Cambridge IGCSE Biology 0610: Biological molecules – Revision Notes

For full explanations and worked examples, read the biological molecules study guide first. These notes are for the final weeks.

They cover topic 4, Biological molecules (sub-topic 4.1), of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. Outcomes 1 to 3 are Core and are tested on every paper. Outcome 4, the structure of DNA, is Supplement content and is Extended only (Papers 2 and 4). Food tests can also appear on the practical papers, Paper 5 and Paper 6.

Links: 0610 course hub · printable checklist · practice questions · enzymes revision notes · diagnostic: Core or Extended.

4.1.1 Elements – Core

Molecule	Elements
Carbohydrates	C, H, O
Fats and oils	C, H, O
Proteins	C, H, O, N (some also S)

Must-know distinction: only proteins contain nitrogen. If a question asks which nutrient contains nitrogen, the answer is protein.

4.1.2 Large molecules from small ones – Core

Large molecule	Built from	Where you meet it
Starch	glucose	energy store in plants
Glycogen	glucose	store in animals and fungi
Cellulose	glucose	plant cell walls
Proteins	amino acids	enzymes are proteins (topic 5)
Fats and oils	fatty acids and glycerol	digested by lipase (topic 7)

Memory hook: “three from glucose, one from amino acids, one from two things”. Starch, glycogen and cellulose all come from glucose. Protein comes from amino acids. Fats and oils need fatty acids plus glycerol.

Link to digestion: each of these is reversed by an enzyme. Amylase: starch to simple reducing sugars. Proteases: protein to amino acids. Lipase: fats and oils to fatty acids and glycerol.

4.1.3 Food tests – Core

Test	For	Method in one line	Negative	Positive
Iodine solution	starch	add drops of iodine solution	orange-brown	blue-black
Benedict’s	reducing sugars	add equal volume of Benedict’s, heat in water bath (about 80 °C)	blue	green, yellow, orange or brick-red
Biuret	protein	add biuret solution, mix	blue	purple
Ethanol emulsion	fats and oils	shake food with ethanol, pour into water	clear	cloudy white emulsion
DCPIP	vitamin C	add food solution drop by drop to DCPIP	stays blue	blue to colourless

Method in steps: Benedict’s

1. Grind solid food with distilled water.
2. Add 2 cm³ of food solution to a test-tube.
3. Add 2 cm³ of Benedict’s solution.
4. Heat in a water bath for a few minutes.
5. Record the colour at the start and at the end.

Method in steps: ethanol emulsion

1. Crush the food and shake it with ethanol.
2. Pour the ethanol into a clean test-tube of water.
3. Look for a cloudy white layer or emulsion.

Method in steps: DCPIP comparison

1. Measure the same volume of DCPIP into each tube (for example 1 cm³).
2. Add juice drop by drop from a syringe or pipette, shaking each time.
3. Record the volume at which the blue colour disappears.
4. Smaller volume needed = more vitamin C.

Semi-quantitative Benedict’s

The colour order shows how much reducing sugar is present:

blue  →  green  →  yellow  →  orange  →  brick-red
none                                      most

Compare foods only if you used the same volume of food solution and Benedict’s, and the same heating time and temperature.

Practical reminders

• Control: test distilled water with each reagent.
• Safety: ethanol is flammable (no flames nearby); use a water bath for heating; wear eye protection.
• Record: a table with food, test, start colour, end colour, conclusion.
• Conclude with “detected” or “not detected”.

A results table that scores full marks

Food	Test	Colour at start	Colour at end	Conclusion
Milk	Biuret	blue	purple	protein detected
Milk	Iodine solution	orange-brown	orange-brown	starch not detected
Distilled water (control)	Biuret	blue	blue	reagent alone gives no change

Notice three things: every column has a heading, both colours are recorded, and the conclusion names the molecule.

Worked reminder: interpreting mixed results

Potato extract gives blue-black with iodine, blue with Benedict’s and blue with biuret.

• Starch detected.
• No reducing sugar detected.
• No protein detected (biuret stayed blue).

Watch the trap here: Benedict’s and biuret both start blue, so “stayed blue” is a negative for both.

Planning a fair food-test comparison

If a question asks you to compare two foods, use this checklist:

1. Independent variable: the food being tested.
2. Dependent variable: the final colour (or, for DCPIP, the volume of juice needed).
3. Keep constant: mass of food ground up, volume of water used to make the extract, volume of reagent, and for Benedict’s the temperature and heating time.
4. Control: distilled water tested the same way.
5. Repeats: test each food more than once and check the results agree.

Command words for this topic

• List / State: a short answer, no explanation. “C, H, O and N” is enough.
• Describe a food test: the reagent, what you do with it (including heating if needed), and the colour change from start to end.
• Explain a DCPIP result: link the volume of juice to the amount of vitamin C.
• Suggest: use the data and your knowledge to give a reasonable reason. More than one answer may be accepted.
• Calculate (Extended DNA questions): show the working, not just the percentage.

4.1.4 DNA structure – Extended only

• Two strands coiled together: a double helix.
• Each strand contains bases.
• Bonds between pairs of bases hold the strands together.
• Pairing is fixed: A–T and C–G. Letters are enough; full names are not required.

Method in steps: base percentages

1. Pair the given base with its partner: they have the same percentage.
2. Double it and subtract from 100.
3. Halve the remainder: that is each of the other two bases.

Worked reminder: 40% G gives C = 40%, then A + T = 100 − 80 = 20%, so A = T = 10%.

Method in steps: complementary strand

Swap each letter using A↔T and C↔G. Keep the order.

T T A G C C G A
A A T C G G C T

Must-know distinctions

• Starch vs glycogen: both from glucose; starch in plants, glycogen in animals and fungi.
• Iodine vs Benedict’s: iodine detects starch; Benedict’s detects reducing sugars and needs heat.
• Biuret vs Benedict’s: both start blue. Biuret goes purple for protein with no heating. Benedict’s goes green to brick-red for reducing sugar with heating.
• Emulsion test order: ethanol first, water second.
• DCPIP reading: a small volume of juice to decolourise means high vitamin C.
• Base pairs vs strands (Extended): the bonds are between bases, and they hold the two strands together.

Quick self-test

1. Name the element found in proteins but not in carbohydrates.
2. Name the smaller molecule from which glycogen is made.
3. State the two types of smaller molecule that make up oils.
4. State the colour of iodine solution when starch is absent.
5. Which food test needs heating?
6. A food solution turns purple with biuret solution. What does it contain?
7. Give one safety precaution for the ethanol emulsion test.
8. Juice A decolourises 1 cm³ of DCPIP with 2.5 cm³; juice B with 1.0 cm³. Which contains more vitamin C, and how many times more per cm³?
9. Two Benedict’s tests give yellow and brick-red. Which solution had more reducing sugar?
10. (Extended) Write the complementary strand for G C A T T A G C.
11. (Extended) In a DNA sample, 15% of the bases are A. Calculate the percentage of C.
12. (Extended) What holds the two strands of DNA together?

Answers

1. Nitrogen (N).
2. Glucose.
3. Fatty acids and glycerol.
4. Orange-brown (it stays orange-brown).
5. Benedict’s test.
6. Protein.
7. Keep the ethanol away from naked flames, because it is flammable.
8. Juice B. It needed 2.5 ÷ 1.0 = 2.5 times less volume, so it has 2.5 times as much vitamin C per cm³.
9. The brick-red one.
10. C G T A A T C G.
11. T = 15%, so A + T = 30%; C + G = 70%; C = 35%.
12. Bonds between pairs of bases.

Where marks are usually lost

• Leaving nitrogen out of the elements in protein, or adding it to carbohydrates.
• Writing only “fatty acids” for the building blocks of fats and oils; glycerol is needed too.
• Stating a colour change with only the end colour. Give both: “blue to purple”.
• Writing “black” instead of “blue-black” for starch.
• Missing the heating step in the Benedict’s test, or adding heating to the biuret test.
• Describing the emulsion test with water added first, or without shaking the food in ethanol.
• Concluding “more drops of juice, more vitamin C” in the DCPIP test. It is the other way round.
• Comparing foods by Benedict’s colour without keeping volumes and heating time the same.
• (Extended) Pairing A with C or G with T, or saying bonds hold “the helix” rather than naming bonds between bases.
• (Extended) In percentage questions, forgetting to halve the remainder between the two other bases.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge International. Topic 4, Biological molecules, sub-topic 4.1.