GCSE Biology - AQA

2.1.6 - Testing For Carbohydrates, Proteins and Lipids

Get tutored by the creator of Mooramo

As well as making Mooramo, I also tutor Science and Maths in London and online.

To find out more, visit my website, georgewellertutoring.co.uk

Testing For Carbohydrates, Proteins and Lipids

Chemical tests can be used to identify many different biological molecules.

Testing for glucose

To test for the presence of glucose in a sample, you use a solution called Benedict's solution. First you add Benedict's solution to the sample, then you heat this mixture in a water bath for a few minutes.

Benedict's solution is blue. When heated with glucose it turns brick-red. Therefore, if the solution turns brick-red this shows that glucose is present, and if it stays blue, this shows that glucose is absent.

Diagram showing the process of testing for glucose using Benedict's solution. The diagram shows that first the sample is placed in a boiling tube and a few drops of Benedict's solution (which is blue) are added to it. Then, it is heated in a water bath for a few minutes. If it stays blue, then glucose is absent. If it turns brick-red, then glucose is present.

Testing for glucose using Benedict's solution.

Photograph of eight test tubes in a test tube rack on a bench in a laboratory. The tubes all contain the same volume of liquid, which comes a couple of centimetres up the test tubes. The test tube on the far left contains blue liquid. As you move to the right through the test tubes, the colour gradually becomes more red. The tube on the far right contains dark red liquid.

Benedict's solution mixed with glucose solutions of a range of different concentrations. In the tube on the far left, the Benedict's solution has stayed blue, indicating that the glucose concentration is very low or zero. Moving to the right, the colour becomes slightly more red in each tube - indicating that the glucose concentration is slightly higher each time.

(Note: there are other substances besides glucose that can turn Benedict's solution brick-red - including other sugars. Therefore, if it turns brick-red this doesn't completely prove that glucose is present. However, if it remains blue this does prove that glucose is absent).

Testing for starch

To test for the presence of starch in a sample, you add a few drops of iodine solution (iodine dissolved in water).

Iodine solution is yellow-brown. In the presence of starch it turns blue-black.

Therefore, if the iodine changes colour to blue-black, this shows that starch is present, and if it stays yellow-brown then this shows that starch is absent.

Diagram showing the process of testing for starch. The title says, "Testing for Starch". Below this, there are labelled drawings illustrating the following steps: 1. Place sample to be tested in a boiling tube. 2. Add a few drops of iodine solution. Results: If the solution stays yellow-brown, starch is absent. If the solution turns blue-black, starch is present.

Testing for starch using iodine solution.

Photograph of the bottom of a beaker containing a small amount of iodine solution. The iodine solution is yellow-brown in colour. The beaker is on top of a paper towel.

Some iodine solution in a beaker. It is yellow-brown. Image: Farhang Amini via Wikimedia Commons (CC BY-SA 4.0 - https://creativecommons.org/licenses/by-sa/4.0/deed.en)

Photograph of a slice of white bread with a drop of iodine solution on it. The iodine solution is a blue-black splodge on the slice of bread.

A drop of iodine solution on a slice of bread. The iodine solution has turned blue-black because the bread contains starch.

Testing for protein

To test for protein, you add a chemical called the Biuret reagent.

The Biuret reagent is blue. In the presence of protein it turns purple.

Therefore, if it turns purple, this shows that protein is present, and if it stays blue this shows that protein is absent.

Diagram of the process of testing for protein. There is a title at the top that says, "Testing for Protein". Underneath that, there is a drawing of a boiling tube containing the sample to be tested. A few drops of Biuret reagent are being added from a dropper. Labels explain, "1 Place sample to be tested in a boiling tube. 2 Add a few drops of Biuret reagent". There is an arrow pointing down from this drawing. The arrow splits in two and points to two different boiling tubes with the word "Results" written between them. In the boiling tube on the left, the sample is blue. A label explains, "If the solution stays blue, protein is absent.". In the tube on the right, the sample is purple. A label explains, "If the solution turns purple, protein is present".

Testing for protein using the Biuret reagent.

Photo of two cups containing solutions. In the cup on the left, the solution is purple. The left cup has a plus sign written on it (to denote the fact that the test result is positive). In the cup on the right, the solution is blue. The right cup has a minus sign written on it (to denote the fact that the test result is negative).

The Biuret reagent has been added to two solutions to test whether they contain proteins. In the solution on the left, the Biuret reagent has turned purple, indicating that proteins are present. In the solution on the right, it has remained blue, indicating that there are no proteins present. Image: Biureedireaktsiooni tulemus.jpg by Ykskaks3 on Wikimedia Commons (CC BY-SA 4.0 - https://creativecommons.org/licenses/by-sa/4.0/deed.en)

Testing for lipids

To test for the presence of lipids in a sample, you carry out a procedure called the emulsion test.

The steps of the emulsion test are as follows:

  • Place the sample in a test tube.
  • Add ethanol to the sample and shake the test tube to mix the ethanol with the sample.
  • Allow the contents of the test tube to settle.
  • Pour the mixture into a test tube containing water.

If lipids are present, a milky-white substance - called an emulsion - will appear in the tube. If lipids are absent, a milky-white emulsion will not appear.

Diagram showing the process of testing for lipids. The title at the top says, "Testing for Lipids". Then are are drawing illustrating the following steps: 1. Place sample to be tested in a test tube. 2 Add ethanol to the sample. 3. Shake to mix the ethanol with the sample. 4. Allow the contents of the tube to settle. 5. Pour the mixture into a test tube containing water. Results: If a milky-white emulsion does not form, lipids are absent. If a milky-white emulsion forms, lipids are present.

Testing for lipids with the emulsion test

Summary

The tests for glucose, starch, protein and lipids are summarised in the table below:

Biological MoleculeTestPositive ResultNegative Result
GlucoseAdd Benedict's solution and heat in a water bath for a few minutesTurns brick-redStays blue
StarchAdd a few drops of iodine solutionTurns blue-blackStays yellow-brown
ProteinAdd Biuret reagentTurns purpleStays blue
LipidsEmulsion test (add ethanol, shake, leave to settle, then add water)Milky-white emulsion formsNo milky-white emulsion

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/4

How do you test for glucose?

2/4

How do you test for starch?

3/4

How do you test for protein?

4/4

How do you test for lipids?

Donate

Please consider donating to support Mooramo. I am one person doing this whole project on my own - including building the site, writing the content, creating illustrations and making revision resources. By making a one-time or repeating donation you will buy me time to work on Mooramo, meaning that I can get new content on here more quickly.

Donate