Module 98. Biomolecules and Biochemistry

Biomolecules are organic compounds that form the structural and functional foundation of all living organisms. They include a diverse range of molecules such as carbohydrates, proteins, lipids, and nucleic acids, which are essential for life processes including growth, metabolism, reproduction, and cellular communication. Biochemistry, the scientific study of these molecules and their chemical interactions, bridges biology and chemistry by explaining the molecular mechanisms that sustain life. Together, biomolecules and biochemistry form the basis for understanding physiology, genetics, and molecular biology.

Nature and Classification of Biomolecules

Biomolecules are composed mainly of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur. The presence of carbon enables the formation of complex and stable covalent structures, giving rise to the immense diversity of organic molecules found in living systems. These molecules can be broadly classified into micromolecules and macromolecules based on their size and molecular weight.

1. Micromolecules: Small, simple molecules such as water, amino acids, sugars, and ions that play key roles in metabolism and cellular reactions.
2. Macromolecules: Large, complex molecules like proteins, polysaccharides, and nucleic acids, formed by polymerisation of smaller subunits (monomers).

Based on chemical composition and function, biomolecules are grouped into four main categories: carbohydrates, lipids, proteins, and nucleic acids. Each class performs specific biological functions vital to maintaining life.

Carbohydrates: Energy Providers and Structural Components

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically in the ratio Cₙ(H₂O)ₙ. They serve as the primary source of energy for cellular activities and as structural materials in plants and animals.
Carbohydrates are classified into three types:

• Monosaccharides: The simplest sugars, such as glucose, fructose, and galactose. Glucose is the chief energy source in cellular respiration.
• Disaccharides: Formed by linking two monosaccharide units, e.g. sucrose (glucose + fructose) and lactose (glucose + galactose).
• Polysaccharides: Complex carbohydrates composed of many monosaccharides, such as starch (energy storage in plants), glycogen (energy storage in animals), and cellulose (structural component of plant cell walls).

Carbohydrates also participate in cell recognition and signalling, as seen in glycoproteins and glycolipids present on cell membranes.

Proteins: Structural Framework and Functional Machinery

Proteins are complex polymers of amino acids linked by peptide bonds. They form the structural framework of cells and perform diverse functional roles, including catalysis, transport, and defence. The chemical composition of proteins includes carbon, hydrogen, oxygen, nitrogen, and sometimes sulphur.
Each protein’s function depends on its unique three-dimensional structure, determined by the sequence of amino acids. Proteins are classified into:

• Fibrous Proteins: Structural and insoluble, e.g. keratin in hair, collagen in connective tissue.
• Globular Proteins: Functional and soluble, e.g. enzymes, hormones, and antibodies.

Key functions of proteins include:

• Enzymatic Activity: Enzymes such as amylase and DNA polymerase accelerate biochemical reactions by lowering activation energy.
• Transport: Haemoglobin carries oxygen in the blood; membrane proteins regulate molecule movement.
• Regulation: Hormones such as insulin control metabolic processes.
• Defence: Antibodies identify and neutralise pathogens.

Proteins are thus indispensable for growth, repair, and regulation within living systems.

Lipids: Energy Storage and Cellular Structure

Lipids are hydrophobic organic compounds composed primarily of carbon, hydrogen, and oxygen. Unlike carbohydrates and proteins, they are insoluble in water but soluble in organic solvents. Lipids include fats, oils, waxes, phospholipids, and steroids.
Their main biological roles are:

• Energy Storage: Fats provide more than twice the energy per gram compared to carbohydrates.
• Structural Function: Phospholipids form the bilayer of cell membranes, maintaining integrity and regulating permeability.
• Insulation and Protection: Adipose tissue stores fat and insulates the body.
• Hormonal Role: Steroid hormones such as testosterone and oestrogen regulate physiological functions.

Lipids also act as signalling molecules, influencing metabolism and inflammation. Excessive lipid accumulation, however, may lead to health disorders such as atherosclerosis and obesity.

Nucleic Acids: Carriers of Genetic Information

Nucleic acids are macromolecules that store and transmit genetic information. They are composed of nucleotides, each consisting of a sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base. There are two main types:

1. Deoxyribonucleic Acid (DNA): The genetic blueprint of life, containing instructions for protein synthesis. DNA has a double-helical structure formed by complementary base pairing (A–T, G–C).
2. Ribonucleic Acid (RNA): Plays a crucial role in decoding and expressing genetic information. Types include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Through transcription and translation, DNA and RNA coordinate the synthesis of proteins, thereby directing cellular structure and function. The discovery of DNA’s structure by Watson and Crick in 1953 marked a milestone in molecular biology and biochemistry.

Enzymes and Metabolic Reactions

Enzymes are biological catalysts that accelerate chemical reactions in cells without being consumed. They exhibit specificity for substrates and operate under mild conditions of temperature and pH. Enzymes function by binding to substrates at their active site, forming an enzyme-substrate complex that converts substrates into products.
Metabolism, the sum of all biochemical reactions in an organism, consists of two processes:

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy (e.g. glycolysis).
• Anabolism: Synthesis of complex molecules from simpler precursors, consuming energy (e.g. protein synthesis).

Energy released during catabolism is stored as adenosine triphosphate (ATP), the universal energy currency of cells. ATP powers various physiological activities such as muscle contraction, nerve impulse transmission, and biosynthesis.

Vitamins, Hormones, and Other Biochemical Molecules

In addition to the major biomolecules, smaller organic compounds such as vitamins, hormones, and secondary metabolites play vital roles in biochemistry.

• Vitamins: Organic micronutrients required in small quantities for enzyme activity and metabolic regulation. Examples include vitamin C (ascorbic acid), vitamin D, and B-complex vitamins.
• Hormones: Chemical messengers that regulate body functions, including metabolism, growth, and reproduction. Examples include insulin, adrenaline, and thyroxine.
• Secondary Metabolites: Compounds such as alkaloids, flavonoids, and terpenoids, which have ecological and pharmacological importance.

Deficiencies or imbalances of these substances can lead to various metabolic disorders and diseases.

Significance of Biochemistry in Modern Science

Biochemistry plays a crucial role in medicine, agriculture, and biotechnology. It helps in diagnosing diseases, developing pharmaceuticals, and understanding genetic disorders. Techniques such as chromatography, electrophoresis, and spectroscopy allow scientists to study biomolecular structures and reactions at the molecular level.
Applications include:

• Medical Diagnostics: Enzyme assays and biomarker detection for diseases like diabetes or cancer.
• Genetic Engineering: Recombinant DNA technology for producing insulin or vaccines.
• Agricultural Improvement: Enhancing crop yield through genetic modification.
• Forensic Science: DNA fingerprinting for identification and crime investigation.

Biochemistry thus provides the molecular understanding required to advance healthcare, environmental conservation, and biotechnology.