List the chemical elements that make up: carbohydrates, fats and proteins.

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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

  1. Carbohydrates – elements C, H, O (≈1 : 2 : 1).
    Monomer = monosaccharide; polymers = starch, glycogen, cellulose.
  2. Fats (Lipids) – elements C, H, O.
    Monomers = glycerol + fatty acids; polymer = triglyceride (plus related lipids).
  3. 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.

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