Testing Various Solutions for Biological Macromolecules using Iodine, Benedict’s Solution, and the Biuret test

Introduction:

This lab will be testing different solutions to see weather they contain

various biological macromolecules. The three tests being preformed are an iodine

test for starch and glycogen, the Benedict’s test for reducing sugars, and the biuret

test for proteins. 12 different solutions will be subjected to each of these three tests

in order so see which of the biological macromolecules that are being tested for are

present. The iodine test will reveal if starch or glycogen is present, because starch

and glycogen are very similar in terms of the atoms and molecules they are made up

with but bond slightly different iodine solution will turn blue-black in the presence

of starch and reddish brown in the presence of glycogen. If the benedict’s test

results in a colour change of yellow, green, red, brown or a combination of these

then a reducing sugar is present in the solution. The final test, the biuret test will

test for proteins turning the solution purple violet if they are present.

Materials and method:

Refer to Bio130L lab manuel 2001 pages 16-20

Results:

Iodine testSolution 1: negSolution 2: negSolution 3: negSolution 4: negSolution 5: negSolution 6: negSolution 7: positive glycogenSolution 8: positive starchSolution 9: negSolution 10: negSolution 11: negSolution 12: positive glycogen

Benedict’s test Solution 1: posSolution 2: negSolution 3: posSolution 4: posSolution 5: negSolution 6: posSolution 7: negSolution 8: negSolution 9: negSolution 10: pos (hard to tell if reaction ordered as solution is already yellow orange)Solution 11: negSolution 12: neg

Biuret test Solution 1: negSolution 2: negSolution 3: negSolution 4: negSolution 5: negSolution 6: negSolution 7: negSolution 8: negSolution 9: posSolution 10: negSolution 11: negSolution 12: neg

Discussion:

The iodine test was as one might expect. The glycogen solution, solution

number seven, reacted by changing colour to reddish brown, and the starch

solution, solution eight, reacted by turning a dark navy blue. The only other solution

to react was the unknown solution, solution 12, which reacted by turning a reddish

brown. From the one can infer that the unknown solution contains glycogen.

Possible sources of error that may have occurred during this test is a contamination

of solutions as many pipettes were used and allowed none were used twice the

testing was done on a spot plat so some solution may have splashed over into other

bowls. Considering that all the tests went, as one would expect it could be

considered unlikely that and significant contamination did occur.

The Benedict’s test reveled that solutions 1, 3, 4, 6, and 10 glucose, maltose,

honey, lactose, and beer respectively, contain sugars. Solutions 1, 3, 4, and 6 turned

orange red as one may expect as they are all sugars or solutions that contain sugars

(BrilliantBiologyStudent, 2012). Solution 10 however turned a light shade of green

when the benedict’s solution was added, then proceeds to turn and orange yellow

after heated slightly darker than the beer solutions original colour. It is unclear

weather or not this solution reacted in the heat bath or simple reverted to its

original colour under the influence of the heat. As it is unknown what brand of beer

was tested it cannot be said for certain wither solution 10 contains any sugars that

would react to the Benedict’s test. Sources of error for the Benedict’s test may have

arose from the heat bath. While all the solutions spent exactly five minutes in the

heat bath while the water was as a boil the exact temperature of the water was not

measured or maintained to be equal throughout the test. Contamination of the

samples may still have occurred but seems unlikely as each solution was transferred

directly form its beaker into its own test tube.

The biuret test reveled only solution nine, protein solution, to contain any

proteins. This is as one may expect because only solution nine is known to contain

proteins. The beer solution again may have reacted depending on what brand of

beer it is, as it is known that some brands contain protein (SparkPeople, 2012), and

it may be impossible to tell because the colour of the beer is masking any results.

Sources of error for this experiment may have arose when adding the solutions to

the test tubes as they were all beside each other in a rack, solution may have spilled

into neighboring tubes. However as stated before this may be unlikely as the results

produced by the test were as one may expect.

In conclusion concerning the unknown solution one may determine based on

the results of this experiment that the solutions contains glycogen, and does not

contain starch, proteins, or any sugars that the Benedict’s solution can test for.

Further tests would be needed to determine the identity of the unknown solution

with a higher level of certainty.

References:Department of biology 2001 Introductory Cell Biology Laboratory Manual. University of Waterloo, Waterloo. Pp. 16-20

SparkPeople. (2012, feb 8). calories in molson canadian. Retrieved from http://www.sparkpeople.com/calories-in.asp?food=molson canadian

BrilliantBiologyStudent. (2012). Benedict's test for reducing sugars. Retrieved from http://www.brilliantbiologystudent.com/benedict-s_test_reducing_sugar.html