Combined Science Biology B4: Biological Molecules

Cambridge IGCSE Combined Science Biology B4.1 links the elements in carbohydrates, fats and proteins to named small and large molecules, then requires four food tests: iodine for starch, Benedict's solution for reducing sugars, biuret for proteins and the ethanol emulsion test for fats and oils. This topic is about composition, building blocks and observation-led identification.

Elements in the three molecule groups

Carbohydrates contain carbon, hydrogen and oxygen.

Fats and oils also contain carbon, hydrogen and oxygen.

Proteins contain carbon, hydrogen, oxygen and nitrogen. Some proteins contain other elements, but nitrogen is the additional element required by this syllabus comparison.

Element lists do not describe molecular arrangement. Carbohydrates and fats can contain the same three elements while having different structures and properties.

Do not say food tests detect carbon, hydrogen, oxygen or nitrogen directly. They test for particular groups of biological molecules through characteristic reagent changes.

Large carbohydrate molecules are built from glucose

Starch, glycogen and cellulose are large molecules made from glucose units.

They are not interchangeable names:

• starch is a glucose-based carbohydrate found as a storage molecule in plants
• glycogen is a glucose-based storage molecule in animals and fungi
• cellulose is a glucose-based structural molecule in plant cell walls

The official B4 boundary requires the building-block relationship. Their detailed bonding and chemical structures are not required here.

Glucose itself is a reducing sugar. Starch is not identified by the Benedict test in the same direct way because it is a different, larger carbohydrate.

Proteins are built from amino acids

Proteins are large molecules made from amino acids.

Different amino-acid sequences allow different proteins to form. The biuret test detects protein rather than identifying which protein or amino-acid sequence is present.

Do not call every nitrogen-containing substance a protein. The required evidence is the appropriate protein test under controlled conditions.

Enzymes are proteins, but their action and temperature or pH response belong to B5.

Fats and oils are built from fatty acids and glycerol

Fats and oils are made from fatty acids and glycerol.

The syllabus groups fats and oils together for this building-block relationship and for the ethanol emulsion test. A positive emulsion result does not distinguish a fat from an oil or identify a particular fatty acid.