1. List the chemical elements that make up:
    • Carbohydrates
    • Fats
    • Proteins


All three biomolecules contain Carbon, Hydrogen and Oxygen.

Proteins also contain Nitrogen. Proteins sometimes contain Sulfur and Phosphorus too.


  1. State that large molecules are made from smaller molecules, limited to:
    • Starch and glycogen from glucose
    • Proteins from amino acids
    • Fats and oils from fatty acids and glycerol

polymer diagramPolymers are large molecules made from small, similar molecules (often referred to as subunits).

In the case of carbohydrates, starch and glycogen are large polymer molecules made of glucose.

Note: the simplest sugar is a glucose molecule (C6H12O6).

In the case of proteins, amino acids are the monomers.

Different amino acids bond together, to form chains known as peptides. The bonds between these amino acids are known as peptide bonds. These peptides come together to form proteins such as enzymes.

igcseaid_diagramsThe most common type of fats and oils are triglycerides. A triglyceride is made up of three fatty acid molecules bonded to a glycerol molecule. The bond between a fatty acid and a glycerol molecule is known as an ester bond. Because the subunits in fats and oils aren’t all similar, they aren’t known as polymers – they are not made of similar subunits (fatty acids and glycerol are rather different, actually).

Fats are triglycerides that are solid at rtp (room temperature and pressure). Remember, rtp is a temperature of 20oC and a pressure of 1 atm.

Oils are liquid at rtp.


  1. Describe the use of:
    • Iodine solution to test for starch

This test is super easy! All that needs to be done is adding a few drops of iodine to the test solution/ test material. If it contains starch, the solution will turn blue-black, if not, it’ll remain an orangey/ brown colour (the colour of iodine).


  • Benedict’s solution to test for reducing sugars

To a known volume of test solution, you add the equal volume of Benedict’s reagent/ solution. Give it a stir and look for any colour changes. If none, try heating it in a warm water bath (about 80oC), and look for any colour changes. If there are no changes, there are no reducing sugars present in the solution.

benedict's solution test

Benedict’s reagent is blue in colour. If there are any sugars present, it’ll change from blue to green, to yellow, to orange, to red (the fire colours). Green means that there are only traces of reducing sugars and red means that the solution has a high concentration of reducing sugars.

benedict's reagent results

Note: sucrose is not a reducing sugar.

  • Biuret test for proteins

Biuret reagent is a mix of two chemicals – copper sulfate (CuSO4) mixed with either sodium hydroxide (NaOH) or potassium hydroxide (KOH).

The biuret test is done to show the presence of peptide bonds. Remember, peptide bonds are the bonds that link the amino acids together in a protein. If peptide bonds are present, the blue biuret reagent will turn mauve or purple.

biuret's testTo perform the test, simply add the biuret reagent to the test solution. (Note, if the test material is solid and not liquid, crush it and mix it with distilled water, to form a solution). The volume of biuret reagent you add should be the same as the volume of the test solution you add it to.

In some cases, you won’t get a ready mixed biuret reagent solution. In this case,

Measure out a known volume of test solution into a test tube. About 1cm3 should be enough. Add the same volume of NaOH (or KOH) to the test tube and stir.

Add a few drops of CuSO4 solution, shaking after each drop.


After you have completed the test, observe the colour of the solution. If it’s mauve or purple, there are peptide bonds in the solution, so there are probably proteins in the solution.

  • Ethanol emulsion test for fats and oils

This test is pretty simple too! You add the test sample to a concentrated ethanol solution. You put the resulting mixture into a test tube of distilled water, close it, and shake it around. If a cloudy emulsion forms, fats are present; if not, there are no fats.

ethanol emulsion test for fats

  1. State that water is important as a solvent

Water is extremely important as a solvent. All reactions in our body occur in water, our cells are 70% water on average, substances are transported around our body after being dissolved in water, etc.

So water is essential for life.





Notes submitted by Sarah

Illustrations by Azmina

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