative
Glucose (beaker # 1)
Yellow (no changed)
Negative
Glucose-1-phosphate (beaker # 2)
Yellow (no changed)
Negative
Maltose (beaker # 3)
Yellow (no changed)
Negative
Honey (beaker # 4)
Yellow (no changed)
Negative
Sucrose (beaker # 5)
Yellow (no changed)
Negative
Lactose (beaker # 6)
Yellow (no changed)
Negative
Glycogen (beaker # 7)
Brown
Positive
Starch (beaker # 8)
Purple Black
Positive
Protein (beaker # 9)
Yellow (no changed)
Negative
Beer (beaker # 10)
Yellow (no changed)
Negative
Distilled Water (beaker # 11)
Yellow (no changed)
Negative
Unknown (beaker # 12)
Brown
Positive

Table 2 – Benedict Test for Reducing Sugars
Samples
Observations
Positive / Negative
Glucose (test tube # 1)
Orange + Precipitate
Positive
Glucose-1-phosphate (test tube # 2)
Blue + No Precipitate
Negative
Maltose (test tube # 3)
Orange + Precipitate
Positive
Honey (test tube # 4)
Brown + Precipitate
Positive
Sucrose (test tube # 5)
Blue + No Precipitate
Negative
Lactose (test tube # 6)
Orange + Precipitate
Positive
Glycogen (test tube # 7)
Blue + No Precipitate
Positive
Starch (test tube # 8)
Blue + No Precipitate
Positive
Protein (test tube # 9)
Blue + No Precipitate
Negative
Beer (test tube # 10)
Yellow brown + Precipitate
Positive
Distilled Water (test tube # 11)
Blue + No Precipitate
Negative
Unknown (test tube # 12)
Orange + Precipitate
Positive

Table 3 – Biuret Test for Protein
Samples
Observations
Positive / Negative
Glucose (test tube # 1)
Clear (no changed)
Negative
Glucose-1-phosphate (test tube # 2)
Clear (no changed)
Negative
Maltose (test tube # 3)
Clear (no changed)
Negative
Honey (test tube # 4)
Clear (no changed)
Negative
Sucrose (test tube # 5)
Clear (no changed)
Negative
Lactose (test tube # 6)
Clear (no changed)
Negative
Glycogen (test tube # 7)
Clear (no changed)
Negative
Starch (test tube # 8)
Clear (no changed)
Negative
Protein (test tube # 9)
Purple
Positive
Beer (test tube # 10)
Clear (no changed)
Negative
Distilled Water (test tube # 11)
Clear (no changed)
Negative
Unknown (test tube # 12)
Clear (no changed)
Negative

Discussion According to Table 1: Iodine Test for Starch, the beaker #7 (Glycogen), beaker #8 (Starch) and beaker #12 (Unknown) were all either brown or dark purple. They all had a positive result. This Iodine test for Starch was a test for polysaccharides as well not just Starch. If any samples turned brown or dark purple, that means they were polysaccharide. Both glycogen and starch were polysaccharides because they had a positive result, same as our unknown beaker #12. The reason that there was a color changed was due to the iodine absorbing all light wavelengths corresponding to colors, just like not letting any light pass through, therefore the sample turned black. It was the absence of light and color all together (Elservier Science Publishers, 1988). On the other hand, referring to Table 1: Iodine Test for Starch again, all the other beakers had a negative result. Base on looking at the distilled water – the control, all the other samples did not have any color changed. The simple reason was because they did not have any starch/polysaccharide in them, the amylase coil was absent (Di Giuseppe Maurice, 2003).
According to Table 2: Benedict’s Test for Reducing Sugar, the beaker #1 (Glucose), beaker # 3 (Maltose), beaker #6 (Lactose) and beaker #12 (Unknown) were all orange color with precipitation. They all had a positive result. The reason that there was a color changed was depended on the quantity of reducing sugar presented. The Benedict’s test for reducing sugar was specific to any one type of reducing sugar, and that the color corresponds to the total reducing sugar presented. Benedict’s solution contains copper (II) sulfate, sodium carbonate and sodium citrate. The blue copper (II) ions from copper (II) sulfate were reduced to red copper (I) ions by the aldehyde groups in the reducing sugars and because of that it accounted for the color changed observed. As the concentration of reducing sugar increases, the nearer the final color is to brick – red and the greater the precipitate formed (Sur, B. K., R. K. Shukla, and V.S Agashe, 1972). On the other hand, referring to Table 2: Benedict’s Test for Reducing Sugar again, looking at the control – distilled water, all the other beakers had a negative result because they did not have any color changed. The reason was because these samples do not have any reducing sugar in them. Especially, for Sucrose, it was the only sugar with a negative result. The reason was because it did not reduce copper sulfate because the Benedict’s solution could not broke down the bonds of sucrose like other sugars (Di Giuseppe Maurice, 2003).
According to Table 3: Biuret’s Test for Protein, only beaker # 9 (Protein) had a color changed into purple. It had a positive result. The reason that protein had a positive result was based on the result on the ability of Copper (II) ions to form a violet – colored chelate complex with peptide bonds in alkaline conditions. The Hydrated Copper Sulfate provided the copper (II) ion to form the chelate complex. Copper (II) ions give the reagents its characteristic blue color. The peptide bonds in biuret give a positive result for the test (Harold A. Scheraga, 1961). On the other hand, referring to Table 3: Biuret’s Test for Protein again, base on the control – distilled water, all the other samples had a negative results and the reason was because they did not have any protein presented, which means they did not have any peptide bonds (Di Giuseppe Maurice, 2003).
In conclusion, all these tests above that we’ve performed seemed to have an accurate result in general. Starch, Glycogen and our unknown both gave the positive result in the Iodine test for Starch. Glucose, Maltose, Lactose and our unknown both gave the positive result in the Benedict’s test for Reducing Sugar. Protein also gave a positive result in the Biuret test for Protein. According all the result above, I could determine that our unknown was a Carbohydrate because it gave a positive result in both the Iodine test and the Benedict’s test.
Sources of Error and Improvements to the Procedure
One thing I would like to improve for this lab would be the using of the Pasteur pipettes. If we messed them up, then our results might not be accurate. So I suggested that we could put the pipettes right beside the correct samples to avoid mixing up. Other than that, there wasn’t any huge problem for this lab.

References Alberts, B. (2010). Sugars are Energy Sources for Cells and Subunits of Polysaccharides. Essential cell biology (3rd ed., pp. 52-53). New York: Garland Science.




Alberts, B. (2010). The Shape and Structure of Protein. Essential cell biology (3rd ed., p. 121). New York: Garland Science.

Department of Biology. (2013). Identification of Some Macromolecules. BIOL 130L Lab Manual (pp. 14-18). University of Waterloo.




 Giuseppe, M. (2003). Carbohydrate. Nelson biology 12 (pp. 29-30). Toronto: Nelson Thomson Learning.




Giuseppe, M. (2003). Carbohydrate . Nelson biology 12 (pp. 31-32). Toronto: Nelson Thomson Learning.




 Giuseppe, M. (2003). Protein . Nelson biology 12 (pp. 44-47). Toronto: Nelson Thomson Learning.Scheraga, H. A. (1961). Hydrodynamic Properties of Protein Solution. Protein structure (pp. 1-2). New York: Academic Press.




Singh, B. R. (2000). Techniques of Protein Structure Analysis. Infrared analysis of peptides and proteins: principles and applications (p. 3). Washington, D.C.: American Chemical Society.




Sur, B. K., Shukla, R. K., & Agashe, V. S. (1972). The Role Of Creatinine And Histidine In Benedict 's Qualitative Test For Reducing Sugar In Urine. Journal of Clinical Pathology, 25(10), 892-895.




Yalpani, M. (1988). Starch. Polysaccharides: syntheses, modifications, and structure/property relations (pp. 112-113). Amsterdam: Elsevier.

References: Alberts, B. (2010). Sugars are Energy Sources for Cells and Subunits of Polysaccharides. Essential cell biology (3rd ed., pp. 52-53). New York: Garland Science. Alberts, B. (2010). The Shape and Structure of Protein. Essential cell biology (3rd ed., p. 121). New York: Garland Science. Department of Biology. (2013). Identification of Some Macromolecules. BIOL 130L Lab Manual (pp. 14-18). University of Waterloo. Giuseppe, M. (2003). Carbohydrate. Nelson biology 12 (pp. 29-30). Toronto: Nelson Thomson Learning. Giuseppe, M. (2003). Carbohydrate . Nelson biology 12 (pp. 31-32). Toronto: Nelson Thomson Learning. Giuseppe, M. (2003). Protein . Nelson biology 12 (pp. 44-47). Toronto: Nelson Thomson Learning.Scheraga, H. A. (1961). Hydrodynamic Properties of Protein Solution. Protein structure (pp. 1-2). New York: Academic Press. Singh, B. R. (2000). Techniques of Protein Structure Analysis. Infrared analysis of peptides and proteins: principles and applications (p. 3). Washington, D.C.: American Chemical Society. Sur, B. K., Shukla, R. K., & Agashe, V. S. (1972). The Role Of Creatinine And Histidine In Benedict 's Qualitative Test For Reducing Sugar In Urine. Journal of Clinical Pathology, 25(10), 892-895. Yalpani, M. (1988). Starch. Polysaccharides: syntheses, modifications, and structure/property relations (pp. 112-113). Amsterdam: Elsevier.