Biological Molecules
Three groups of large molecules make up most of the material in living organisms: carbohydrates, fats and oils (lipids), and proteins. This topic asks three things about each — which elements it contains, which smaller units it is built from, and how to test for it.
The food tests in particular are close to free marks, provided the colours are learned precisely.
1. The elements
| Molecule | Elements |
|---|---|
| Carbohydrates | Carbon, hydrogen, oxygen |
| Fats and oils | Carbon, hydrogen, oxygen |
| Proteins | Carbon, hydrogen, oxygen, nitrogen — and sometimes sulfur |
Nitrogen is what distinguishes proteins. A question giving you a list of elements and asking which molecule it is turns entirely on whether nitrogen appears. Carbohydrates and fats both contain only C, H and O — you cannot tell them apart from elements alone.
This is also why plants need nitrate ions from the soil: to make amino acids, and from those, proteins.
2. Large molecules and their basic units
Large biological molecules are built from smaller units joined together.
| Large molecule | Built from |
|---|---|
| Starch | Glucose (simple sugar) |
| Glycogen | Glucose |
| Cellulose | Glucose |
| Protein | Amino acids |
| Fats and oils | Fatty acids and glycerol |
Note that starch, glycogen and cellulose are all made from the same unit — glucose. They differ in how the glucose molecules are arranged, and that difference gives them completely different properties and roles:
| Molecule | Role | Found in |
|---|---|---|
| Starch | Energy storage | Plants |
| Glycogen | Energy storage | Animals (mainly liver and muscle) |
| Cellulose | Structural — makes the cell wall | Plants |
Glycogen is not glucagon. Glycogen is stored carbohydrate; glucagon is a hormone. They belong to different topics and are constantly swapped.
Proteins and their shape
Amino acids join in long chains. There are about 20 different amino acids, and the order in which they are joined determines the protein produced.
Because the chain then folds into a specific shape, and because a different sequence gives a different shape, an enormous variety of proteins is possible — enzymes, antibodies, haemoglobin, keratin, and many hormones.
A useful comparison: 26 letters produce every word in the language; 20 amino acids produce every protein.
Scope note. Cambridge IGCSE 0610 requires the elements, the basic units, and the fact that different sequences of amino acids give different shapes and functions. It does not require the structural formula of an amino acid, peptide bonds by name, condensation and hydrolysis mechanisms, the chemical formulae of mono- and disaccharides, or primary/secondary/tertiary/quaternary protein structure. That material belongs to A Level and to some other specifications — if you are sitting 0610 you can leave it.
3. The food tests
Five tests are examinable. Learn the method, the starting colour and the final colour for each — questions ask for all three.
| Test for | Reagent | Method | Colour change |
|---|---|---|---|
| Starch | Iodine solution | Add iodine solution to the sample | Orange-brown → blue-black |
| Reducing sugars | Benedict’s solution | Add Benedict’s solution and HEAT in a water bath | Blue → brick red (via green, yellow, orange) |
| Protein | Biuret solution | Add biuret solution — no heating | Blue → purple |
| Fats and oils | Ethanol | Add ethanol, shake, then add water | Clear → cloudy white emulsion |
| Vitamin C | DCPIP | Add the sample drop by drop to blue DCPIP | Blue → colourless |
The details examiners look for
Benedict’s is the only test that requires heating. If a question asks how you would test for reducing sugars and you omit “heat in a water bath”, you lose a mark. Conversely, adding “heat” to the biuret test is wrong.
Benedict’s gives a graded result. Blue means no reducing sugar; green, yellow and orange indicate increasing amounts; brick red indicates the most. This makes it useful for comparing concentrations — an experiment question may ask you to rank samples by colour.
The ethanol emulsion test needs the water. Ethanol alone does nothing visible. Fat dissolves in ethanol, and adding water then makes it come out of solution as tiny droplets — the cloudy white emulsion. Missing out the water step loses the mark.
DCPIP works the opposite way round to the others. You are looking for the blue colour to disappear, and the volume of vitamin C solution needed to decolourise it tells you the concentration — the less needed, the more vitamin C present.
A memory hook for the two blue-start tests: Biuret → purple for protein. Benedict’s is the one you heat.
A note on safety and method
Ethanol is flammable, so it must not be heated with a naked flame. Benedict’s test uses a water bath rather than direct heating for the same reason.
4. The structure of DNA
DNA is made of two strands coiled together to form a double helix.
Each strand is made of units containing a base. There are four bases, usually written as A, T, C and G.
The base-pairing rule: A pairs with T, and C pairs with G.
These are complementary base pairs — a base will only ever pair with its partner. So a strand reading A T G C pairs with a strand reading T A C G.
The sequence of bases in a gene determines the sequence of amino acids in a protein, which is what links this topic to inheritance.
What you do not need for 0610: the number of hydrogen bonds between each base pair, the sugar and phosphate names, nucleotides in structural detail, or the mechanism of DNA replication.
5. Mistakes that cost marks
Forgetting to heat Benedict’s solution.
Heating the biuret test. It doesn’t need heat.
Leaving the water out of the ethanol emulsion test.
Saying iodine goes “black”. It is blue-black, from orange-brown.
Giving “blue to red” for Benedict’s without “brick”. Brick red is the expected wording.
Saying proteins contain only C, H and O. They contain nitrogen too.
Confusing glycogen with glucagon, or with cellulose.
Saying starch and cellulose are made of different units. Both are made of glucose — the difference is in the arrangement.
Learning A-level detail instead of the syllabus content. Structural formulae and protein folding levels are not examined at 0610; the food tests and basic units are, heavily.
Frequently asked questions
Which food test needs heating? Only Benedict’s (for reducing sugars).
What colour does Benedict’s solution go if there is a lot of reducing sugar? Brick red. Green, yellow and orange indicate smaller amounts; it stays blue if there is none.
How do I test for protein? Add biuret solution; it changes from blue to purple. No heating.
Why do you add water in the fat test? Fat dissolves in ethanol. Adding water makes it come out of solution as droplets, producing the cloudy white emulsion that shows fat is present.
What is the difference between starch, glycogen and cellulose? All three are made from glucose. Starch stores energy in plants, glycogen stores energy in animals, and cellulose is structural, forming the plant cell wall.
Which element tells you a molecule is a protein? Nitrogen.
What are the base pairs in DNA? A with T, and C with G.
Quick revision checklist
- I know the elements in carbohydrates, fats and proteins, and that nitrogen marks out proteins
- I can name the basic units of starch, glycogen, cellulose, protein and fats
- I know starch, glycogen and cellulose are all made from glucose
- I can state the role and location of each of those three
- I can explain how different amino acid sequences give different proteins
- I can give the reagent, method and both colours for all five food tests
- I know Benedict’s is the only test requiring heating
- I know the ethanol test needs water added
- I know DCPIP goes blue → colourless, and how that indicates concentration
- I can describe DNA as two strands in a double helix with four bases
- I know A–T and C–G, and can write a complementary strand
- I know which of this topic’s extra detail is beyond 0610
These notes cover topic 4 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
Finished this topic?
Saved on this device — no account needed.