Biological Molecules

Summary: The four main groups of biological molecules are carbohydrates, lipids (fats/oils), proteins, and DNA. Each group has a specific chemical composition and functions essential to life. Food tests (Benedict’s, iodine, Biuret, emulsion test) are used to identify the presence of these molecules. Water is also an essential biological molecule due to its role as a universal solvent. Tags: igcse biology biological-molecules carbohydrates lipids proteins dna food-tests Created: 2026-07-16 Last Updated: 2026-07-16

1. Carbohydrates

Carbohydrates contain the elements carbon (C), hydrogen (H), and oxygen (O). The ratio of hydrogen to oxygen is 2:1 — the same ratio as in water (H2O), hence the name “carbo-hydrate”.

Carbohydrates are classified by the number of sugar units:

Key polysaccharides:

2. Benedict’s Test for Reducing Sugars

What it tests for: Reducing sugars (all monosaccharides and some disaccharides, e.g. glucose, fructose, maltose — but NOT sucrose, which is a non-reducing sugar).

Method:

1. Add an equal volume of Benedict’s solution (blue) to the food sample in a test tube
2. Place the test tube in a boiling water bath for 2-5 minutes
3. Observe any colour change

Results:

Benedict’s test is semi-quantitative — the colour indicates how much reducing sugar is present (not just whether it is present or absent). The more brick-red the precipitate, the higher the concentration of reducing sugar.

3. Iodine Test for Starch

What it tests for: Starch (a polysaccharide).

Method:

1. Add a few drops of iodine solution (orange-brown) to the food sample
2. Observe any colour change

Results:

Iodine solution is orange-brown, not yellow. The positive result is blue-black, not just black or purple.

4. Lipids (Fats and Oils)

Lipids contain the elements carbon (C), hydrogen (H), and oxygen (O) — but they contain much less oxygen than carbohydrates, which makes them a more energy-dense storage molecule.

Structure:

• Made up of 3 fatty acid molecules joined to 1 glycerol molecule
• This is why lipids are sometimes called triglycerides
• Fatty acids are long hydrocarbon chains (C and H)

Functions of lipids:

• Long-term energy storage — lipids contain more than twice the energy per gram compared to carbohydrates
• Thermal insulation — fat layer under the skin (adipose tissue) reduces heat loss
• Protection of vital organs — fat around kidneys, heart
• Structural component of cell membranes — phospholipids form the bilayer of all cell membranes
• Waterproofing — oils on skin, fur, feathers

5. Emulsion Test for Lipids

Method:

1. Add ethanol to the food sample and shake vigorously (this dissolves any lipids present)
2. Pour the mixture into an equal volume of cold water in a clean test tube
3. Observe the result

Results:

Why this works: Lipids dissolve in ethanol but not in water. When the ethanol-lipid solution is added to water, the lipid comes out of solution and forms tiny droplets that scatter light, creating a milky white emulsion.

6. Proteins

Proteins contain the elements carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S).

Structure:

• Proteins are made of amino acids joined together in long chains
• A chain of amino acids is called a polypeptide
• Each protein has a unique sequence of amino acids, folded into a specific 3D shape
• The 3D shape is held by bonds between different parts of the polypeptide chain (hydrogen bonds, ionic bonds, disulfide bridges)
• The shape determines the function — if the shape changes (denaturation), the protein can no longer function

Functions of proteins:

7. Biuret Test for Proteins

Method:

1. Add an equal volume of sodium hydroxide solution (NaOH) to the food sample
2. Add a few drops of copper(II) sulfate solution (CuSO4, dilute)
3. Shake gently and observe any colour change

Results:

Biuret reagent is sometimes used as a ready-mixed solution (NaOH + CuSO4). The positive result is violet/purple, not blue.

8. DNA (Deoxyribonucleic Acid)

DNA is the molecule that carries the genetic code — the instructions for making all the proteins in an organism.

Structure:

• DNA is a double helix (two strands twisted around each other)
• Each strand is made of nucleotides joined together
• A nucleotide consists of: a phosphate group, a sugar (deoxyribose), and a nitrogenous base
• There are four bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
• The bases pair specifically: A pairs with T, C pairs with G (complementary base pairing)
• The two strands are held together by hydrogen bonds between the complementary bases

Function:

• Carries the genetic information (the sequence of bases codes for the sequence of amino acids in proteins)
• Located in the nucleus of cells (in chromosomes)
• Controls which proteins are made (protein synthesis) — one gene codes for one protein

9. Water — An Essential Biological Molecule

Water is vital for all living organisms. Its properties make it essential for life:

Sources

• BBC Bitesize GCSE Biology — Biological molecules, BBC (free educational resource)
• OpenStax Biology 2e — Ch. 2 The Chemical Foundation of Life, Ch. 3 Biological Macromolecules, Rice University (free, CC BY 4.0)
• Cambridge IGCSE Biology 0610 — Syllabus 4: Biological molecules, Cambridge Assessment International Education
• CK-12 Biology for High School — Organic Compounds / Biochemistry, CK-12 Foundation (free, CC BY-NC 3.0)

Related Notes

• Enzymes — Enzymes are proteins; their specific 3D shape determines which reaction they catalyse
• Human Nutrition — The digestive system breaks down carbohydrates, lipids, and proteins; food tests are applied to nutrition
• Inheritance — DNA structure and how genes code for proteins
• Movement Into and Out of Cells — How water (osmosis) and solutes (diffusion) move across membranes
• Cell Structure and Organisation — Where these biological molecules are found in cells (e.g. DNA in nucleus, ribosomes make proteins)
• IGCSE-Bio-Index — Full IGCSE Biology index

Common Misconceptions