Tests for Biological Molecules (OCR Gateway GCSE Biology: Combined Science)

Revision Note

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Benedict's Test (Reducing Sugars)

  • A wide range of biological molecules can be tested for using simple chemical tests
    • This allows the biological molecules within a sample (such as a food substance) to be determined
  • Reducing sugars include simple sugars like glucose that are just made from one unit (monomer), and some sugars made from two units joined together like maltose
    • The presence of reducing sugars can be tested for with a Benedict's test

Test for a reducing sugar

  • Add Benedict's reagent (which is blue) to the sample solution in a test tube
  • Heat in a water bath that has been set at 75C for 5 minutes
  • Take the test tube out of the water bath and observe the colour
  • A positive test will show a colour change from blue to orange/dark red
    • An orange/dark red precipitate (solid particles suspended in the solution) will form
  • A negative test will remain blue
  • The amount of colour change is an indication of the amount of reducing sugar present
    • From: blue → green → yellow → orange → red

The Benedict's test for glucose, IGCSE & GCSE Biology revision notes

The Benedict's test for glucose

Iodine Test (Starch)

  • We can use iodine to test for the presence or absence of starch in a food sample
  • Add drops of iodine solution to the food sample
  • A positive test will show a colour change from orange-brown to blue-black
  • The colour remains orange-brown if there is no starch present

Testing for starch in solution, downloadable AS & A Level Biology revision notes

In the presence of starch, iodine will turn from brown to blue-black

Biuret Test (Protein)

  • The biuret test can be used to identify if any protein is present in a sample  
  • Add drops of biuret solution (a mixture of sodium hydroxide and copper sulphate) to the food sample
  • A positive test will show a colour change from blue to violet / purple
  • If there is no protein present, the solution will stay blue

The Biuret test for protein, IGCSE & GCSE Biology revision notes

The biuret test for protein

Emulsion Test (Lipids)

  • An emulsion test can be used to identify if any lipids are present in a sample  
  • Mix the food sample with 4cm3 of ethanol and shake
  • Allow time for the sample to dissolve in the ethanol
  • Strain the ethanol solution into another test tube
  • Add the ethanol solution to an equal volume of cold distilled water (4cm3)
  • A positive test will show a cloudy emulsion forming
    • The more lipid is present, the stronger the milky colour will be

The ethanol test for lipids, IGCSE & GCSE Biology revision notes

The ethanol test for lipids

Food Test Results Table

Food test colour changes table, downloadable IGCSE & GCSE Biology revision notes

Practical investigation safety tips

  • If you were carrying out these tests you should try to identify the main hazards and think of ways to reduce harm
  • Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles
  • Iodine is also an irritant to the eyes
  • Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately
  • Ethanol is highly flammable; keep it away from any Bunsen burner
  • The Bunsen burner itself is a hazard due to the open flame

Worked example

The table below shows the results of testing a range of foods - can you identify the positive tests - which biological molecules are present?

Table_Food-Tests-Analysis, downloadable AS & A Level Biology revision notes

Exam Tip

When describing food tests in exam answers, make sure you give the starting colour of the solution and the colour it changes to for a positive result.

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.

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