All living organisms are made of the same basic chemical elements — carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur. The molecules formed from these elements are called biomolecules. Studying them helps us understand how life works at the molecular level.
Types of Biomolecules
- Biomolecules are broadly classified as:
- Carbohydrates — energy sources and structural components
- Proteins — structural, enzymatic, transport, hormonal, and immunological functions
- Lipids — energy storage, membrane structure, signalling
- Nucleic acids — storage and transfer of genetic information
Carbohydrates
- Carbohydrates have the general formula (CH2O)n. They are classified as:
- Monosaccharides: simplest units, e.g., glucose (C6H12O6), fructose, ribose (C5H10O5)
- Disaccharides: two monosaccharides joined by a glycosidic bond, e.g., sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose)
- Polysaccharides: many monosaccharides, e.g., starch (storage in plants), glycogen (storage in animals), cellulose (structural in plants), chitin (in fungal cell walls and insect exoskeletons)
Starch has two components: amylose (linear, alpha-1,4 linkages) and amylopectin (branched, alpha-1,6 linkages at branch points).
Proteins
Proteins are polymers of amino acids joined by peptide bonds. There are 20 standard amino acids; each has an amino group (-NH2), a carboxyl group (-COOH), a hydrogen, and a variable R group. The sequence of amino acids is called the primary structure. Coiling into an alpha-helix or folding into beta-sheets gives secondary structure. The 3D shape of the polypeptide is tertiary structure, and association of multiple subunits gives quaternary structure (e.g., haemoglobin has 4 subunits).
Enzymes are biological catalysts — mostly proteins. They lower activation energy without being consumed. Key terms: substrate, active site, enzyme-substrate complex. Factors affecting enzyme activity: temperature, pH, substrate concentration, inhibitors.
Lipids
- Lipids are not polymers in the strict sense. They include:
- Fats and oils: triglycerides (glycerol + 3 fatty acids). Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more double bonds.
- Phospholipids: glycerol + 2 fatty acids + phosphate group; amphipathic — form cell membranes
- Steroids: e.g., cholesterol, bile acids, sex hormones; ring structures
- Waxes: protective coatings on leaves and insect cuticles
Nucleic Acids
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polynucleotides. Each nucleotide = nitrogenous base + pentose sugar + phosphate group.
DNA bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T). RNA replaces thymine with Uracil (U) and uses ribose instead of deoxyribose.
Chargaff's rule: In DNA, A = T and G = C. If A content is 30%, then T = 30%, and G + C = 40%, so G = C = 20%.
The Concept of Metabolic Pool and Macromolecules
The cell contains thousands of small molecules (metabolites) in a dynamic pool. Large molecules (macromolecules) like proteins, polysaccharides, and nucleic acids are made by polymerising these small molecules using energy. Acids soluble in trichloroacetic acid (TCA) are small biomolecules; the acid-insoluble fraction contains macromolecules.
Enzymes — Detailed
Enzyme activity is measured as the rate of conversion of substrate to product. The Michaelis-Menten model describes how reaction rate (V) relates to substrate concentration [S]: V = Vmax x [S] / (Km + [S]). Km (Michaelis constant) is the substrate concentration at half-maximum velocity. A low Km means high affinity.
Inhibitors: Competitive inhibitors resemble the substrate and block the active site reversibly. Non-competitive inhibitors bind elsewhere (allosteric site) and change enzyme shape.
Cofactors: Non-protein components needed for enzyme activity. Metal ions (Mg2+, Zn2+) are cofactors; small organic molecules are coenzymes (e.g., NAD+, FAD, Coenzyme A).
Common mistakes
- Confusing glycosidic bond (joins sugars) with peptide bond (joins amino acids) — they are entirely different types of covalent bonds.
- Assuming all lipids are fats — phospholipids, steroids, and waxes are also lipids.
- Mixing up Vmax and Km — Vmax is the maximum rate; Km is the substrate concentration for half-Vmax. Lower Km = greater affinity for substrate.
- Forgetting that enzymes are not consumed or permanently changed; they emerge from the reaction unchanged and can act again.
Summary
Biomolecules are the chemical foundation of life. Carbohydrates provide energy and structure; proteins drive almost every cellular process; lipids form membranes and store energy; nucleic acids carry genetic information. Enzymes, as biological catalysts, control the rate of metabolic reactions. Understanding their structure-function relationships is central to biology.