Lab 2: Recognizing Certain Macromolecules By Conducting the Benedict’s test, Iodine test, and Biuret testPrapti Patel20723924Partner: Priya VyasTA’s: Lawrentina D’Souza and Xinyi LiExperiment performed on September 26, 2018BIOL 130L section 019Wednesday 2:30PM to 5:20PM, STC 4008Due on October 16, 2018Introduction The motive of this lab was to was to recognize certain macromolecules and to identify what the unknown sample (sample 106) was (Alberts et al.
, 2014). This can be done by comparing and contrasting the results from the multiple tests carried out in the lab, which were the Iodine test, Benedict’s test and the Biuret test (Alberts et al., 2014). All the tests had experimental controls to assure that the required results were being met; The controls were disparate for all 3 tests (Alberts et al., 2014). A positive control provides a positive result and a negative control provides a negative result (Alberts et al., 2014).
Since the results of the known macromolecules are given at the end, it’s easy to identify if the expected reaction has taken place or not (Alberts et al., 2014). Firstly, the Iodine test is performed to identify starch and glycogen from all of the sample solutions. Starch and glycogen are all polymers of glucose, both are energy storage molecules, and they also have similar glycosidic linkages. They differ in that starch is a plant product whereas glycogen is an animal product (Edwards et al., 2011).
Secondly, biuret test is performed to identify proteins in the solutions. A protein is a polymer built from amino acids that supplies cell with their structure and does most of their activities (Alberts et al., 2014). Thirdly, the benedict’s test is performed to identify the reducing sugars from the sample solutions. Materials and MethodsPlease refer to “Lab 2: Identification of Some Macromolecules” on pages 15-20 in the BIOL 130L lab manual for the procedure. (Department of Biology, 2018). All steps were followed without deviationResultsTable 1: Qualitative data for the Iodine test, Biuret test, and Benedict’s testTest Beaker/ Test Before After Test results tube numberIodine Test 1 Clear, colourless, Pale yellow, – and transparent transparent, and bright 2 Clear, colourless, Pale yellow, – and transparent transparent, and brighter than number 1 3 Clear, colourless, Bright pale yellow, – and transparent transparent 4 Clear, slight- yellow, Bright pale yellow, – and transparent transparent 5 Clear, slight-yellow, Bright pale yellow, – and transparent transparent 6 Clear, slight-yellow, Bright pale yellow, – and transparent transparent 7 Slightly white, Dark yellowish- + colourless, and brown, translucent translucent 8 Clear, colourless, Opaque, navy blue, + and transparent dark 9 Cloudy, colourless, Bright neon yellow, – and has fizz bubbly, translucent 10 Dark yellowish- Translucent, mild – brown, and yellow, no fizz translucent 11 Clear, colourless, Translucent, mild – and transparent yellow 12 Slightly cloudy Translucent,orangish- + and translucent brownBenedict’s Test 1 Clear, colourless, Saucy red, opaque + and transparent 2 Clear, colourless, Cerulean blue, – and transparent translucent 3 Clear, colourless, Opaque, maroon + and transparent 4 Clear, slight- yellow, Opaque, dark brown + and transparent 5 Clear, slight-yellow, Opaque, brown + and transparent 6 Clear, slight-yellow, Cerulean blue, – and transparent translucent 7 Slightly white, Cerulean blue, – colourless, and translucent translucent 8 Clear, colourless, Cerulean blue, – and transparent translucent 9 Cloudy, colourless, Cerulean blue, – and has fizz translucent 10 Dark yellowish- Opaque, solid + brown, and yellow, fizz present translucent 11 Clear, colourless, Cerulean blue, – and transparent translucent 12 Slightly cloudy Cerulean blue, – and translucent translucentBiuret Test 1 Clear, colourless, Translucent, light – and transparent blue 2 Clear, colourless, Translucent,light – and transparent blue 3 Clear, colourless, Translucent, light – and transparent blue 4 Clear, slight- yellow, Translucent, light – and transparent blue 5 Clear, slight-yellow, Translucent, light – and transparent blue 6 Clear, slight-yellow, Translucent, light – and transparent blue 7 Slightly white, Translucent, light – colourless, and blue translucent 8 Clear, colourless, Translucent, light – and transparent blue 9 Cloudy, colourless, Slight opaque, dark + and has fizz navy blue, fizz present at surface 10 Dark yellowish- Light blue at top, – brown, and purplish at bottom, translucent translucent 11 Clear, colourless, Translucent, light – and transparent blue 12 Slightly cloudy Translucent, light – and translucent blue DiscussionThe iodine test, Benedict’s test, and biuret test, all had positive and negative controls.
The controls were necessary in order to compare and contrast the results from the multiple tests carried out in the lab. To find if a sample of the solutions had a reducing sugar, the benedict’s test was used . Glucose, maltose, honey, and beer all tested positive for the benedict’s test (we know this because a colour change occurred), however, honey and beer are not sugar but contain a reducing sugar (Alberts et al., 2014), so just glucose is the positive control for this test. Glucose was initially clear and colourless but after the experiment, it changed to an opaque, red colour. Maltose was also clear and colourless but changed to an opaque red. Honey was initially clear and pale yellow but changed to an opaque, dark brown .
However, beer which was initially yellowish brown, changed to an opaque solid yellow; This is due to the cuprous oxide that occurred. Precipitate occurred in glucose, maltose, honey, and beer. Sucrose was tested to be positive, however, it should have been a negative control.
Similarly, lactose was tested to be negative and it should have been a positive control. The misconduct could have happened due to contamination between different samples. The unknown sample was a negative test because no colour change occurred in that test tube, which tells us that it is not a reducing sugar.To find if a sample of the solutions had protein present in it, the biuret test was performed. Protein tested positive for the biuret test.
The protein changed from a cloudy, colourless solution to a dark navy blue colour therefore, protein was the positive control. Rest of the 11 test tubes tested negative for the biuret test because they had no colour change. The unknown sample was a negative test because no colour change occurred in that test tube, which tells us that it is not a protein.The last test performed was to determine starch and glycogen from the solutions.
This is the iodine test. Glycogen and starch both tested positive for this experiment, however the unknown solution also tested to be positive. The sample that had starch changed to navy blue whereas the sample that had glycogen changed to a dark yellowish- brown colour. Furthermore, the unknown solution had a colour change from a cloudy colour to an orangish-brown colour. This means that the unknown test tube is glycogen.
Observed result was attained for the unknown solution, however, two of my control samples did not produce expected results, I would question whether or not the unknown test was correctly identified. Some sources of error that may have caused variations in the results could be due to not allowing the water in the beaker to a complete boil, adding way more or way less drops of iodine (Lugol’s) solution and/or 1% copper sulfate. Another error that could have cause inaccuracy with the results could have been due to cross contamination of the solutions. Furthermore, their was very less distance between the test tubes in the test tube rack and therefore contamination may have taken place while adding certain solutions to the test tubes.
To avoid these errors, I would take in regard that all the test tubes are put in at the same time for the same amount of time, and seal the top of the test tubes so no contamination occurs and do a few trials of the experiment for better accurate results. Once all the tests were performed, we could identify the positive and negative controls from the sample of solutions. According to the observational data and the results, the unknown solution is glycogen. This was due to the fact that the unknown solution reacted to iodine test similarly to as the glycogen reacted with the iodine test (colour changed to orangish- brown). Therefore sample 106 seems to be glycogen. ReferencesDepartment of Biology.
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