Mini-Lesson

Biological molecules

This mini-lesson covers Topic 4 — Biological molecules: the elements and building blocks of carbohydrates, proteins and fats, the food tests (iodine, Benedict's, biuret, ethanol emulsion and DCPIP) and the structure of DNA.

Work through each screen, answer the questions as you go and collect ⭐ stars. Watch for the Supplement flag. Press Start when you're ready.

Molecules · elements & building blocks

Carbohydrates, proteins & fats

Large biological molecules are built from smaller ones:

Carbohydrates — contain carbon, hydrogen and oxygen (C, H, O). Starch, glycogen and cellulose are all built from glucose.
Proteins — contain carbon, hydrogen, oxygen and nitrogen (C, H, O, N; some also contain sulfur). Proteins are built from amino acids.
Fats and oils (lipids) — contain carbon, hydrogen and oxygen (C, H, O). They are built from fatty acids and glycerol.

glucose → starch · amino acids → protein · fatty acids + glycerol → fatlarge molecules are assembled from these smaller sub-units

Exam tip: the extra element in proteins is nitrogen — that's the easy way to tell proteins apart from carbohydrates and fats, which are all just C, H and O.

Quick check

The extra element

?Which element is always present in proteins but not in carbohydrates or fats?

Match it

Match the molecule to its building block

Tap a large molecule on the left, then the sub-unit it is built from on the right.

Large molecule

Built from

Food tests

The food tests

Each nutrient has a chemical test with a colour change from negative to positive:

Starch — add iodine solution: orange-brown → blue-black if starch is present.
Reducing sugars (e.g. glucose) — add Benedict's solution and heat: blue → brick-red/orange if present.
Protein — add biuret solution: blue → purple/violet if protein is present.
Fats & oils — the ethanol emulsion test: a milky-white cloudy emulsion forms if fat is present.
Vitamin C — add to blue DCPIP: it decolourises (goes colourless) if vitamin C is present.

Positive results: starch blue-black · sugar brick-red · protein purple · fat milky emulsion · vitamin C decolourises DCPIP.

Watch out: only Benedict's needs heating. Benedict's detects reducing sugars — not all sugars — and starch gives a negative Benedict's result.

Quick check

Reading a food test

?A student adds iodine solution to a food sample and it turns blue-black. What does this show?

Sort it

Which nutrient does each test find?

Tap a test result, then tap the nutrient it detects.

🍞 Starch

🍬 Reducing sugar

🥚 Protein

Quick check

The odd one out

?Which food test requires the sample to be heated (e.g. in a water bath) to get a result?

Supplement · DNA structure

The structure of DNA

DNA is the molecule that carries genetic information. Its structure:

Two strands coiled together to form a double helix.
Each strand contains chemicals called bases.
Bonds between pairs of bases hold the two strands together.
The bases always pair up in the same way: A with T, and C with G (complementary base pairing).

Complementary base pairing: A always pairs with T, and C always pairs with G.

Supplement · Calculate

Your turn — base pairing

1One strand of a short piece of DNA has the base sequence A T C G A T. Because A pairs with T and C pairs with G, how many of the bases on the opposite strand are T?

bases

Hint: each A on this strand pairs with a T opposite. Count the A's: A T C G A T.

Supplement · Quick check

Which base pairs with C?

?In DNA, which base always pairs with cytosine (C)?

Recap

The big ideas to know

Carbohydrates: C, H, O · from glucose (starch, glycogen, cellulose)

Proteins: C, H, O, N · from amino acids

Fats/oils: C, H, O · from fatty acids + glycerol

Food tests: iodine→starch (blue-black) · Benedict's + heat→reducing sugar (brick-red) · biuret→protein (purple) · ethanol emulsion→fat (milky) · DCPIP→vitamin C (decolourises)

DNA (Supp): double helix, two strands, bases pair A–T and C–G

You've covered all of Cambridge IGCSE Topic 4. Press Finish to see your score.

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