What are the main types of biological molecules covered in the Cambridge IGCSE Coordinated Science syllabus?

The main types of biological molecules covered in the Cambridge IGCSE Coordinated Science syllabus include carbohydrates, proteins, lipids, and nucleic acids. These molecules are essential for various biological processes and structures in living organisms.

How do carbohydrates function in living organisms according to the IGCSE Coordinated Science curriculum?

In the IGCSE Coordinated Science curriculum, carbohydrates are described as a primary source of energy for living organisms. They are also important for structural purposes, such as cellulose in plant cell walls. Carbohydrates are made up of sugar units and can be simple sugars like glucose or complex carbohydrates like starch and glycogen.

What role do proteins play in biological systems as per the Cambridge IGCSE syllabus?

Proteins play a crucial role in biological systems by acting as enzymes, hormones, and structural components. They are made up of amino acids and are essential for growth, repair, and maintenance of body tissues. Enzymes, which are proteins, catalyze biochemical reactions, making them vital for metabolism.

Why are lipids important in biological molecules, and how are they studied in the IGCSE Coordinated Science course?

Lipids are important biological molecules because they serve as long-term energy storage, make up cell membranes, and act as signaling molecules. In the IGCSE Coordinated Science course, students learn about the structure of lipids, including fats and oils, and their role in maintaining cell integrity and energy balance.

How are nucleic acids significant in the study of biological molecules in the Cambridge IGCSE Coordinated Science curriculum?

Nucleic acids, such as DNA and RNA, are significant because they store and transmit genetic information in living organisms. In the Cambridge IGCSE Coordinated Science curriculum, students explore how nucleic acids are involved in protein synthesis and heredity, highlighting their role in the continuity of life.

Why is "Forgetting to state that the Benedict's test requires heating" a common mistake in Biological Molecules?

Why it happens: Students describe the reagent but omit the heating step, which is essential for the reaction to occur. How to avoid it: Always write: 'add Benedict's reagent AND heat (in a water bath at ~80°C)'.

Why is "Stating the positive Biuret result is blue" a common mistake in Biological Molecules?

Why it happens: Copper sulfate is blue, and students confuse the reagent colour with the positive result. How to avoid it: Positive Biuret = **purple/violet**. Blue = the colour of CuSO₄ itself (negative result).

Why is "Expecting sucrose to give a positive Benedict's test" a common mistake in Biological Molecules?

Why it happens: Students think all sugars are detected by Benedict's, but sucrose is a non-reducing sugar. How to avoid it: Sucrose is non-reducing — must be hydrolysed to glucose + fructose first (by boiling with HCl, then neutralising) before a positive Benedict's test is obtained.

Why is "Saying animals store energy as starch" a common mistake in Biological Molecules?

Why it happens: Students remember starch as the plant polysaccharide and over-generalise. How to avoid it: Animals store carbohydrate as **glycogen** (in liver and muscle). Plants store carbohydrate as **starch**.

Biological MoleculesCambridge IGCSE Coordinated Sciences 0654

Biological Molecules

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

PreviousNext

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Short Study Notes — Biological Molecules

Start with these resources to cover the key concepts, then work through the practice questions.

Page 1 / 0

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Biological Molecules, ready to print or save offline.

Biological Molecules — Cambridge IGCSE Coordinated Science 0654

Carbohydrates, proteins, and lipids form the structural and functional basis of all living things. Food tests are mandatory practical knowledge. The elements in each molecule and the distinction between reducing and non-reducing sugars catch students out every year.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

B4.1 — Describe the chemical elements present in carbohydrates, proteins, and lipids.
B4.2 — Describe the structure and functions of carbohydrates, proteins, and lipids.
B4.3 — Carry out and interpret food tests for reducing sugars, starch, protein, and lipids.
B4.4 — Describe the biological importance of water.

Carbohydrates — From Simple Sugars to Complex Polymers

Elements: C, H, O. Three types: monosaccharides, disaccharides, polysaccharides.

Elements always present: Carbon (C), Hydrogen (H), Oxygen (O) — with H:O ratio of 2:1 (same as water).

Three levels of carbohydrate:

Type	Examples	Function
Monosaccharides (1 sugar unit)	Glucose (C₆H₁₂O₆), fructose, galactose	Direct energy source; building blocks for larger carbohydrates
Disaccharides (2 sugar units joined)	Sucrose (glucose + fructose), maltose, lactose	Transport form (sucrose in plant phloem)
Polysaccharides (many sugar units)	Starch, glycogen, cellulose	Storage and structural roles

Key polysaccharides:

Starch: storage form in PLANTS (insoluble — does not affect osmosis; compact for storage)
Glycogen: storage form in ANIMALS — stored in liver and muscle cells
Cellulose: structural component of plant CELL WALLS — insoluble, rigid, cannot be digested by most animals (dietary fibre)

Note on storage: Starch and glycogen are INSOLUBLE, which means they do not affect water potential inside cells and can be stored in large quantities without drawing water in by osmosis.

C, H, O only — H:O ratio 2:1.
Glucose = primary energy source for respiration.
Starch = plant storage; glycogen = animal storage.
Cellulose = plant cell wall structural material (indigestible by humans).

Proteins — Built from Amino Acids, Function Determined by Shape

Elements: C, H, O, N (always). Amino acids joined by peptide bonds. 3D shape = function.

Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N) — always present. Often also Sulphur (S).

The nitrogen tells you it's a protein — carbohydrates and lipids do NOT contain nitrogen.

Structure:

Building blocks = amino acids (there are 20 different ones)
Two amino acids join via a peptide bond → dipeptide
Many amino acids → polypeptide → folded → protein
The specific sequence of amino acids determines the protein's unique 3D shape
Shape determines function (e.g. enzyme active site)

Functions of proteins — enormous diversity:

Function	Example
Enzymes (biological catalysts)	Amylase, pepsin, lipase
Structural	Collagen (connective tissue), keratin (hair, nails)
Transport	Haemoglobin (carries O₂ in blood)
Hormones	Insulin (controls blood glucose)
Antibodies	Immunoglobulins (immune defence)
Membrane proteins	Carrier and channel proteins

Key point: If a protein is DENATURED (shape changed by heat or extreme pH), it can no longer function.

Elements: C, H, O, N (always) — the N distinguishes proteins from carbs and lipids.
Built from amino acids joined by peptide bonds.
Protein's 3D shape determines its specific function.
Denaturation = irreversible change in shape → loss of function.

Lipids — Energy-Dense and Multi-Functional

Elements: C, H, O (less O than carbohydrates). Structure: 1 glycerol + 3 fatty acids.

Elements: Carbon (C), Hydrogen (H), Oxygen (O) — but MUCH LESS oxygen than in carbohydrates.

Structure: Triglyceride = 1 glycerol molecule + 3 fatty acid molecules (joined by ester bonds).

Saturated vs unsaturated fatty acids:

Saturated: no C=C double bonds; solid at room temperature; mainly animal fats
Unsaturated: one or more C=C double bonds; liquid at room temperature; mainly plant oils

Functions of lipids:

Function	Example
Long-term energy store	Adipose (fat) tissue under skin
Thermal insulation	Subcutaneous fat in mammals
Protection of organs	Fat surrounding kidneys
Cell membranes	Phospholipid bilayer
Fat-soluble vitamins	Vitamins A, D, E, K dissolved in lipids

Why lipids store more energy than carbohydrates: Lipids have more C–H bonds and less oxygen per gram → more energy released per gram when oxidised. Lipids release ~39 kJ/g versus ~17 kJ/g for carbohydrates.

C, H, O only — but proportionally LESS oxygen than carbohydrates.
1 glycerol + 3 fatty acids → triglyceride.
Lipids store more energy per gram than carbohydrates (~39 kJ/g vs ~17 kJ/g).
Phospholipids form cell membranes (phospholipid bilayer).

Food Tests — Essential Practical Knowledge

Know reagent, procedure, positive result, and negative result for all four tests.

These are tested directly in Paper 6 (Alternative to Practical) and described in Paper 4. Know every detail.

Test For	Reagent	Procedure	Positive Result	Negative Result
Reducing sugars (glucose, maltose, fructose)	Benedict's solution (blue)	Add 2 cm³ Benedict's + food solution, heat in water bath ~80°C for 3 min	Orange/brick-red precipitate forms	Remains blue
Starch	Iodine solution (yellow-brown)	Add 2-3 drops of iodine to food	Blue-black colour	Stays yellow-brown
Protein	Biuret reagent (NaOH + dilute CuSO₄, blue)	Add NaOH, then add dilute CuSO₄	Purple/violet colour	Stays blue
Lipids	Ethanol emulsion test	Shake food with ethanol; pour solution into water	White milky emulsion forms	Stays clear

IMPORTANT — Sucrose (non-reducing sugar): Sucrose does NOT give a positive Benedict's test directly. To test for sucrose:

First test with Benedict's (negative = no reducing sugar)
Add dilute HCl (hydrochloric acid) and heat to hydrolyse sucrose → glucose + fructose
Neutralise with sodium hydrogencarbonate
Re-test with Benedict's — now positive (orange/red) confirms sucrose was present

Controls: Always run a control (same procedure with distilled water) to compare colour changes objectively.

Benedict's (heated) → orange/red = reducing sugar present.
Iodine → blue-black = starch present.
Biuret (NaOH + CuSO₄) → purple = protein present.
Ethanol + water → white emulsion = lipid present.
Sucrose: negative Benedict's → hydrolyse with HCl → positive Benedict's.

How it’s examined

Paper 2: multiple choice on food test results, molecular composition, or functions. Paper 4: 'State the elements found in a protein but NOT in a lipid' (answer: Nitrogen); 'Describe the test for starch and the expected result' (2 marks); table-completion questions on food test procedures. Paper 6: practical questions on interpreting food test results from a table of observations.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B4); 0654/42 May/Jun 2023 — Q1 (biological molecules). Last reviewed 2026-05-14.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Biological Molecules, ready to print or save as PDF.

Step-by-step worked examples — Biological Molecules

Step-by-step solutions to past-paper-style questions on biological molecules, written exactly the way a tutor would explain them at the board.

1Interpret iodine solution food test result
Core• food test, iodine, starch
Question
A food sample gives a blue-black colour with iodine solution. State what this result indicates.
Step-by-step solution
1. Step 1
  Iodine solution (yellow-brown) turns blue-black in the presence of starch. No colour change (remains yellow-brown) = starch absent.
Answer
Starch is present in the food sample.
Examiner tip
State the result AND the conclusion. Writing only 'positive result' or only 'starch' without linking them loses marks.
2Describe the test for protein using Biuret reagent
Extended• food test, Biuret, protein
Question
Describe how you would test a food sample for the presence of protein. State the result for a positive test.
Step-by-step solution
1. Step 1
  Add dilute sodium hydroxide solution (NaOH) to the food sample in a test tube.
2. Step 2
  Add a few drops of dilute copper(II) sulfate solution ( $\text{CuSO}_4$ , blue).
3. Step 3
  Observe the colour: a purple/violet colour indicates protein is present (positive Biuret test). If the solution remains blue, no protein is present.
Answer
Add NaOH then dilute $\text{CuSO}_4$ to the sample. A purple/violet colour indicates protein is present.
Examiner tip
Examiners require both reagents (NaOH and CuSO₄) in the correct order for full marks. The positive colour is purple or violet — NOT blue (that is the colour of CuSO₄ itself).
3Monomers of proteins and how they are joined
Extended• protein, amino acids, peptide bonds
Question
State the monomers of proteins and explain how they are joined together.
Step-by-step solution
1. Step 1
  The monomers of proteins are amino acids. There are 20 different amino acids used in living organisms.
2. Step 2
  Amino acids are joined together by peptide bonds, formed during condensation reactions (water is released). A chain of amino acids is called a polypeptide.
3. Step 3
  One or more polypeptide chains fold into a specific 3D shape to form a functional protein.
Answer
The monomers of proteins are amino acids. They are joined by peptide bonds formed during condensation reactions, producing polypeptide chains.
4Benedict's test — interpret brick red result
Core• food test, Benedict's, reducing sugar
Question
A student tests an unknown solution with Benedict's reagent and heats it. The solution turns brick red. What does this show?
Step-by-step solution
1. Step 1
  Benedict's reagent is blue. When heated with a reducing sugar (e.g. glucose, maltose), it changes colour from blue → green → yellow → orange → brick red, depending on concentration.
2. Step 2
  A brick red colour indicates a high concentration of reducing sugar is present.
Answer
A reducing sugar (e.g. glucose) is present in high concentration. The blue Benedict's solution has been reduced to a brick red precipitate of copper(I) oxide.
Examiner tip
Always name the type of sugar (reducing sugar) not just 'sugar'. Sucrose is a non-reducing sugar and does NOT give a positive Benedict's test without prior hydrolysis.
5Compare roles of starch and glycogen
Extended• starch, glycogen, storage
Question
Compare the roles of starch and glycogen in living organisms.
Step-by-step solution
1. Step 1
  Starch: the main carbohydrate storage molecule in plants. Made of glucose monomers. Stored in chloroplasts and storage organs (potato tubers, seeds).
2. Step 2
  Glycogen: the main carbohydrate storage molecule in animals (and fungi). Made of glucose monomers. Stored in liver and muscle cells.
3. Step 3
  Both are insoluble (so they do not affect osmosis) and can be hydrolysed back to glucose when energy is needed. Glycogen is more branched than starch, allowing faster mobilisation of glucose.
Answer
Starch: energy storage in plant cells (e.g. seeds, tubers). Glycogen: energy storage in animal cells (liver and muscles). Both are polymers of glucose; both insoluble so do not affect osmotic potential of the cell.

Key Definitions and Keywords — Biological Molecules

Definitions to memorise and the exact keywords mark schemes credit for biological molecules answers — sharpened from recent examiner reports for the 2026 Cambridge IGCSE sitting.

Carbohydrate
Examiner keyword
A biological molecule made of carbon, hydrogen and oxygen (ratio of H:O approximately 2:1). Includes monosaccharides (e.g. glucose), disaccharides (e.g. maltose), and polysaccharides (e.g. starch, glycogen, cellulose).
Protein
Examiner keyword
A large biological molecule made of one or more chains of amino acids linked by peptide bonds. Proteins have structural and functional roles (e.g. enzymes, antibodies, haemoglobin, collagen).
Lipid
Examiner keyword
A biological molecule made of fatty acids and glycerol. Functions include energy storage, thermal insulation, and as components of cell membranes.
Monomer
Examiner keyword
A small molecule that can join with many identical units to form a polymer. E.g. glucose (monomer of starch/glycogen/cellulose), amino acids (monomers of proteins).
Peptide bond
Examiner keyword
The chemical bond formed between two amino acids during a condensation reaction, linking them into a polypeptide chain.
Reducing sugar
Examiner keyword
A sugar that can donate electrons to (reduce) another compound; detected by Benedict's test (blue → brick red on heating). Examples: glucose, fructose, maltose.
Emulsion test
A food test for lipids: dissolve sample in ethanol, add water; a milky-white emulsion indicates lipids are present.

Common Mistakes and Misconceptions — Biological Molecules

The traps other students keep falling into on biological molecules questions — taken from recent Cambridge IGCSE examiner reports and mark schemes — and how to avoid them.

✕Forgetting to state that the Benedict's test requires heating
Why it happens
Students describe the reagent but omit the heating step, which is essential for the reaction to occur.
How to avoid it
Always write: 'add Benedict's reagent AND heat (in a water bath at ~80°C)'.
✕Stating the positive Biuret result is blue
Why it happens
Copper sulfate is blue, and students confuse the reagent colour with the positive result.
How to avoid it
Positive Biuret = purple/violet. Blue = the colour of CuSO₄ itself (negative result).
✕Expecting sucrose to give a positive Benedict's test
Why it happens
Students think all sugars are detected by Benedict's, but sucrose is a non-reducing sugar.
How to avoid it
Sucrose is non-reducing — must be hydrolysed to glucose + fructose first (by boiling with HCl, then neutralising) before a positive Benedict's test is obtained.
✕Saying animals store energy as starch
Why it happens
Students remember starch as the plant polysaccharide and over-generalise.
How to avoid it
Animals store carbohydrate as glycogen (in liver and muscle). Plants store carbohydrate as starch.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Biological Molecules — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Type

Examples

Function

Monosaccharides (1 sugar unit)

Glucose (C₆H₁₂O₆), fructose, galactose

Direct energy source; building blocks for larger carbohydrates

Disaccharides (2 sugar units joined)

Sucrose (glucose + fructose), maltose, lactose

Transport form (sucrose in plant phloem)

Polysaccharides (many sugar units)

Starch, glycogen, cellulose

Storage and structural roles

Function

Example

Enzymes (biological catalysts)

Amylase, pepsin, lipase

Structural

Collagen (connective tissue), keratin (hair, nails)

Transport

Haemoglobin (carries O₂ in blood)

Hormones

Insulin (controls blood glucose)

Antibodies

Immunoglobulins (immune defence)

Membrane proteins

Carrier and channel proteins

Function

Example

Long-term energy store

Adipose (fat) tissue under skin

Thermal insulation

Subcutaneous fat in mammals

Protection of organs

Fat surrounding kidneys

Cell membranes

Phospholipid bilayer

Fat-soluble vitamins

Vitamins A, D, E, K dissolved in lipids

Test For

Reagent

Procedure

Positive Result

Negative Result

Reducing sugars (glucose, maltose, fructose)

Benedict's solution (blue)

Add 2 cm³ Benedict's + food solution, heat in water bath ~80°C for 3 min

Orange/brick-red precipitate forms

Remains blue

Starch

Iodine solution (yellow-brown)

Add 2-3 drops of iodine to food

Blue-black colour

Stays yellow-brown

Protein

Biuret reagent (NaOH + dilute CuSO₄, blue)

Add NaOH, then add dilute CuSO₄

Purple/violet colour

Stays blue

Lipids

Ethanol emulsion test

Shake food with ethanol; pour solution into water

White milky emulsion forms

Stays clear

Biological Molecules

Short Study Notes in the form of Slides

Short Study Notes — Biological Molecules

Detailed Notes

What you’ll learn

How it’s examined

1Interpret iodine solution food test result

2Describe the test for protein using Biuret reagent

3Monomers of proteins and how they are joined

4Benedict's test — interpret brick red result

5Compare roles of starch and glycogen

Carbohydrate

Protein

Lipid

Monomer

Peptide bond

Reducing sugar

Emulsion test

✕Forgetting to state that the Benedict's test requires heating

✕Stating the positive Biuret result is blue

✕Expecting sucrose to give a positive Benedict's test

✕Saying animals store energy as starch

Practice questions

Past paper style quiz

Assess your understanding

Biological Molecules — frequently asked questions

Biological Molecules

Short Study Notes in the form of Slides

Short Study Notes — Biological Molecules

Detailed Notes

What you’ll learn

How it’s examined

1Interpret iodine solution food test result

2Describe the test for protein using Biuret reagent

3Monomers of proteins and how they are joined

4Benedict's test — interpret brick red result

5Compare roles of starch and glycogen

Carbohydrate

Protein

Lipid

Monomer

Peptide bond

Reducing sugar

Emulsion test

✕Forgetting to state that the Benedict's test requires heating

✕Stating the positive Biuret result is blue

✕Expecting sucrose to give a positive Benedict's test

✕Saying animals store energy as starch

Practice questions

Past paper style quiz

Assess your understanding

Biological Molecules — frequently asked questions