Table of Contents
IGCSE Biology Topic 4.1 — The chemical elements in biological molecules, how large molecules are built, and how to test for each molecule.
Big Idea: All living things are made of the same types of biological molecules — carbohydrates, lipids, proteins, and DNA. These molecules are all built from smaller “building block” units joined together.
1. Chemical Elements in Biological Molecules #
Every biological molecule is made of chemical elements. You need to know which elements are in each type of molecule.
| Molecule | Elements it contains |
|---|---|
| Carbohydrates | Carbon (C), Hydrogen (H), Oxygen (O) |
| Lipids (fats and oils) | Carbon (C), Hydrogen (H), Oxygen (O) |
| Proteins | Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), and often Sulfur (S) |
| DNA | Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P) |
Exam Tip — How to remember: Carbohydrates and lipids only contain C, H, and O. Proteins add Nitrogen (and sometimes Sulfur). DNA adds both Nitrogen and Phosphorus.
2. Large Molecules Built from Smaller Ones #
Large biological molecules are not made all at once. They are built by joining many small molecules together — like connecting pieces of a chain. You need to know which small molecules make up each large molecule.
| Large Molecule | Built from (small molecules) |
|---|---|
| Starch | Glucose molecules joined together |
| Cellulose | Glucose molecules joined together |
| Glycogen | Glucose molecules joined together |
| Proteins | Amino acids joined together |
| Lipids | One glycerol + three fatty acids joined together |
| DNA | Nucleotides joined together |
Note: Starch, cellulose, and glycogen are all made from glucose — but they have different structures and different jobs. Starch stores energy in plants, glycogen stores energy in animals, and cellulose makes plant cell walls.
IMAGE NEEDED: Simple diagram showing glucose joining to form starch, and amino acids joining to form a protein chain
Google Images Search: “IGCSE biology monomer polymer glucose starch amino acid protein diagram educational”
3. Chemical Tests for Biological Molecules #
In biology, we can use chemical tests to find out which biological molecules are present in a sample. You must know the reagent, method, and result for each test.
Test 1: Starch — Iodine Solution Test #
How to do the test:
- Add a few drops of iodine solution to the sample
- Observe the colour change
- Positive result (starch present): Blue-black colour
- Negative result (no starch): Stays orange-brown (the colour of iodine solution)
Test 2: Glucose and Maltose — Benedict’s Solution Test #
This test detects reducing sugars, which includes glucose and maltose. Benedict’s solution is blue when cold.
How to do the test:
- Add Benedict’s solution to the sample in a test tube
- Place the test tube in a hot water bath for about 5 minutes
- Observe the colour change
- Positive result (reducing sugar present): Turns brick-red / orange-red
- Negative result (no reducing sugar): Stays blue
Important: The test must be heated. If you forget to heat it, you will not get a colour change even if glucose is present.
Test 3: Protein — Biuret Test #
How to do the test:
- Add sodium hydroxide (NaOH) solution to the sample
- Then add a few drops of copper sulfate (CuSO₄) solution (or use ready-made Biuret reagent)
- Observe the colour change (no heating needed)
- Positive result (protein present): Turns purple / violet
- Negative result (no protein): Stays blue
Test 4: Lipids — Ethanol Emulsion Test #
How to do the test:
- Dissolve the sample in ethanol (alcohol) — mix well
- Pour this mixture into a test tube of cold water
- Observe the result
- Positive result (lipid present): A white, cloudy (milky) emulsion forms
- Negative result (no lipid): The water stays clear
The cloudiness happens because lipids do not dissolve in water. When you pour the ethanol-lipid mixture into water, the fat droplets spread through the water and scatter light, making it look milky.
Summary: Chemical Tests at a Glance #
| Molecule to test | Reagent used | Method | Positive result |
|---|---|---|---|
| Starch | Iodine solution | Add drops directly | Blue-black |
| Glucose / Maltose | Benedict’s solution | Add + heat in water bath | Brick-red / orange |
| Protein | NaOH + copper sulfate (Biuret) | Add reagents, no heat | Purple / violet |
| Lipid | Ethanol + water | Dissolve in ethanol, pour into water | White cloudy emulsion |
Exam Tip: For each test, make sure you know three things: the reagent, whether heating is needed, and the exact colour of a positive result. Exams often ask for specific colours — “blue-black” is more precise than just “dark colour.”
