Chemical Test To Identify Biochemical Molecules Biology Essay



The aims of these experiments are to use chemical test to distinguish some biochemical molecules. As these molecules are very important with our lives, it is necessary to test them and understand their behavior


Biology chemical molecules play an important role in our lives, storage of energy such as starch (poly glucose), protecting structures such as protein, insulation material such as lipid, the fast energy providing molecules such as glucose, etc. Starch is a macromolecule which is easily tested. Starch is macromolecules made up of glucose units connect together with glycosidic bonds [Google Web Definition, 2009]. According to the information from virtual chembook [2009], iodine solution form I3- anion and can bind to a starch to make a helix structure. The color appears blue because all other light is absorbed

The basic way to distinguish these macromolecules is use some other chemical solution to react or mix with them and watch any color changes. Testing reducing sugar is another experiment that can be used to watch color changes, but it needs to be heated.

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Reducing sugar is the sugar molecule that behaves like reducing agent, it contains the aldehyde group (COOH) [Google web definition, 2009]. Glucose is heated with Benedict's reagent to form a brick red precipitate [Chemistry comes alive, 2009]. In fact, the Benedict's reagent is made from copper sulphate and sodium carbonate and it provides the Cu2+ anion and the Cu2+ was reduced to Cu+, which in turn has its aldehyde group oxidesied to acid (COOH).

Protein is another kind of macromolecules, and it needs more steps to test it. Protein is made of amino acids linked together, the signal part of protein and the bond for this test focus on is the peptide bonds. The website [Guangdong Biology Teaching, 2009] states that when the test solution includes peptide bond, the test reagent, Buiret Reagent, copper ions can react with the polypeptide and produce the complex compound with a purple color observed.

Lipid is a long chain of carbohydrate which has a hydrophilic ionic ‘head' and hydrophobic organic ‘tail'. The experiment here looks at this significance of lipid by using water and ethanol, to control its solubility and partitioning in both.


Equipments and Materials: several test tubes, pipettes, a bung, iodine, Fehling reagent, Biuret Reagent A and B, ethanol, distilled water

Solution A: starch solution Solution B: Glouse solution

Solution C: egg albumen (protein) Solution D: lipid (fat)


1 cm³ of solution A was added into a test tube. 3-4drops of iodine was added into the test tube. The color change in the tube was noted.

Reducing sugar

2 cm³ of solution B was added into a test tube. Before the test tube was heat in a water to bath about 60'C for 2 minutes, 5 cm³ of Fehling reagent was dropped into

the tube. The color change in the tube was noted.


2 cm³ of solution C is added into a test tube. Before 1 cm³ of Biuret Reagent B was trickled into the test tube by using a pipette, 2 cm³ of Buiret Reagent A was also than added into the test tube. The color changes in the tube were observed.


2 cm³ of solution D was added into a test tube. Then, 2 cm³ of ethanol was added tube, after the tube was shaken vigorously (use a bang), 2 cm³ of cold water was added into the tube. The change in the tube was noted.


Table 1 shows the results of these experiments, with all the color changes , they show that all the experiments are achieved.

Observation of the experiments










Fehling reagent

Fehling A

Biuret B


Cold water


Clear test solution to dark blue

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Yellow or red precipitate appear


Blue suspension on the bottom

Cloudy white suspension forms

Table 1


The observations from the tests show clearly that changes occurred in all the biology molecules after using some reagent. However, many errors were possible during the experiments. For instance, when the protein test was being done, the reagents may have been dropped in the wrong order, the color of the solution did not change and show light blue. After 2 more experiments, the correct one was procedure was established.

At the same time, some more new ideas from other students were gathered. Classmate Crystal (2009) suggested that while doing the reducing sugar test, a change in the order of the Fehling reagent could be tried, but the results came from them were the same!

It was also found that before the tube with lipid was shaken, there were two white layers and a clean layer between them. After the test was shaken, the wonderful cloudy suspension appears. It may be that before the shaking, the oil was in the middle and the hydrophilic head react with the water and the hydrophobic react with the ethanol. After the tube was shaking, they mixed.

Meanwhile, there were several differences between the results we obtained and the ones from the theory. Such as the color of the changes of the test tube which with starch. The color seen was much darker than the description of the color which was got from the theory. Maybe the iodine solution was added too much because it was not measured accuracy.


In brief, it is easy to test the biology molecules and distinguish each one by watching the color changes. In addition, there are many things to be careful to during the experiments such as the order of the reagents, the time that some tubes should be shaken or even heated –failure is possible! Remembering the steps to do them in order then success is possible.


Virtual chembook ( (16 November 2009)

Chemistry comes alive! ( RGANIC/ORG18/TRAM18/B/MENU.HTM) (16 November 2009)

Guangdong Biology Teaching ( (16 November 2009)

Google web definition ( en&q=define:starch&ei=inYKS8W0NouNkAX-oNzeCQ&sa=X&oi=glossary_ definition&ct=title&ved=0CAcQkA E) (23 November 2009)

Google web definition ( define%3Areducing+sugar) (23 November 2009)