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Running head: TEST FOR BIOLOGICAL MOLECULES 1

Test for Biological Molecules

Student’s Name

Institutional Affiliation

TEST FOR BIOLOGICAL MOLECULES 2

Test for Biological Molecules

Introduction

The experiment aimed to test for the presence of certain biological molecules in

unnamed substances (liquids). The molecules that were being investigated include proteins,

starch, lipids, and a reducing sugar (glucose). The identification of these molecules is made

possible by their ability to react with certain reagents and produce predetermined chemical

changes. Each specific molecule reacts with a particular reagent, and a catalyst in some cases,

leading to the change in a particular property of the reagent, such as color or solubility. The

transformation in color or other properties of the solution is based on the occurrence of a

chemical reaction between the test solution and the reagent. For the starch test, the required

reagent is a solution of iodine, which upon reaction with starch, changes color from brown to

blue-black. For the protein test, the reagent is Biuret solution, which changes color from blue

to purple. For the reducing sugar test, Benedict’s solution is used, and it changes color from

blue to a reddish orange or green color in presence of glucose. Lastly, for the lipids test,

Sudan VII dye is sued and the presence of these molecules is confirmed if the test solution

dissolves the dye. To confirm the efficacy of the reagent, a control test is performed using a

solution of known properties, so that the excepted chemical change is elicited. If the control

test does not yield the required results, it implies that the experiment is faulty, and something

must be corrected until the outcomes appear as expected.

Hypothesis

Upon the addition of iodine, solutions containing starch should turn blue-black. The

test solutions containing proteins should change Biuret reagent from blue to purple. The

solutions containing lipids should dissolve Sudan IV dye, while those having reducing sugars

should transform Benedict’ solution from blue to green or orange color.

Materials

TEST FOR BIOLOGICAL MOLECULES 3

Test tubes

Droppers

Reagents: Iodine, Biuret reagent, Benedict’s solution and Sudan IV dye.

Water bath

Sodium Hydroxide solution

Methods

Starch Test

 Put a small amount (10 drops) of test solution in a test tube.

 Add 5 drops of iodine solution.

 Shake and check for any color change.

 Repeat the above three steps for both the positive and negative control test solutions,

each at a time and in separate tubes.

Test for Reducing Sugars

 Put a small amount (10 drops) of test solution in a test tube.

 Add 5 drops of Benedict’s solution.

 Boil the solution in a water bath for 3-5 minutes.

 Shake and check for any color change.

 Repeat the above three steps for both the positive and negative control test solutions,

each at a time and in separate tubes.

Protein Test

 Put a small amount (1 ml) of test solution in a test tube.

 Add 5 drops of Biuret reagent.

 Add 4 drops of Sodium Hydroxide (NaOH) solution

 Shake and check for any color change.

TEST FOR BIOLOGICAL MOLECULES 4

 Repeat the above four steps for both the positive and negative control test solutions,

each at a time and in separate tubes.

Lipids Test

 Put a small amount (2 ml) of test solution in a test tube.

 Add one toothpickful of Sudan IV dye reagent.

 Shake thoroughly and observe if the dye dissolves completely.

 Repeat the above three steps using one test tube filled with 2 ml of the control test

solution and another with 2 ml of water, each at a time

Results/ Data

Starch Test

Name and

Amount of

test

solution

used

Name, color,

and Amount

of Reagent

used

Catalyst

End

Color

Presence of

Starch

Color of the

Positive

Control Test

Solution

Color of

the

Negative

Control

Test

Solution

10 drops of

water

5ml of

Iodine(brown)

None

brown

blue-black

brown

10 drops of

starch

Blue-

black

Yes

10 drops

(unknown

solution)

blackish

Yes

10 drops

(unknown

solution)

Deep

brown

TEST FOR BIOLOGICAL MOLECULES 5

Test for Reducing Sugars

Test for Proteins

Test for Lipids

Discussion

Explanation for the Observed Changes

In the test for starch, Iodine solution (I2KI) reacts with the starch in the test solution.

Starch is a polymer of glucose, and it makes iodine to change color after reacting with the

coils of the polymer. Thus, the iodine changes color from brown to blue-black, and

Name and

Amount of

test

solution

used

Name, color,

and Amount

of Reagent

used

Catalyst

End

Color

Presence of

Reducing

Sugars

Color of the

Positive

Control

Test

Solution

Color of

the

Negative

Control

Test

Solution

Benedict’s

Solution

Heat

Name and

Amount of

test

solution

used

Name, color,

and Amount

of Reagent

used

Catalyst

End

Color

Presence of

Proteins

Color of the

Positive

Control

Test

Solution

Color of

the

Negative

Control

Test

Solution

Biuret

Reagent

NaOH

Name

and

Amount

of test

solution

used

Name,

color, and

Amount

Reagent

used

Catalyst

Solubility

of the

dye

End

color

Presence

of Lipids

Color of

the

Positive

Control

Test

Solution

Color of

the

Negative

Control

Test

Solution

Sudan IV

dye

None

TEST FOR BIOLOGICAL MOLECULES 6

sometimes a precipitate is formed. In the absence of starch, the color of the test solution

mixed with iodine remains brown.

In the test for reducing sugars, Benedict’s solution is reduced by glucose and other

reducing sugars, causing it to change color from blue to reddish orange. Benedict’s solution is

an oxidizing agent; hence it can be reduced by these sugars. Essentially, it contains cupric

ions, which gives it the blue color, mixed with citrate in an alkaline solution. The cupric ions

are reduced to cupric oxide, causing the change in color. Sometimes the color changes to

green, which means that the amount of reducing sugars is small. For this chemical reaction to

happen, a catalyst is required in the form of heat, and, thus, when Benedict’s solution is

mixed with the test solution, it must be heated in a water bath for 3-5 minutes to catalyze the

reaction.

In the test for protein, Biuret reagent is used. It is predominantly composed of CuSO4,

which explains the light blue color. The peptide bonds in proteins have an affinity for this

reagent, and they bond with it causing a change in color (Handelsman et al., 2004). After

exposure to proteins, Biuret reagent changes color from light blue to purple. In order for the

reaction to happen efficiently, a catalyst in the form of NaOH is required.

In the test for lipids, the primary concept that lipids are non-polar, hence they dissolve

the red Sudan IV dye. On the other hand, polar liquids like water do not dissolve the dye

(Handelsman et al., 2004). Therefore, on the addition of Sudan Iv dye to a lipid solution, the

mixture is entirely miscible. Thus when a test solution dissolves Sudan IV dye it means lipids

are present.

Restatement of hypothesis

Upon the addition of iodine, solutions containing starch should turn blue-black. The

test solutions containing proteins should change Biuret reagent from blue to purple. The

TEST FOR BIOLOGICAL MOLECULES 7

solutions containing lipids should dissolve Sudan IV dye, while those having reducing sugars

should transform Benedict’ solution from blue to green or orange color.

How the outcomes compared to the expected results

The outcomes of the experiment were as expected since the color changes happened

perfectly. Additionally, the control tests confirmed that these reagents were working properly.

Sources of errors

Some of the things that could have altered the results include the use of unclean

apparatus, using disproportional volumes of reagents and test solutions, and lack of catalysts

in the reactions that needed some (Bonini, Plebani, Ceriotti, F., & Rubboli, 2002). If I did the

experiment again, I would try to warm all the mixtures of reagent and test solution to see if

there would be further color changes. This would help me to know whether heat can trigger

further reactions in the tests that did not optimize through increasing temperature, such as the

starch and protein test.

TEST FOR BIOLOGICAL MOLECULES 8

References

Bonini, P., Plebani, M., Ceriotti, F., & Rubboli, F. (2002). Errors in laboratory

medicine. Clinical chemistry, 48(5), 691-698.

Handelsman, J., Ebert-May, D., Beichner, R., Bruns, P., Chang, A., DeHaan, R., ... & Wood,

W. B. (2004). Scientific teaching. Science, 304(5670), 521-522.

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