Pearson Edexcel International GCSE in Biology · 4BI1 · Free · no account needed
Carbohydrates, Proteins and Lipids
The three big food molecules — what they are made of, what they are built from, and how to test for them.
Topic 2 · Structures and functions in living organisms — one of 34 lessons in this topic, and one of 68 in Biology.
This lesson ends with Emil Fischer. Learn Carbohydrates, Proteins and Lipids, teach it, then defend it against Emil Fischer.
The definition this lesson teaches, word for word — every Biology definition.
- Protein
- A large molecule built from a chain of amino acids. Proteins contain carbon, hydrogen, oxygen and nitrogen, and it is the number and order of the amino acids that decides which protein it is.
One of the 16 Biology required practicals.
What this lesson covers in the specification
Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.
- 2.7 — The chemical elements present in carbohydrates, proteins and lipids
- 2.8 — Carbohydrates, proteins and lipids as large molecules built from smaller basic units
- 2.9 — Practical: test food samples for glucose, starch, protein and fat (required practical)
Specification section 2(c) Biological molecules — also taught in Comparing Cells, and Stem Cells, Enzymes: Catalysts With a Shape.
1 · Understand it
No exam language yet. The only question this section answers is: do I actually understand what is happening?
Big biological molecules are made by joining small ones in a chain — the way a necklace is made from beads. Change the beads and you change everything about the necklace. That single idea explains why carbohydrates, proteins and lipids behave so differently despite all being 'food'.
The three, and their beads
- Carbohydrates — made from simple sugars. Join many glucose molecules and you get starch (plant store) or glycogen (animal and fungal store).
- Proteins — made from amino acids. There are about twenty different amino acids, so the possible chains are effectively endless. That is why proteins do so many different jobs.
- Lipids (fats and oils) — made from fatty acids and glycerol. One glycerol holds three fatty acids.
The elements matter and are easy marks. Carbohydrates and lipids both contain only carbon, hydrogen and oxygen. Proteins contain those three plus nitrogen — and that nitrogen is why plants need nitrate ions from the soil, and why protein is the one food group you cannot simply build out of sugar.
Think of it like building materials
Carbohydrate is firewood: burn it fast for energy. Lipid is coal: more energy per gram, stored for later, also useful for insulation and waterproofing. Protein is bricks: you build with it. That is why the body burns carbohydrate first, stores fat second, and only breaks down protein when it is desperate — you do not demolish the walls to heat the house unless there is nothing else left.
2 · Grade 9 Notes
A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.
Elements — learn this exactly
- Carbohydrates: carbon, hydrogen, oxygen
- Lipids: carbon, hydrogen, oxygen
- Proteins: carbon, hydrogen, oxygen AND nitrogen (some also contain sulfur)
Each large molecule and the small units it is built from
- Starch and glycogen
- built from simple sugars (glucose)
- Protein
- built from amino acids — about twenty different ones
- Lipid
- built from three fatty acids and one glycerol
Required practical: food tests
- Glucose (a reducing sugar) — add Benedict's solution and heat in a water bath. Blue → brick red / orange if present.
- Starch — add iodine solution. Orange-brown → blue-black if present.
- Protein — add Biuret solution (sodium hydroxide then copper sulfate). Blue → purple/lilac if present.
- Fat — the emulsion test: dissolve in ethanol, then add water. A cloudy white emulsion forms if present.
Variables
Independent (changed) — which food sample is being tested (and which reagent is used on it)
Dependent (measured) — the final colour — a qualitative result, present or absent, not a measurement
| Control variable | Why it must be held constant |
|---|---|
| Volume of food sample | more sample gives a stronger colour, so two foods could not be compared |
| Volume of reagent | too little reagent gives a weak or absent colour even when the food is present |
| Heating time and water-bath temperature for Benedict's | an unheated or briefly heated tube reads negative whatever is in it |
| Grinding and filtering the sample the same way | an unfiltered suspension is cloudy already, which hides the emulsion test result |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Benedict's solution not heated, or not heated long enough. The tube reads negative even when reducing sugar is present, and it does so every time. | Heat every tube in an 80 °C water bath for a fixed five minutes. |
| Judgement | Deciding whether a colour counts as 'brick red' or 'orange', or whether a lilac tinge is really purple. | Compare against a printed colour standard, or use a colorimeter. One person judges every tube in the same light. |
| Systematic | A cloudy or coloured food sample makes the emulsion test unreadable, since a positive result IS cloudiness. | Filter the sample first, and run an ethanol-and-water control with no food in it. |
| Random | Benedict's gives a graded colour — green, yellow, orange, brick red — with the amount of sugar, so a weak positive is easily called negative. | Note that the test is SEMI-quantitative and record the colour reached rather than only 'positive' or 'negative'. |
The words this page is examined on
- Amino acid
- the basic unit a protein is built from; about twenty different kinds exist
- Simple sugar
- the basic unit a carbohydrate is built from — glucose is the one to name
- Fatty acid and glycerol
- the units a lipid is built from: THREE fatty acids to ONE glycerol
- Reducing sugar
- a sugar that gives a positive Benedict's test — glucose is one; starch is not
- Emulsion
- the cloudy white suspension formed in the fat test when ethanol solution is poured into water
Learn this definition · Protein
A large molecule built from a chain of amino acids. Proteins contain carbon, hydrogen, oxygen and nitrogen, and it is the number and order of the amino acids that decides which protein it is.
The three molecules — elements, units, and how to test for them
| Carbohydrate | Protein | Lipid | |
|---|---|---|---|
| Elements | carbon, hydrogen, oxygen | carbon, hydrogen, oxygen AND nitrogen (some also sulfur) | carbon, hydrogen, oxygen |
| Built from | simple sugars, e.g. glucose | amino acids — about twenty different ones | three fatty acids and one glycerol |
| Large examples | starch (plants), glycogen (animals and fungi) | enzymes, haemoglobin, antibodies | fats (solid) and oils (liquid) |
| The test | Benedict's, heated — for reducing sugar; iodine — for starch | Biuret solution | emulsion test: ethanol, then water |
| Positive result | blue → brick red (Benedict's); orange-brown → blue-black (iodine) | blue → purple or lilac | cloudy white emulsion |
The four food tests, in the order the marks are given
- REDUCING SUGAR — add Benedict's solution and HEAT in a water bath. Blue → green → yellow → orange → brick red as the amount rises.
- STARCH — add iodine solution at room temperature. Orange-brown → blue-black.
- PROTEIN — add Biuret solution (sodium hydroxide, then copper sulfate). Blue → purple or lilac.
- FAT — dissolve the sample in ethanol, then pour into water. A cloudy white emulsion forms.
- In every case: name the reagent, name any condition (heat for Benedict's), and give BOTH colours — from and to. Giving only the final colour loses a mark.
Model answer [3 marks]
A food sample gives a brick-red colour with Benedict's solution and a lilac colour with Biuret solution. State what the sample contains, and explain how you know. [3]
It contains a reducing sugar such as glucose, because Benedict's solution turns from blue to brick red only when a reducing sugar is present and the mixture has been heated. It also contains protein, because Biuret solution turns from blue to purple or lilac only in the presence of protein. The two tests are independent, so a positive result in each shows both are present.
Model answer [3 marks]
Explain why nitrogen is the element that makes protein different from carbohydrate and lipid, and give one consequence for a plant. [3]
Carbohydrates and lipids contain only carbon, hydrogen and oxygen, all of which a plant can obtain from carbon dioxide and water. Proteins also contain nitrogen, which cannot be obtained from either. A plant must therefore absorb nitrate ions from the soil in order to make amino acids and so build proteins, which is why a nitrate deficiency stunts growth.
Not this: 'Benedict's test goes purple.'
This: Benedict's goes brick red for sugar. Biuret goes purple for protein. Mixing up the two colours is the most common food-test error.
Why the nitrogen matters more than it looks
A. Carbohydrate and lipid are carbon, hydrogen and oxygen only — everything a plant can get from air and water.
B. Protein needs NITROGEN as well, which must be absorbed as nitrate. That single extra element is why protein cannot simply be built out of sugar, in a plant or in you.
Mark-losing trap. Only protein contains nitrogen — that is why plants need nitrate ions and why excess is removed as urea.
Mark-losing trap. Benedict's must be HEATED. An unheated tube reads negative however much sugar is in it.
Mark-losing trap. Give BOTH colours — from and to. 'It goes brick red' scores less than 'blue to brick red'.
Mark-losing trap. A lipid is three fatty acids AND one glycerol. Naming only the fatty acids is half the answer.
Mark-losing trap. Starch and glycogen are both built from glucose. What differs is how the chains are arranged, not the unit.
3 · Prove it — the five questions
The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.
- Grade 6 · State [1 mark] — Which element is present in proteins but not in carbohydrates or lipids?
- Grade 7 · Describe [2 marks] — Describe how you would test a food sample for the presence of starch, and state the result if starch is present.
- Grade 8 · Explain [3 marks] — A student tests a food sample with Benedict's solution and records 'no sugar present'. The teacher points out the student never used a water bath. Explain why the result cannot be trusted.
- Grade 9 · Explain [4 marks] — Explain why proteins can perform an enormous range of different functions in the body, whereas starch essentially does one job.
- 9+ · Analyse [4 marks] — A food sample gives a brick-red Benedict's result, a blue-black iodine result and a purple Biuret result, but no emulsion. Which conclusion is best supported?
The people behind this science
Two ways into the same idea — the one who showed food chemistry is testable, and the one who read structure from indirect evidence. Inside Incandio each of them answers knowing exactly which lesson you have just finished.
Louis Pasteur — the one who showed food chemistry is testable
Your food tests are colour changes that reveal an invisible composition. Pasteur built his career on exactly that move — using a specific, repeatable test to prove what was present in a liquid when nobody could see it, and insisting on controls before believing any result.
- “How do you prove a substance is present when you cannot see or taste it?”
- “Why does a control matter as much as the test itself?”
- “What would you do if a test gave a result you did not expect?”
- “How can a colour change tell you anything about a molecule?”
Rosalind Franklin — the one who read structure from indirect evidence
This lesson's central idea is that large biological molecules are chains of small repeating units, and that the units decide everything. Franklin spent her career deducing exactly that kind of structure — in DNA, in coal, in viruses — from evidence that never showed the molecule directly.
- “How do you work out the structure of a molecule you can never see?”
- “Why does it matter so much which small units a large molecule is built from?”
- “Why can proteins do thousands of jobs when starch does essentially one?”
- “How do you know when your evidence is strong enough to claim a structure?”
Think you understand it?
Reading is the start. On Incandio a topic counts as mastered only once you have taught it and defended it — the two things a revision guide cannot ask of you.
Incandio this topic → Prove it. Teach it. Defend it. What does “Incandio it” mean?
- Prove it — five questions on Carbohydrates, Proteins and Lipids, marked on the spot, that tell you what was missing rather than just what was wrong.
- Teach it — Explain Carbohydrates, Proteins and Lipids in your own words to Ember, an AI pupil who does not have it yet and asks the awkward question. Understanding is credited; wording is not required.
- Defend it — Argue Carbohydrates, Proteins and Lipids against Emil Fischer — three rounds, marked against descriptors you can read before you begin.
- Then the exam — read all 10 authored exam-style questions on this unit, each with its command word and its tariff, then have them marked on Incandio with the mistake you actually made named.
Completely free. No subscriptions, no paywalls, no account needed to start.
Carry on through the course
- Previous lesson: Comparing Cells, and Stem Cells
- Next lesson: Enzymes: Catalysts With a Shape
- Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
- Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
- Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
- Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
- Diffusion, Osmosis and Active Transport — The three ways anything gets into or out of a cell — which need energy, which do not, and what makes each one fast or slow.
- All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification