Cambridge IGCSE Biology 0610 · Topic 4.1
Biological Molecules
Chapter 4 asks a narrower question than it first looks like it does: not what living things are made of in general, but which specific elements sit inside carbohydrates, fats and proteins, and which smaller molecules those larger ones are built from. Once that groundwork is in place, the chapter turns practical — five food tests you need to describe precisely, reagent by reagent — before Extended candidates take one further step into the basic structure of DNA.
Cambridge places every part of this under a single syllabus point, 4.1 Biological molecules, but it rewards a fairly linear way of thinking about it: chemical elements, then the smaller building blocks those elements form, then the chemical tests that let you detect those molecules in an unknown sample. Extended candidates add one further strand — the physical structure of DNA and the fixed rule that governs how its bases pair — though Cambridge deliberately keeps that structural detail simple at this stage, saving genes and protein synthesis for Topic 17, Inheritance.
Marks in this chapter are lost less through not knowing the biology, and more through blurring lines the mark scheme keeps sharp. Carbohydrates and fats share exactly the same three elements, so “contains carbon, hydrogen and oxygen” does not distinguish between them — only nitrogen does that. Starch, glycogen and cellulose are three different molecules built from the very same small unit, so naming glucose tells an examiner what a molecule is made from, not which one you mean. And each food test has one reagent, one method and one specific colour or appearance change; borrowing a step from a different test is usually where a food-test answer starts to fall apart. The two pages below are built around getting each of those distinctions exactly right.
What’s in this chapter
| Page | Level | What it covers |
|---|---|---|
| Food tests | Core | Iodine, Benedict’s, biuret, ethanol emulsion and DCPIP — the exact reagent, method and colour change for each, plus how to read a results table. |
| DNA structure and base pairing | Supplement | The double helix, bases along each strand, and the fixed A–T / C–G pairing rule that lets you work out one strand from the other. |
Core or Supplement?
Everything on this hub page itself — the elements in carbohydrates, fats and proteins, and the smaller molecules that build them — is Core, along with the whole of the food tests page. DNA structure is Supplement only, required for Extended candidates and not examinable for Core-only candidates. If you’re not certain which tier you’re sitting, that’s worth confirming before spending revision time on the DNA structure page, since nothing on the rest of this chapter depends on it.
The elements in carbohydrates, fats and proteins
Biological molecules can be compared by the chemical elements they contain. For this chapter, Cambridge limits that comparison to three food groups:
| Food group | Elements it contains |
|---|---|
| Carbohydrates | carbon, hydrogen, oxygen (C, H, O) |
| Fats and oils | carbon, hydrogen, oxygen (C, H, O) |
| Proteins | carbon, hydrogen, oxygen, nitrogen (C, H, O, N) |
Two things are worth reading directly off that table. Carbohydrates and fats and oils contain exactly the same three elements, so you cannot use elements alone to tell them apart — you would need a food test for that instead. Nitrogen, on the other hand, is the element that marks protein out from the other two: it is the only one of the three food groups that contains it. If a question asks specifically for the distinguishing element in protein, nitrogen alone earns the mark; if it asks you to state the elements in protein, Cambridge wants the complete set — carbon, hydrogen, oxygen and nitrogen — not just the one that happens to be unique to it. Some proteins do also contain sulfur, but sulfur sits outside the required answer and should never be substituted for one of those four elements.
How larger molecules are built from smaller ones
The second comparison Cambridge wants is structural rather than elemental: which smaller molecules combine to form each of the larger ones.
| Large molecule | Built from |
|---|---|
| Starch | glucose |
| Glycogen | glucose |
| Cellulose | glucose |
| Protein | amino acids |
| Fats and oils | fatty acids and glycerol |
Starch, glycogen and cellulose are three separate large molecules, but all three are built from exactly the same small unit — glucose. What differs between them is how those glucose molecules are arranged, not which building block is used, so naming glucose correctly answers “what is starch made from” without telling an examiner which of the three glucose-based molecules you actually mean if the question asks you to identify one specifically. Protein follows a simpler pattern: it is built from amino acids, full stop, with no second component to remember. Fats and oils are the one exception to that one-building-block pattern — they are formed from two different components, fatty acids and glycerol, and an answer that names only fatty acids is incomplete even though it is not wrong.
The functional side of these molecules — starch as a plant energy store, cellulose in plant cell walls, and the dietary sources and importance of carbohydrates, fats and proteins — belongs to later chapters rather than to this one; you can pick that up in diet and dietary sources. What Chapter 4 wants from you is narrower and more exact: the elements each molecule contains, and the smaller molecules it is built from — the two things the tables above set out. Once those are secure, the food tests give you a practical way to detect several of these molecules in an actual sample, and Extended candidates can move on to DNA structure, where the same instinct for precise wording matters just as much.