The Use of Controls and Indicators to Find What Macromolecules are in an Unknown

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The Use of Controls and Indicators to Find What Macromolecules are in an Unknown

Name: Erin

Teaching Assistants: Laura and Polina, Section # 013

Course # 4677 Day: Monday, Time: 7:00PM – 9:50PM, Room #: B2 151

Lab Performed: Monday September 15th, 2008


The objective of this lab was to discover what macro molecules were in the unknown using different substances as experimental controls. Experimental controls are samples used in a lab that have a known outcome. This outcome can show what an unknown is comprised of. A positive control shows what the outcome of the experiment should look like if the experiment were to work. A negative control shows what would happen to the substance if the experiment were to go wrong. When compared to the control, what type of macromolecules in the unknown can be discovered (Enlexica, Inc, 2008).

Indicators are usually used to expose a certain substance within a substance and normally shown through a colour change.  These indicators were used to create the controls and used to identify certain macro molecules in different substances. The three different indicators that were used in this lab were Lugol’s iodine solution, Benedict’s solution and Biuret solution (Buckley Jr., 2003).

Lugol’s solution, otherwise known as iodine solution is an indicator used to find starch in a substance. This solution is made of 5% iodine, 10% potassium and 85% distilled water. The positive outcome of this indicator shows the presence of starch in an organic compound by turning a dark-blue or black colour in the presence of starch or reddish-brown in the presence of glycogen from its original pale yellow colour (Answers Corporation, 2008). The reason the substance turns these colours is because of the staining of Lugol’s solution on coiled polysaccharide molecules in the starch and glycogen. Lugol’s solution does not stain simple sugars because it only stains polysaccharides which are starches and glycogen, or complex sugars.

Benedict’s solution is used to indicate the presence of an aldehyde or ketone functional group. An aldehyde functional group is found in a reducing sugar when in a basic solution. A reducing sugar is a sugar that has a potentially free aldehyde group that is used for reducing alkaline solutions. Examples of reducing sugars are monosaccharides and most disaccharides (Answers Corporation, 2008). Benedict’s solution is an excellent indicator of reducing sugars because it is an alkaline. This solution contains cupric sulphate which is reduced to cuprous ions by the aldehyde group in the reducing sugar. The cuprous ions, when heated, form a precipitate of cuprous oxide, which can range from yellow to green or red to brown. The precipitate shows that the test is positive and the substance is in fact a reducing sugar (Pearson Education, 2007).

Biuret solution is used to indicate proteins and peptides in a substance. Biuret solution is a mixture of copper II sulphate and potassium hydroxide. The copper atoms will react with the peptide bonds, which are the bonds within proteins, to produce a colour change. A violet colour change occurring from the original light blue means that the substance contains protein. Proteins are composed of different arrangements of the 20 different amino acids. Amino acids are held together by peptide bonds (Holland, 1999).

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Materials and Methods

All steps were carried out as stated in BIOL 130L Lab Manual, for FALL 2008, pages 18 – 22.


Table 1: Results of Iodine Test for Starch and Glycogen

Name of Control                Positive/Negative Result                 Colour                

1% Glucose solution                Negative                                Pale yellow        

0.3% Glucose-1-phosphate        Negative                                Pale yellow

1% Maltose solution                Negative                                Pale yellow

Honey solution                Negative                                Pale yellow

1% Sucrose solution                Negative                                Pale yellow

1% Lactose solution                Negative                                Pale yellow

1% Glycogen solution                Positive                                Brown

1% Starch                        Positive                                Dark blue

Protein                                Negative                                Pale yellow

Beer                                Negative                                Yellow

Distilled water                        Negative                                Pale yellow

Unknown # 281                Positive                                Brown


One drop of iodine solution was added to each of these solutions. Table 1 shows that ...

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