IGCSE Biology Year10 0610 Biochemical tests

Testing for Organic Molecules or Biochemicals

This part of the syllabus is really fun. There are many different tests that help us differentiate between the main groups of organic molecules; Lipids (fats and oils), Carbohydrates (sugars, starches etc.) Proteins and Nucleic acids (found in genetic material such as DNA).

Test for Protein

This is the test for protein which we did today.

First, we mixed a testing solution called Biuret Reagent. 

You can buy it, but it's much more fun to mix our own. 

Biuret Reagent.

Ask the teacher to premix 0.5 molar solution of potassium hydroxide, KOH. That's 28g solid KOH diluted in water to 1 litre (1 dm cubed).   

Before the test, add about 0.1g copper sulfate to a few drops of water (about 1/4 of a spatula) in a test-tube.

Add the copper sulfate solution to about 2 ml KOH in a large test-tube. It turns a nice bright blue colour.  That's it!!!  Biuret Reagent.

You can't store this solution for more than a few days, so just make enough for the lesson.

The test:

We need to make sure the test works, so we test it on a 2ml solution of peptone (the smelly solid we use to make agar plates to grow bacteria) or biuret solution, which contains the same bonds found in proteins that make the test work. This is a POSITIVE TEST.

Simply gently pour the Biuret Reagent you made into the test solution. Swirl gently, and you should see a beautiful violet colour just like here in our picture.

Choose a colour pencil closest to the test result and copy it into your notebook. Simple, and it helps you to remember.

Carbohydrate Tests

The tests for carbohydrates are: The Starch Test and Testing for Reducing Sugars.

Starch Test.

This is the easiest test to do. Simply take a small sample of starch in a test-tube as a POSITIVE TEST.

-Add about 2 ml distilled water and shake. Starch doesn't dissolve in water, but will give a milky white suspension.

-Add a few drops of 1% Iodine solution, a red/orange colour.

In a positive test it will turn a dark blue, almost black colour as in the photo.

The dark colour is made as Iodine atoms become trapped inside the starch molecules, forming a complex.

Test for Reducing Sugars

Reducing sugars are monosaccharides (single sugars) such as glucose and fructose.

Caution: Lactose, a disaccharide found in milk, also gives a positive test!! As does sucrose treated with acid!

This is a very dramatic test.

Method:  We dissolved a sample of glucose in distilled water in a test-tube, A, as a positive test, and dissolved a similar amount of sucrose, B, as a negative test.

We added  a similar volume of Benedict's Reagent to each test-tube.                                                       https://en.wikipedia.org/wiki/Benedict%27s_reagent 

Here's a photo:

Both samples are a bright blue due to the presence of copper (ii) ions.

Next, both test-tubes are heated gently in a Bunsen-burner flame for about 2 minutes. Always point the test-tube away from people and never look down into it.

Test-tube B, the negative test, didn't change colour, it stayed blue.

Test-tube A, the positive test with glucose solution, changed to a bright brick-red colour.

The red colour comes from the copper (ii) ions being reduced to copper(i) ions by glucose.

A really cool test!