4.1 Biological Molecules
Objective
By the end of this section students will be able to:
- List the chemical elements that constitute the three major classes of biological macromolecules – carbohydrates, fats (lipids) and proteins.
- State that large biomolecules are assembled from simpler building‑blocks (e.g., starch, glycogen and cellulose are polymers of the monosaccharide glucose).
- Identify the monomer(s) for each class and give at least one common polymer.
- Recall the five qualitative tests used to recognise each macromolecule, including the principle behind each test and a practical tip.
- Describe the main biological role(s) of each class.
Elements Present in Each Class
| Macromolecule |
Elements present |
Typical empirical formula (illustrative) |
| Carbohydrates |
C, H, O |
CnH2nOn (e.g. glucose C6H12O6) |
| Fats (Lipids) |
C, H, O |
C55H98O6 (typical triglyceride) |
| Proteins |
C, H, O, N, S (S in some proteins) |
CxHyNzOwSs (variable) |
Monomers and Polymers
- Carbohydrates
- Monomer: simple sugars (monosaccharides) – e.g. glucose, fructose.
- Common polymers: starch, glycogen, cellulose (long chains of glucose units).
- Fats (Lipids)
- Monomers: glycerol (a three‑carbon alcohol) and fatty acids (long‑chain carboxylic acids).
- Common polymer: triglyceride (one glycerol + three fatty‑acid chains). Related lipids such as phospholipids and sterols are built from the same basic units.
- Proteins
- Monomer: amino acid (contains –NH2, –COOH, a side‑chain R, plus H). Twenty standard amino acids are used in proteins.
- Common polymer: peptide chain (protein) formed by peptide bonds between amino acids. Examples: albumin, enzymes, keratin, antibodies.
Qualitative Tests (Core Requirement)
| Test |
Macromolecule detected |
Principle (why it works) |
Result (colour/precipitate) |
Practical tip |
| Iodine test |
Starch (a carbohydrate) |
Iodine molecules insert into the helical amylose chains, forming a charge‑transfer complex. |
Blue‑black colour |
Add a few drops of iodine solution to a small amount of the solid or aqueous sample in a clean test tube. |
| Benedict’s test |
Reducing sugars (monosaccharides and some disaccharides) |
Cu2+ ions are reduced to Cu+ by the aldehyde/ketone group, producing a coloured precipitate. |
Brick‑red precipitate (colour change: blue → green → yellow → orange → red) |
Heat the mixture gently in a boiling water bath; observe the colour change after cooling. |
| Biuret test |
Proteins (peptide bonds) |
Copper(II) ions form a violet‑coloured complex with the –NH‑ groups of peptide bonds. |
Violet / purple colour |
Mix equal volumes of sample and Biuret reagent, then gently swirl; colour appears within a minute. |
| Ethanol‑emulsion test |
Fats / lipids (hydrophobic molecules) |
Lipids are insoluble in ethanol; when water is added they form a milky emulsion. |
Milky white emulsion |
Dissolve a tiny amount of the sample in ethanol, then add a few drops of water and shake vigorously. |
| DCPIP test |
Vitamin C (often linked with carbohydrate‑rich foods) |
Blue DCPIP (oxidised) is reduced to colourless DCPIP by the antioxidant vitamin C. |
Solution becomes colourless (blue disappears) |
Use a fresh DCPIP solution; add the sample dropwise until the blue colour vanishes. |
Biological Functions (Key Points)
- Carbohydrates
- Quick source of energy – glucose.
- Energy storage – starch in plants, glycogen in animals.
- Structural role – cellulose in plant cell walls.
- Fats (Lipids)
- Long‑term energy storage (high caloric value).
- Insulation and protection of vital organs.
- Major component of cell membranes (phospholipids) and precursors of hormones.
- Proteins
- Catalysis – enzymes.
- Transport – haemoglobin, membrane transport proteins.
- Structural support – collagen, keratin.
- Defence – antibodies.
- Regulation – many hormones are proteins.
Summary
- Carbohydrates – elements C, H, O (≈1 : 2 : 1).
Monomer = monosaccharide; polymers = starch, glycogen, cellulose.
- Fats (Lipids) – elements C, H, O.
Monomers = glycerol + fatty acids; polymer = triglyceride (plus related lipids).
- Proteins – elements C, H, O, N, (sometimes S).
Monomer = amino acid; polymer = peptide chain (protein).
Optional Supplementary Note
Although not part of the core IGCSE syllabus, the structure of DNA illustrates how biological macromolecules are inter‑related: DNA is a polymer of nucleotides, each containing a carbohydrate (deoxyribose), a phosphate group, and a nitrogenous base.