Biology 0610Chapter 4 of 21

Biological molecules

Cambridge IGCSE Biology 0610 Topic 4 revision chapter covering the whole of subtopic 4.1 Biological molecules for the 2026-2028 syllabus, version 2, with Core and Supplement content clearly separated. Topic 4 contains three Core statements and one Supplement statement, and this chapter maps every one of them to visible teaching. Core 4.1.1 is taught as the chemical elements that make up exactly three named families: carbohydrates and fats contain carbon, hydrogen and oxygen; proteins additionally contain nitrogen. The shorthand CHO and CHON is introduced only after the full element names, no claim is made that every protein must contain sulfur, and phosphorus does not appear in any required element list. Core 4.1.2 is taught as the syllabus writes it, limited to three cases: starch, glycogen and cellulose from glucose; proteins from amino acids; fats and oils from fatty acids and glycerol. The chapter is built around keeping those two questions apart, because answering one when the other was asked is the commonest way marks are lost here. Core 4.1.3 teaches all five required food tests as full laboratory methods with realistic apparatus diagrams, stated starting and final colours, negative results and matched hazards: iodine solution for starch turning yellow-brown to blue-black; Benedict's solution heated in a hot-water bath for reducing sugars, running blue through green, yellow and orange to a brick-red precipitate; the biuret method using either centre-supplied biuret reagent or sodium hydroxide followed by a small amount of dilute copper(II) sulfate, turning blue to lilac or purple for proteins; the ethanol emulsion test producing a milky-white emulsion rather than a precipitate for fats and oils; and the DCPIP test for vitamin C, in which the sample is added drop by drop to a measured volume of blue DCPIP until the colour is lost. The DCPIP test is given particular attention because it is the only test in the topic whose positive result is a colour disappearing rather than appearing, because three of the five reagents are blue and all three stay blue on a negative result, and because it is never heated, since vitamin C is destroyed by heat. It is also the one test that can rank samples, through the volume needed to reach the end-point, and that inverse-proportion reasoning is worked through with verified arithmetic. Supplement 4.1.4 is a dedicated, clearly labelled section on the structure of a DNA molecule, held to the four parts the syllabus states: two strands coiled together into a double helix, each strand containing chemicals called bases, bonds between pairs of bases holding the strands together, and the fixed pairing of A with T and C with G, with the full base names and the internal chemistry of a strand explicitly excluded. Every result is labelled with words and symbols as well as colour, so no interpretation depends on colour alone. A dedicated strand of the chapter trains the AO3 scientific reasoning that Papers 5 and 6 assess rather than recall: the difference between an observation and an inference, why a positive result supports presence while a negative result only means not detected under these conditions, why an uncontrolled colour cannot be read as a concentration, and which variables must be equalised for a fair comparison, together with a full eleven-point planning contract worked through on a vitamin C investigation including results table, graph choice, conclusion logic and matched improvements. A worked unknown-sample investigation reasons through all five test results step by step. Interactive identification and test-selection activities, a summary matrix, a mistake clinic, a Core retrieval check, a separate Extended retrieval check, a mixed exam-style challenge set, a mastery checklist and a spaced-review plan support both first-pass learning and last-week revision.Show more

Recommended next

Revision notes

Interactive notes with exam tips and worked examples.

Start notes

Study path

Chapter overview

A summary of this Biology chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.

Key ideas to remember

Elements, smaller molecules, test. Three questions, three molecule families, five tests — and one rule that governs all of them: say what you saw before you say what it means.
Carbohydrates and fats stop at three: carbon, hydrogen, oxygen. Proteins add nitrogen. Three families, two lists, one difference — and no phosphorus anywhere in statement 4.1.1.
Glucose makes three — starch, glycogen, cellulose. Amino acids make one. Fats and oils need two. That is the whole list, and it is limited to those three. Elements are atoms; building blocks are molecules — never swap the two answers.
Plants store starch and build walls of cellulose. Animals store glycogen. Proteins include the enzymes. Fats and oils store energy. Learn this to keep the 4.1.2 list apart — and remember that the roles themselves are examined in Topics 5, 6 and 7, not here.
Iodine solution: orange-brown in, blue-black out, starch found. No heat, no precipitate, and nothing said about any other carbohydrate.
Blue in, heat in a water bath, and read the colour you reached plus the precipitate. Blue means no reducing sugar detected. Green through brick-red all mean detected — and none of them means a number.
Biuret: blue in, lilac or purple out, protein found. No heat, no precipitate — and if the tube stays blue, the protein was not detected. Of the three blue reagents, this is the one that gains a colour without being heated.
Ethanol, then water, then look at it against something dark. Clear means not detected. Milky-white means a fat or oil — an emulsion, never a precipitate, and never near a flame.

What you need to be able to do

Core 4.1.1 I can list the chemical elements that make up carbohydrates, fats and proteins, using the full element names.
I can explain why proteins carry one element that carbohydrates and fats do not, and name it.
Core 4.1.2 I can state the smaller molecules that starch, glycogen, cellulose, proteins, and fats and oils are made from.
I can separate a molecule's elements from the smaller molecules it is made from, and answer whichever one the question actually asks for.
I know that the syllabus list of large molecules stops at five, and I do not add a sixth.
Core 4.1.3a I can describe the iodine solution test for starch, stating the starting colour and the final colour.
Core 4.1.3b I can describe the Benedict's solution test for reducing sugars, including heating in a hot-water bath, and give the full colour sequence and the precipitate.
Core 4.1.3c I can describe the biuret test for proteins using either the supplied biuret reagent or sodium hydroxide followed by a small amount of dilute copper(II) sulfate.
Core 4.1.3d I can describe the ethanol emulsion test for fats and oils, and call the positive result an emulsion rather than a precipitate.
Core 4.1.3e I can describe the DCPIP test for vitamin C, and I know that here the positive result is the blue colour being lost.
I can state the negative result of each of the five tests, not only the positive one.
I can name a genuine hazard for each test and a matching precaution.
I can write an observation and an inference as two separate statements.
I can explain why a positive result supports presence, while a negative result only shows the substance was not detected under those conditions.
I can explain why a Benedict's or DCPIP result cannot be read as an exact concentration unless the comparison has been controlled.
I can list the variables that must be equalised when comparing two foods, and explain why cross-contamination ruins a result.
I can interpret a set of five test results on an unknown food sample and state only the conclusions the evidence supports.
Supplement 4.1.4a I can describe a DNA molecule as two strands coiled together to form a double helix.
Supplement 4.1.4b I can state that each strand contains chemicals called bases.
Supplement 4.1.4c I can state that bonds between pairs of bases hold the two strands together.
Supplement 4.1.4d I can state that the bases always pair up in the same way, A with T and C with G, and I can write the complementary strand for any given sequence.
I can give all four parts of the statement as a four-line answer, and I stop there rather than adding the chemistry that is not required.

Common mistakes to avoid

“Protein is made from glycerol.” Repair Proteins are made from amino acids. Fats and oils are made from fatty acids and glycerol. Why it happens Glycerol and amino acid both sound like chemistry words attached to the food groups, and both appear in the same paragraph of most notes. Anchor glycerol to fats by remembering that fats and oils are the only case in 4.1.2 needing two kinds of smaller molecule, and glycerol is the second one.
“Fats are made from amino acids.” Repair Fats and oils are made from fatty acids and glycerol. The word acid appears in both “amino acid” and “fatty acid”, and that is the whole source of the confusion. Test yourself Which family contains nitrogen? Proteins. Amino acids build the family with nitrogen; fatty acids build the family without.
“Every protein contains sulfur.” Repair For 0610 the element list for protein is carbon, hydrogen, oxygen and nitrogen. Do not add sulfur to a “list the elements” answer. Why it happens Some proteins do contain sulfur, and other courses mention it. Extra detail that contradicts the required list is not a bonus.
“Benedict's solution tests for all carbohydrates.” Repair It tests for reducing sugars, of which glucose and maltose are the examples you will meet. Iodine solution tests for starch. Starch does not give a positive Benedict's result, and glucose does not turn iodine solution blue-black. Why it happens Both are carbohydrates, so one reagent feels like it should cover both. Two different molecules, two different reagents.
“Add Benedict's solution and observe the colour.” (No heating.) Repair Add Benedict's solution and heat the tube in a hot-water bath before reading the result. Without heating there is no colour change to observe. Cost This loses the method mark and produces a false negative on Paper 5 or Paper 6, which then wrecks the conclusion built on it.
“It turns red.” Repair “The blue Benedict's solution turned brick-red and a precipitate formed.” State the starting colour, the final colour and the precipitate. Why it matters The intermediate colours — green, yellow, orange — are also positive. A student who only knows “red” will record a green tube as negative.
“Benedict's turns purple when protein is present.” Repair Biuret reagents turn lilac or purple for protein, with no heating. Benedict's detects glucose and maltose, with heating. Discriminator Heat. Only one of the two blue reagents is heated, and it is not the protein one.
“Heat the ethanol mixture to speed up the emulsion.” Repair Ethanol is flammable. Keep it away from flames and strong heat. The emulsion test involves no heating at all. Cost A method that heats a flammable liquid is not a safe method, so it cannot be a correct description of this test — and it is genuinely dangerous in a laboratory.
“A white precipitate forms, showing fat.” Repair A milky-white emulsion forms. An emulsion is fine droplets suspended through the liquid; a precipitate is an insoluble solid that can settle out. Why it matters The two words describe different physical things. A description that names the wrong one has not described what happens in the tube.
“The solution stayed blue, so the food contains no sugar.” Repair “No reducing sugar was detected under these conditions.” A concentration below the detection limit gives the same appearance as none at all. Habit to build Write detected in every negative conclusion, automatically. It costs one word and it is never wrong.
“It went orange, so it contains about 3% glucose.” Repair The colour is qualitative. It can rank tubes within a controlled, calibrated series; it cannot be converted into a concentration. What you can say “More reducing sugar was detected in tube B than in tube A, because all other variables were kept the same.”
“I used the same dropper for all five samples to save time.” Repair Use a clean dropper for each sample and each reagent. Cross-contamination produces false positives that look exactly like real results. Related habit Never pour unused reagent back into the stock bottle — that contaminates the supply for everyone.
“Starch turned the iodine blue-black.” (Written in the observation column.) Repair Observation: “the orange-brown solution turned blue-black”. Inference: “starch is present”. You saw a colour, not a molecule. Where it costs The observation and the conclusion are two different statements doing two different jobs. Merged into one sentence, neither has been made.
“There are four food tests.” Repair There are five. Statement 4.1.3 lists iodine solution for starch, Benedict's for reducing sugars, biuret for proteins, the ethanol emulsion test for fats and oils, and DCPIP for vitamin C. Why it happens Four is the number in most older notes and in a lot of textbook material written for other syllabuses. A candidate who has learned four has no answer at all to a vitamin C question — not a weak answer, no answer.
“The DCPIP stayed blue, so the juice is full of vitamin C.” Repair Inverted. Blue is the DCPIP negative. The positive result is the blue being lost so that the tube becomes colourless. Why it happens Benedict's and biuret both start blue and gain a new colour, so “blue means nothing yet” becomes a habit — and then DCPIP, which also starts blue, gets read by the same habit. Learn DCPIP as the exception rather than as a fifth example of the pattern.
“The DCPIP stayed blue, so the food contains no vitamin C.” Repair “No vitamin C was detected under these conditions.” Same detection-limit rule as every other test — and here there is a second possibility worth naming: the vitamin C may have been destroyed before the test, because heat breaks it down. Where it costs If the question tells you the sample was cooked, it has handed you the reason: answer with the heat, not with the detection limit.
“I warmed the DCPIP tube to speed the reaction up.” Repair Never heat this test. Vitamin C is destroyed by heat, so warming the tube removes the very substance you are testing for and manufactures a false negative. Why it matters Notice that this is a different kind of reason from “do not heat the ethanol”. That one is about your safety; this one is about the chemistry of the result. A question asking why a test is not heated wants the right one of the two.
“A fourth large molecule belongs in the list.” Repair Statement 4.1.2 says limited to and names three cases: the glucose group (starch, glycogen and cellulose), proteins from amino acids, and fats and oils from fatty acids and glycerol. There is no fourth. Why it happens DNA is the molecule students reach for, because it belongs in this family of ideas and is taught alongside them elsewhere. In 0610 Topic 4, DNA appears only as a Supplement statement about its structure, which asks nothing about what it is made from.
“DNA is a ladder, and the two strands are identical.” Repair Two strands coiled together into a double helix, and they are complementary, not identical — where one strand carries A the other carries T. Why it happens Every teaching diagram, including Figure 4.11 in this chapter, unwinds the molecule into a ladder to show the pairing. The ladder is the explanation; the helix is the answer. If a question says describe the structure, the word coiled has to be there.
“A pairs with C.” Or: “A pairs with U.” Repair A with T, and C with G. Those two pairs are the only ones that exist, in either order. U belongs to RNA, which you meet in Topic 17, not to DNA. Habit to build Check every pair you write by reading it back as a pair. Any rung that is not A–T or C–G, in some order, is wrong — there is nothing else to remember and no exception to look out for.

Examiner tips

Where the tiering bites in Topic 4 — and it is unusually simple. Three of the four statements in this topic are Core, and they are the three that most of the chapter is about: the element lists, the smaller molecules, and all five food tests. Everything a Core candidate needs for a practical paper is Core. The entire Extended delta in Topic 4 is one statement, 4.1.4, the structure of a DNA molecule, and it has one dedicated section.
And one zone that is not about biology at all. If you are an Extended candidate, do not treat the Supplement statement as optional extra reading. Supplement content is examined on Papers 2 and 4 exactly as Core content is. In this topic it is a single four-part statement — the cheapest marks in the chapter, and the easiest to throw away by assuming they are a bonus.
If all five came easily, you have finished the Supplement content of Topic 4. There is no more of it. Spend the time you saved on the Core material instead — Topic 4 has three Core statements against this one Supplement statement, and all three are examined on your papers exactly as they are on a Core candidate's.

How Biological molecules is examined

Which components you sit depends on your route. A Core candidate takes Paper 1 and Paper 3. An Extended candidate takes Paper 2 and Paper 4. Every candidate, on either route, also takes Paper 5 or Paper 6. Topic 4 can appear in all of them, and it looks different in each.
Topic 4 is one of the few topics that can be tested through recall, through data handling and through experimental skills alike, which is why the reasoning cards in this chapter get as much space as the recall ones. The practical components are the same two papers whichever route you are on, so the observation, control and inference work in this chapter is Core content for everybody.
Do not assume the papers are interchangeable. Core and Extended do not share a theory paper. If you are entered for the Extended route you sit Papers 2 and 4, and the Supplement statement in this topic is fair game in both.
Answer-length trap. A short “describe the test for protein” question wants two things: the method and the colour change. Copper ions and peptide bonds are outside 4.1.3 altogether, so a paragraph on them answers a question nobody asked — and if it crowds out the starting colour, part of what was asked has gone missing.

Syllabus reference and sources

Written against: Cambridge IGCSE Biology (0610) syllabus for 2026, 2027 and 2028, version 2 published December 2025 (Subject Content, Topic 4: Biological molecules).

Written by: Academiq Edu Instructor Panel

Source documents

All educational content, structured explanations, diagrams, worked examples, and pedagogical materials contained within this chapter revision note are the exclusive intellectual property of Academiq Edu. Unauthorized reproduction, distribution, resale, or extraction of this content without prior written permission is strictly prohibited under international copyright laws. Cambridge Assessment International Education (CAIE) is a registered trademark of Cambridge University Press & Assessment. This revision guide is independently authored by the Academiq Edu Instructor Panel for educational purposes and is not affiliated with or endorsed by Cambridge Assessment International Education.

Verified content

Academiq Edu Review Team

Biology content review

Every chapter note, MCQ explanation and structured mark scheme is checked by Cambridge curriculum specialists.