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Learn0610Papers 1, 3 · CoreBiological moleculesBiological molecules and food tests
Biological molecules · Subtopic 1 of 1

Biological Molecules and Food Tests

Carbohydrates (C, H, O) are built from simple sugars: glucose joins to form starch (plant store), glycogen (animal store) and cellulose (cell walls). Proteins (C, H, O, N, sometimes S) are chains of amino acids. Fats and oils (C, H, O) are made from fatty acids and glycerol. Food tests: iodine turns blue-black with starch; Benedict's (heated) turns brick-red with reducing sugars; biuret turns purple with protein; ethanol emulsion test gives a cloudy white emulsion with fat; DCPIP decolourises with vitamin C. DNA is a double helix with base pairs A–T and C–G.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Name the elements and building blocks of carbohydrates, proteins and fats.
  • Describe the food tests and their results.
  • Describe the structure of DNA.
Pause and think

Starch, glycogen and cellulose are all made of glucose. Why do they do such different jobs?

Section 2 of 14

Core idea

Start hereThree kinds of building block

Bread, meat and butter look nothing alike, but each is mostly one type of molecule: carbohydrate, protein or fat. Simple chemical tests tell them apart.

The big idea

Large biological molecules are built from smaller units: sugars, amino acids, or fatty acids with glycerol. Coloured reagents detect each one.

1. Building blocks

Large molecules from small units

Digestion breaks these links; cells rebuild them.

2. Food tests

Worked exampleAnalysing milk

Milk: Benedict's (heated) → orange; biuret → purple; emulsion test → cloudy; iodine → stays orange-brown.

Goal
What does milk contain?
Strategy
Interpret each test.
  1. orange⇒reducing sugar\text{orange}\Rightarrow\text{reducing sugar}
    Lactose.
  2. purple⇒protein; cloudy⇒fat\text{purple}\Rightarrow\text{protein};\ \text{cloudy}\Rightarrow\text{fat}
    Casein and milk fat.
  3. no blue-black⇒no starch\text{no blue-black}\Rightarrow\text{no starch}
    Iodine stays orange-brown.
AnswerReducing sugar, protein and fat; no starch.
✓ CheckMatches milk's composition ✓.

3. DNA

DNA has two strands twisted into a double helix. The strands are held by pairs of bases: A with T, C with G. The sequence of bases is a code for making proteins.

Common misconceptionHeating Only Benedict's is heated. Biuret, iodine and the emulsion test work at room temperature.
Pause and think

Why does a brick-red Benedict's result mean more sugar than a green one?

Summary

  • Sugars, amino acids, fatty acids + glycerol.
  • Iodine, Benedict's, biuret, emulsion, DCPIP.
  • DNA: A–T, C–G.
Key words
Reducing sugar
Sugar (e.g. glucose) that turns Benedict's brick-red when heated.
Amino acid
Building block of proteins.
Double helix
Twisted ladder shape of DNA.
HistoryRosalind Franklin. Rosalind Franklin's X-ray photograph "Photo 51" revealed DNA's helical shape, which Watson and Crick used to build their 1953 double-helix model.
Fun factCellulose fibre. Cotton is almost pure cellulose. Most animals can't digest it; cows rely on microbes in their stomachs to break it down.
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