Biological Molecules

This guide covers Topic 4, Biological molecules (subtopic 4.1), for Cambridge O Level Biology 5090, 2026–2028 series.

Where this fits in 5090

Every structure described in Cell Structure and Organisation is built from the molecules covered here, and the food tests in this subtopic are among the most commonly examined practical skills in the whole syllabus — a genuine foundation topic in both senses.

Syllabus coverage

CAMBRIDGE O LEVEL BIOLOGY 5090

• List the chemical elements that make up carbohydrates, lipids (fats and oils), proteins, and DNA (4.1)
• State that large molecules are made from smaller molecules: starch, cellulose and glycogen from glucose; proteins from amino acids; lipids from fatty acids and glycerol; DNA from nucleotides (4.1)
• Describe, and be able to carry out, chemical tests for starch (iodine solution), glucose and maltose (Benedict’s solution), protein (biuret test), and lipids (ethanol emulsion test) (4.1)

5090 is not tiered — every candidate covers all of the above.

Chemical elements in biological molecules

Molecule	Elements present
Carbohydrates	Carbon, hydrogen, oxygen
Lipids (fats and oils)	Carbon, hydrogen, oxygen (a much smaller proportion of oxygen than carbohydrates)
Proteins	Carbon, hydrogen, oxygen, nitrogen (often sulfur too)
DNA	Carbon, hydrogen, oxygen, nitrogen, phosphorus

Nitrogen is the element that distinguishes proteins and DNA from carbohydrates and lipids — a useful quick check when identifying which category a molecule belongs to from its elemental composition.

Large molecules from small building blocks

Large biological molecules are built by joining many smaller molecules together:

Large molecule	Built from
Starch, cellulose, glycogen	Glucose (many glucose units joined together)
Proteins	Amino acids
Lipids	Fatty acids and glycerol
DNA	Nucleotides

Starch, cellulose and glycogen are all built from the same small molecule (glucose) but joined in different arrangements — which is why they have different properties and roles despite a shared building block: starch stores energy in plants, cellulose forms rigid plant cell walls, and glycogen stores energy in animals. Glycogen’s glucose chains are more branched than starch’s, which allows the stored energy to be released faster when an animal needs it.

Lipids are built when one glycerol joins with three fatty acids, linked by ester bonds. Proteins are built when amino acids join in a chain; the specific sequence of amino acids determines how the chain folds into a 3D shape, and that shape determines the protein’s function — an enzyme’s or antibody’s precisely-shaped binding site would be destroyed by the wrong sequence. DNA’s nucleotides are held together as two strands in a double helix, with the two strands linked by specific base pairing.

Food tests

Each food test targets a specific molecule and produces a specific, recallable colour change:

Test	Target molecule	Method	Positive result
Iodine test	Starch	Add iodine solution directly to the sample	Blue-black (from orange-brown)
Benedict’s test	Glucose and maltose (reducing sugars)	Add Benedict’s solution, heat in a water bath	Brick-red precipitate (from blue)
Biuret test	Protein	Add biuret solution (or sodium hydroxide then copper sulfate) at room temperature	Purple/violet (from blue)
Emulsion test	Lipids	Shake sample with ethanol, then pour into water	Cloudy white emulsion

Worked example — describing a procedure. To test an unknown solution for starch: add a few drops of iodine solution directly to the sample at room temperature. A colour change from orange-brown to blue-black indicates starch is present; no colour change indicates starch is absent.

Notice which tests require heating (Benedict’s, in a water bath) and which are done at room temperature (iodine, biuret, emulsion) — this distinction is a specific, common exam-question detail.

Why each molecule matters

Molecule	Function
Carbohydrate	Main energy source; structural role in plants (cellulose)
Lipid	Energy store (more energy per gram than carbohydrate), insulation, protection, membranes
Protein	Enzymes, antibodies, haemoglobin, growth and repair
DNA	Stores the genetic information a cell reads to build proteins

Common mistakes

• Naming the wrong colour change for a test, or reversing which colour indicates a positive vs negative result.
• Forgetting Benedict’s test requires heating, while the iodine, biuret and emulsion tests are performed at room temperature.
• Confusing the biuret test’s reagents — it uses biuret solution (or sodium hydroxide followed by copper sulfate solution), not iodine or Benedict’s solution.
• Assuming Benedict’s test detects all sugars. It specifically detects reducing sugars, of which glucose and maltose (both named in this syllabus) are examples.
• Mixing up the small-molecule building blocks — amino acids build proteins, not lipids; fatty acids and glycerol build lipids, not carbohydrates; nucleotides build DNA specifically.

Quick revision checklist

• The chemical elements present in carbohydrates, lipids, proteins and DNA
• Which small molecules build each large molecule
• All four food tests: reagent, method (including which needs heating), and positive result colour
• Nitrogen as the element that distinguishes proteins and DNA from carbohydrates and lipids

Related resources

• Cell Structure and Organisation — the cell structures these molecules build and are found in
• Enzymes — proteins acting as biological catalysts
• Cambridge O Level Biology subject hub

Written against Cambridge O Level Biology 5090, 2026–2028 series. Always check the current syllabus for your examination year.