Chapter 9 — Biomolecules

What are biomolecules and their chemical basis?
Biomolecules are organic compounds synthesised and used by living organisms to build structure, store energy, transmit information and catalyse reactions. Chemically, they are dominated by carbon chemistry and include functional groups such as hydroxyl, carbonyl, carboxyl, amino, phosphate and sulphhydryl. These functional groups determine reactivity, intermolecular interactions and three-dimensional shape. Studying biomolecules from a chemical perspective means identifying these groups, understanding how they bond and react, and linking molecular structure to biological function.

Levels of organisation and building blocks
Life builds complexity by assembling small monomeric units into macromolecules: monosaccharides into polysaccharides, amino acids into proteins, fatty acids into complex lipids and nucleotides into nucleic acids. Polymerisation reactions are typically condensations that form specific linkages (glycosidic, peptide, ester, phosphodiester), while hydrolysis reverses polymerisation. The chemical properties of monomers — size, polarity, charge and stereochemistry — determine polymer properties like solubility, flexibility and reactivity.

Importance of non-covalent forces
While covalent bonds form the backbone of biomolecules, non-covalent interactions — hydrogen bonds, ionic interactions, hydrophobic effect and van der Waals forces — govern folding, assembly and recognition. For instance, hydrogen bonds stabilise secondary structures of proteins and base pairing in nucleic acids; hydrophobic interactions drive protein folding and membrane formation. These weak forces are directional and reversible, allowing dynamic behaviour required for biological function.

Chemical diversity and biological specificity
Small changes in chemical structure can have large biological consequences. A single stereochemical inversion or a small modification (methylation, phosphorylation, glycosylation) can change how an enzyme recognises a substrate or how a receptor responds. This chemical specificity underlies metabolism, signalling, immunity and heredity. Understanding these principles equips students to predict reactivity, interpret experiments and connect molecular chemistry to physiology and disease.

Applications and methods
Chemical analysis techniques such as chromatography, electrophoresis and spectroscopy allow characterisation of biomolecules. In applied contexts, knowledge of biomolecular chemistry is used in drug design, diagnostics, food science and biotechnology. Thus, learning biomolecules in Class 12 prepares students for higher studies and practical contexts where molecular chemistry explains life processes and technological solutions.

Water as the medium of life
Water is the universal solvent in biological systems and its chemical properties shape biomolecular behaviour. The polar O–H bonds create a dipole moment; oxygen carries partial negative charge and hydrogens carry partial positive charges. This polar character allows water to form extensive hydrogen-bond networks with itself and to solvate polar and charged groups on biomolecules. Solvation by water stabilises ions and polar groups, defines hydration shells around proteins and nucleic acids and affects reaction kinetics by modulating the free energy landscape of processes.

Hydrogen bonding and structure
Hydrogen bonds occur when an H atom covalently bound to an electronegative atom (O or N) interacts with a lone pair on another electronegative atom. In biomolecules, hydrogen bonding is directional and specific, providing structural order: it stabilises α-helices and β-sheets in proteins, contributes to secondary structures in RNA, and underpins Watson–Crick base pairing in DNA. Multiple hydrogen bonds collectively provide significant stabilising energy while remaining reversible, enabling dynamic biological processes like folding and binding.

Hydrophobic effect and self-assembly
Nonpolar groups disrupt water’s hydrogen-bond network; to minimise this disruption, water excludes hydrophobic surfaces, driving nonpolar groups together — this is the hydrophobic effect. It is a major driving force in protein folding (formation of a hydrophobic core), membrane assembly (formation of lipid bilayers with tails inward), and in formation of micelles. The hydrophobic effect is entropically driven: aggregation reduces ordered water around nonpolar surfaces, increasing global entropy.

Electrostatics, ionic strength and pH
Electrostatic interactions occur between charged groups and are screened by dissolved ions. Ionic strength of the medium affects the range and strength of electrostatic attractions and repulsions; high ionic strength can shield charges and reduce long-range interactions, affecting protein–protein and protein–nucleic acid associations. pH controls protonation states of ionisable groups (carboxyl, amino, imidazole), thus altering net charge, solubility and conformation. For example, changes in pH can disrupt salt bridges and hydrogen bonds leading to denaturation.

Van der Waals and stacking interactions
Van der Waals forces, though weak individually, are significant when many atoms are in close proximity; they contribute to tight packing in protein cores. Aromatic π–π stacking interactions stabilise nucleic acid structures via base stacking and are important in ligand binding. Understanding the balance of these forces—hydrogen bonds, hydrophobic effect, electrostatics and van der Waals—allows prediction of how biomolecules fold, bind and function in aqueous environments.

Definition and broad categories
Carbohydrates are polyhydroxy aldehydes or ketones and their derivatives, commonly represented by the empirical formula (CH2O)n. They are classified by complexity: monosaccharides (single sugar units), disaccharides (two linked sugars), oligosaccharides (3–10 units) and polysaccharides (long polymers). Functionally, they are also described as reducing or non-reducing depending on the availability of a free anomeric carbon capable of open-chain aldehyde formation and oxidation.

Monosaccharides
Monosaccharides range from trioses to hexoses and beyond. Important biological monosaccharides include glucose (a hexose aldose), fructose (a hexose ketose) and ribose (a pentose in nucleic acids). Monosaccharides show stereoisomerism due to multiple chiral centres, and the D/L nomenclature is assigned relative to glyceraldehyde. Their ring forms (pyranose and furanose) arise by intramolecular hemiacetal/hemi-ketal formation, creating anomeric centres with α and β configurations. Monosaccharide chemistry includes oxidation to aldonic acids, reduction to alditols, formation of glycosides and derivatisation at hydroxyl groups.

Disaccharides and oligosaccharides
Disaccharides form via condensation between the anomeric hydroxyl of one monosaccharide and a hydroxyl of another, producing a glycosidic bond. Examples: maltose (glucose–glucose α-1,4), lactose (galactose–glucose β-1,4), sucrose (glucose–fructose α-1,2). The nature of the glycosidic linkage determines reducing ability, digestibility and enzyme specificity. Oligosaccharides serve as recognition signals when attached to proteins and lipids on cell surfaces, forming glycoproteins and glycolipids that mediate cell–cell and cell–pathogen interactions.

Polysaccharides
Polysaccharides are diverse: homopolysaccharides contain one monosaccharide type (starch, cellulose, glycogen) while heteropolysaccharides contain different monosaccharides (peptidoglycan, glycosaminoglycans). Structural differences hinge on linkage type (α or β), positions involved (1→4, 1→6), and degree of branching. Amylose (starch) is largely linear with α-1,4 linkages, while amylopectin and glycogen are branched with α-1,6 linkages at branch points. Cellulose is a linear β-1,4 polymer forming rigid microfibrils via interchain hydrogen bonding, conferring mechanical strength to plant cell walls.

Biological roles and digestion
Carbohydrates provide immediate energy (monosaccharides), storage (starch, glycogen) and structure (cellulose, chitin). Digestive enzymes are specific to linkage types: amylase hydrolyses α-1,4 linkages in starch, whereas humans lack cellulase to hydrolyse β-1,4 linkages in cellulose. The chemistry of linkages and branching explains solubility, physical properties and biological accessibility of carbohydrates.

Basic structure and ring formation
Monosaccharides possess multiple stereocentres and a carbonyl functional group (aldehyde in aldoses, ketone in ketoses). In solution many monosaccharides exist predominantly in cyclic forms created by intramolecular hemiacetal (aldoses) or hemiketal (ketoses) formation: for hexoses this yields pyranose (six-membered) or furanose (five-membered) rings. The carbon that was the carbonyl becomes the anomeric carbon and serves as a key reactive centre for glycosidic bond formation and for distinguishing α and β anomers by orientation of the substituent at that carbon.

Stereochemical notation
D/L configuration is assigned by relation to the chiral centre furthest from the carbonyl and is historically based on glyceraldehyde. Most biological monosaccharides are D-isomers. Epimers are monosaccharides differing in configuration at a single carbon (e.g., glucose and galactose differ at C-4). Conformations of cyclic sugars are represented by chair and boat forms that show axial and equatorial positions; substituent orientation affects steric hindrance and stability, with equatorial positions generally more favourable for bulky substituents.

Anomerism and mutarotation
Anomers interconvert in aqueous solution through the open-chain form, a process called mutarotation that leads to a change in optical rotation until an equilibrium mixture of α and β anomers is reached. The equilibrium composition depends on ring stability; for D-glucose the β-pyranose form predominates due to equatorial placement of most hydroxyl groups. Anomeric reactivity is central to formation of glycosides and polysaccharide linkages because once the anomeric hydroxyl forms a glycosidic bond its ability to undergo mutarotation is lost.

Chemical reactions and derivatisation
Monosaccharides can be oxidised at the aldehyde to give aldonic acids, reduced to give alditols (e.g., sorbitol), converted to deoxy sugars by replacement of hydroxyl groups, and form sugar esters and ethers at hydroxyls. Derivatisation is useful for analytical procedures that separate isomers and determine configuration. Reactivity at specific hydroxyls under controlled conditions allows formation of protective groups and selective linkage in synthetic carbohydrate chemistry.

Biological implications
Stereochemistry is crucial for enzyme recognition: enzymes and transporters often distinguish between epimers and anomers. Small stereochemical differences translate into specificity in metabolism and signalling. Understanding stereochemical principles helps explain why some sugars are substrates for enzymes while closely related isomers are not.

Formation and notation of glycosidic linkages
Glycosidic bonds form when the anomeric hydroxyl group of one monosaccharide condenses with a hydroxyl group of another, producing an O-glycosidic linkage and releasing water. Such bonds are described by the configuration (α or β) at the anomeric carbon and the carbon numbers involved, for example α(1→4), β(1→4) or α(1→6). This notation precisely captures the stereochemistry and regiochemistry of the linkage, which are crucial determinants of the three-dimensional shape, enzymatic recognisability and physical properties of the resulting oligo- or polysaccharide.

Disaccharide properties and digestibility
Common disaccharides illustrate how linkage affects chemistry: sucrose (glucose α-1,2 fructose) is non-reducing because both anomeric carbons are engaged in the glycosidic bond so no free aldehyde can form; maltose (glucose α-1,4 glucose) is reducing because one anomeric carbon remains free and can open to an aldehyde; lactose (galactose β-1,4 glucose) is reducing and requires β-galactosidase (lactase) for hydrolysis. Enzymes that hydrolyse disaccharides are highly specific for the configuration and position of the glycosidic link.

Structural and storage polysaccharides
Polysaccharides vary widely: starch (plant energy reserve) consists of amylose (mostly linear α-1,4 linked glucose) and amylopectin (branched α-1,6 linkages). Glycogen (animal energy reserve) is similar to amylopectin but more highly branched, facilitating rapid enzymatic degradation from multiple chain ends. Cellulose is a linear β-1,4 polymer whose chains align and form hydrogen-bonded fibrils that provide tensile strength to plant cell walls. Chitin, a β-linked N-acetylglucosamine polymer, forms exoskeletons in arthropods and structural elements in fungi.

Functional consequences of architecture
Branching increases solubility and accessibility to enzymes; highly branched glycogen allows rapid glucose release. Linear polymers with rigid hydrogen-bond networks yield insoluble, strong materials such as cellulose and chitin. Chemical modifications—acetylation, sulfation, phosphorylation—modify physical properties and interactions; for instance, sulfated glycosaminoglycans in extracellular matrix bind water and resist compression in cartilage.

Biotechnological and industrial roles
Polysaccharides serve as thickeners, stabilisers, dietary fibre and biodegradable materials. Their chemical tunability allows derivatisation to produce films, hydrogels and drug-delivery systems. Understanding how bonding and branching affect properties is key to both biological function and industrial application.

Core structure and classification
An amino acid has a central (α) carbon bonded to an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom and a distinctive side chain (R). This simple scaffold yields great chemical diversity through R groups that can be nonpolar (alkyl or aromatic), polar uncharged (hydroxyl, amide), acidic (carboxyl) or basic (amine). Classification by side-chain properties helps predict solubility, reactivity and location in proteins (surface vs core).

Zwitterionic form and pH behaviour
At physiological pH many amino acids exist as zwitterions where the amino group is protonated (–NH3+) and the carboxyl group is deprotonated (–COO–). The net charge varies with pH: at low pH both groups are protonated (net positive), at high pH both are deprotonated (net negative). The isoelectric point (pI) is the pH where net charge is zero; for neutral amino acids pI is the average of pKa values of the amino and carboxyl groups. Ionisable side chains introduce additional pKa values and influence pI and buffer behaviour.

Stereochemistry and chirality
Except for glycine, amino acids are chiral because the α-carbon bears four different substituents. Biological proteins use L-amino acids almost exclusively. Chirality affects folding, enzyme recognition and the assembly of higher-order structures. Epimeric and stereochemical considerations are crucial in peptide synthesis and metabolic pathways.

Chemical reactivity of side chains
Side chains confer chemical functionality: cysteine thiol groups form disulfide bonds stabilising extracellular protein structure; serine, threonine and tyrosine hydroxyls are targets for phosphorylation regulating activity; histidine’s imidazole ring can act as both proton donor and acceptor near physiological pH facilitating acid–base catalysis. Carboxylate and amino side chains participate in salt bridge formation affecting tertiary structure and substrate binding.

Titration and buffering
Titration curves of amino acids display plateaus near pKa values and buffering capacity where pH changes are resisted. Titration data allow determination of pKa values and pI, useful for protein purification and separation techniques like isoelectric focusing. Chemical modifications including amidation, acetylation and glycosylation alter properties and biological roles of amino acids and peptides.

Formation and nature of the peptide bond
Peptide bonds form by condensation between the carboxyl group of one amino acid and the amino group of another with elimination of water. The peptide linkage (–CO–NH–) is an amide bond with resonance delocalisation between the carbonyl oxygen and the amide nitrogen, giving it partial double-bond character. This restricts rotation around the C–N bond, imposes planarity on the peptide unit and constrains backbone conformations; rotation is therefore largely limited to the bonds adjacent to the α-carbon (φ and ψ angles), which determine secondary structure.

Polypeptide directionality and sequence
Polypeptides have directionality with a free amino (N-) terminus and a free carboxyl (C-) terminus. Sequences are conventionally written from N- to C-terminus. The primary structure — the exact sequence of amino acids — determines how the chain will fold and what functional sites will be formed. Changes at single residues can alter stability, activity or interactions, as seen in many genetic diseases and enzyme variants.

Sequence information and post-translational modification
The primary sequence arises from genetic coding and is subject to post-translational modifications that expand chemical diversity and functionality: signal peptide cleavage, glycosylation, phosphorylation, methylation, acetylation and formation of disulfide bonds. These modifications influence localisation, activity and lifespan of proteins. For example, glycosylation can affect folding and stability of secreted proteins, while phosphorylation commonly modulates enzyme activity.

Analytical determination of sequence
Historically protein sequencing used Edman degradation that sequentially removed N-terminal residues for identification; modern methods include mass spectrometry and DNA sequencing to infer protein sequence. Chemical cleavage at specific residues (e.g., cyanogen bromide at methionine) aids fragmentation for analysis. Knowledge of primary structure enables prediction of domains, motifs and potential functional sites through bioinformatics tools.

Chemical consequences of sequence
The sequence dictates local patterns such as helix-forming propensities, β-strand preferences and locations of turn-inducing residues like proline and glycine. It also positions catalytic residues within active sites and determines oligomerisation interfaces in multimeric proteins. Thus the peptide bond’s chemistry and the primary sequence together underpin all higher structural levels and biological functions.

Secondary structure: local order from backbone interactions
Secondary structures arise from repetitive backbone hydrogen bonding patterns. The α-helix is stabilised by hydrogen bonds between the backbone carbonyl oxygen of residue i and the amide hydrogen of residue i+4, producing a right-handed helical structure with characteristic pitch and side-chain projection. β-sheets consist of extended strands aligned side-by-side with hydrogen bonds between carbonyl and amide groups of neighbouring strands; sheets can be parallel or antiparallel, affecting hydrogen-bond geometry. Turns and loops connect secondary elements and often contain residues like proline and glycine that impose conformational constraints.

Tertiary structure: overall fold from side-chain interactions
Tertiary structure is the three-dimensional arrangement of the entire polypeptide chain stabilised by multiple interactions among side chains: hydrophobic interactions driving burial of nonpolar residues, hydrogen bonds between side chains, ionic interactions or salt bridges between oppositely charged residues, disulfide bonds covalently linking distant cysteines, and van der Waals contacts in tightly packed cores. The fold creates active sites, binding pockets and structural motifs with specific chemical microenvironments required for catalysis, ligand recognition and regulation.

Quaternary structure: assembly of subunits
Many proteins function as multimers; quaternary structure describes spatial arrangement of subunits and their interfaces. Interactions can be noncovalent (hydrophobic patches, hydrogen bonds, salt bridges) or involve covalent disulfide bonds. Quaternary organisation allows cooperative phenomena (e.g., oxygen binding in haemoglobin), regulation through allosteric transitions, and structural stability. Subunit composition and stoichiometry are critical for function.

Folding pathways and chaperones
Folding is a guided process reducing free energy to reach native conformation. Folding intermediates can lead to misfolding and aggregation; molecular chaperones assist correct folding by preventing inappropriate interactions and providing isolated environments. In vivo folding is co-translational for many proteins, with the nascent chain sampling conformations as it emerges from ribosomes. Kinetic and thermodynamic factors determine folding rates and stability.

Structure determination and functional implications
X-ray crystallography, NMR spectroscopy and cryo-electron microscopy reveal structures at atomic resolution. Structural knowledge links sequence to mechanism, informs drug design and helps interpret mutations that perturb stability or function. Understanding all structural levels and the chemical interactions that stabilise them is essential for grasping how proteins perform their diverse biological roles.

Definition and molecular basis
Denaturation is the process by which a protein loses its native three-dimensional structure, and often its biological function, while the primary sequence remains intact. Molecularly, denaturation disrupts the non-covalent interactions—hydrogen bonds, hydrophobic packing, ionic interactions and van der Waals contacts—that stabilise secondary, tertiary and quaternary structures. Denaturants, heat, extremes of pH, detergents, organic solvents and heavy metals perturb these stabilising forces, leading to unfolding and, in many cases, aggregation.

Mechanisms of common denaturing agents
Heat increases molecular motion and disrupts weak bonds; pH shifts alter protonation states critical for salt bridges and hydrogen bonding; detergents and organic solvents insert into hydrophobic cores or the membrane, disrupting hydrophobic interactions; chaotropic agents like urea and guanidinium chloride interfere with hydrogen bonding networks and protein solvation; heavy metal ions bind to thiol or carboxylate groups causing conformational disruption or cross-linking. Each agent targets stability differently, but the result is loss of the native fold.

Reversible vs irreversible denaturation
Some proteins can refold to their native structure once the denaturing condition is removed, demonstrating that the primary sequence encodes folding information (Anfinsen’s dogma). However, irreversible denaturation occurs when aggregates form, disulfide bonds are scrambled, or chemical modifications (deamidation, oxidation) happen during denaturation. Aggregation can bury hydrophobic patches and form insoluble precipitates, preventing refolding.

Biological protection and chaperones
Cells maintain homeostasis to protect proteins: molecular chaperones and heat-shock proteins assist folding, refolding and prevention of aggregation; the ubiquitin–proteasome system targets irreversibly damaged proteins for degradation. Organisms adjust membrane composition and express stress-response proteins to tolerate temperature and chemical stresses.

Measurement and implications
Denaturation can be monitored by spectroscopic changes (loss of secondary structure by circular dichroism, changes in fluorescence of aromatic residues), by activity assays and by calorimetry that detects heat capacity changes. Denaturation underlies everyday phenomena (cooking denatures egg proteins), industrial processes, and diseases (protein misfolding disorders like Alzheimer’s involve toxic aggregates). Understanding chemical causes and prevention strategies is vital in biotechnology and medicine.

Enzymes as specialised catalysts
Enzymes are biological catalysts—mainly proteins, though some RNAs (ribozymes) catalyse reactions—that accelerate chemical reactions by lowering activation energy and stabilising transition states. Their catalytic power derives from precisely arranged active site residues that provide complementary interactions to substrates, orient reactants correctly and create microenvironments with specific polarity, protonation states and electronic properties. Enzymes can increase reaction rates by many orders of magnitude while remaining highly specific and operating under mild cellular conditions.

Active site architecture and specificity
The active site is a unique pocket formed by residues that may be distant in sequence but adjacent in the folded structure. Substrate binding results from multiple weak interactions—hydrogen bonds, ionic interactions, hydrophobic contacts and van der Waals forces—providing specificity. Models such as lock-and-key and induced fit explain binding: induced fit emphasises conformational changes upon substrate binding that optimise interactions and lower activation barriers. Many enzymes require cofactors—metal ions or organic coenzymes—to carry out chemistry beyond the capabilities of amino acid side chains.

Catalytic strategies
Enzymes use several chemical strategies: proximity and orientation to bring substrates into favourable geometry; acid–base catalysis where side chains donate or accept protons to stabilise intermediates; covalent catalysis where a transient covalent bond forms between enzyme and substrate; transition-state stabilisation where binding energy is used to preferentially stabilise high-energy states; and metal ion catalysis where metals stabilise charges or participate in redox chemistry. These strategies often act in concert in enzyme mechanisms.

Kinetics and the Michaelis–Menten model
Enzyme kinetics describe how reaction rate depends on substrate concentration. The Michaelis–Menten model treats formation of an enzyme–substrate complex followed by product formation, yielding parameters Vmax (maximum rate at saturation) and Km (substrate concentration at half Vmax, reflecting affinity under steady-state conditions). Kinetic experiments provide insight into catalytic efficiency (kcat/Km) and mechanisms, and are fundamental for comparing enzymes and studying inhibition.

Biological regulation and applications
Enzyme activity is tightly regulated by allosteric effectors, covalent modification (phosphorylation), compartmentalisation and controlled expression. In industry and medicine enzymes are exploited for catalysis, biotransformations and as drug targets. Understanding the chemical basis of catalysis enables rational design of inhibitors, engineered enzymes and artificial catalysts for biological and chemical applications.

Types of inhibition and chemical basis
Enzyme inhibitors reduce catalytic activity and can act reversibly or irreversibly. Competitive inhibitors bind the active site, competing with substrate; chemically they mimic substrate structure and engage similar interactions. Non-competitive inhibitors bind to an allosteric site and alter enzyme conformation, reducing catalytic turnover without preventing substrate binding in many cases. Uncompetitive inhibitors bind only to the enzyme–substrate complex, stabilising it and preventing product formation. Irreversible inhibitors form covalent bonds with active-site nucleophiles (e.g., serine, cysteine), permanently inactivating the enzyme.

Kinetic signatures and practical interpretation
Each inhibition type has characteristic kinetic effects: competitive inhibition increases apparent Km (lower apparent affinity) while Vmax remains unchanged because sufficiently high substrate can outcompete inhibitor; non-competitive inhibition reduces Vmax without changing Km; uncompetitive inhibition lowers both Km and Vmax. Lineweaver–Burk plots (double reciprocal) help distinguish types: competitive inhibition lines intersect at the y-axis, non-competitive lines intersect at the x-axis, and uncompetitive inhibition gives parallel lines. These kinetic patterns derive from how the inhibitor affects formation of product from enzyme–substrate complexes.

Allosteric regulation and cooperativity
Many enzymes are regulated allosterically: binding of an effector at a site distinct from the active site induces conformational changes that increase or decrease activity. Cooperativity, found in multimeric enzymes, produces sigmoidal kinetics and provides a switch-like response to changing substrate concentration, important in metabolic control. Chemical effectors include small metabolites, nucleotides and ions that modulate activity by stabilising different conformational states.

Physiological and pharmacological relevance
Feedback inhibition, where end-products inhibit rate-limiting enzymes, maintains metabolic balance. Drugs often function as enzyme inhibitors (antibiotics, antivirals, cancer therapies). Knowledge of chemical mechanisms allows design of selective inhibitors and strategies to overcome resistance. Understanding inhibition chemistry also guides toxicology: many poisons irreversibly inhibit essential enzymes.

Reversibility, specificity and therapeutic design
Reversible inhibitors are useful for temporary modulation of enzymes; irreversible inhibitors can be potent drugs when targeted to diseased cells but risk side effects. Rational drug design uses structural and kinetic information to optimise potency, selectivity and pharmacokinetic properties. Thus enzyme inhibition chemistry is central to biochemistry and medicinal chemistry.

Overview and chemical diversity
Lipids are a chemically diverse group of hydrophobic or amphipathic molecules including fatty acids, glycerides (mono-, di- and triacylglycerols), phospholipids, sphingolipids and sterols. Their unifying feature is poor solubility in water and solubility in organic solvents. Lipid diversity arises from chain length, degree and position of unsaturation, head-group composition and presence of polar substituents. This chemical diversity underlies their varied biological roles in energy storage, membrane structure, signalling and insulation.

Fatty acids: saturation and stereochemistry
Fatty acids are carboxylic acids with long hydrocarbon chains; saturation refers to presence or absence of carbon–carbon double bonds. Saturated fatty acids have straight chains that pack tightly and form solids at room temperature. Unsaturated fatty acids usually have cis-double bonds that introduce kinks, preventing tight packing, lowering melting points and making oils liquid. Trans fatty acids, uncommon naturally, have straighter geometry and can have adverse health effects. The position of double bonds (ω-3, ω-6) is biologically significant, affecting membrane properties and metabolic pathways.

Triacylglycerols and energy storage
Triacylglycerols (triglycerides) are esters of glycerol and three fatty acids and are the primary form of energy storage in animals. They are dense energy reserves, metabolised via lipolysis and β-oxidation to produce acetyl-CoA and reducing equivalents. The chemical inertness of ester bonds under physiological conditions makes triacylglycerols stable storage molecules until enzymatically mobilised.

Phospholipids, bilayers and membrane chemistry
Phospholipids contain two fatty acid chains and a phosphate-containing polar head group attached to glycerol. Their amphipathic nature leads to spontaneous formation of bilayers in aqueous environments: hydrophobic tails face inwards while hydrophilic heads interact with water. Bilayer properties such as fluidity, permeability and curvature depend on fatty acyl saturation, chain length and cholesterol content. Membrane proteins are embedded or associated with this dynamic matrix, and lipid composition influences protein function and cell signalling.

Sterols and signalling lipids
Sterols like cholesterol have rigid four-ring structures that modulate membrane order and serve as precursors for steroid hormones, bile acids and vitamin D. Other lipids function as local signalling molecules (eicosanoids, prostaglandins) derived from polyunsaturated fatty acids and act via receptor-mediated pathways. Dysregulated lipid metabolism is implicated in diseases like atherosclerosis, obesity and metabolic syndrome, linking chemical lipid properties to health outcomes.

Nucleotides as chemical units
Nucleic acids are polymers of nucleotides, each comprising three parts: a nitrogenous base (purines: adenine, guanine; pyrimidines: cytosine, thymine in DNA, uracil in RNA), a five-carbon sugar (deoxyribose in DNA, ribose in RNA) and one or more phosphate groups attached at the 5′ position. The base provides coding information, the sugar determines the polymer backbone chemistry, and the phosphate confers negative charge and establishes linkage chemistry via phosphodiester bonds.

Phosphodiester backbone and directionality
Nucleotides are joined by 3′→5′ phosphodiester bonds between the 3′-hydroxyl of one sugar and the 5′-phosphate of the next, producing a sugar–phosphate backbone with inherent directionality (5′ to 3′). The negative charge of the backbone contributes to solubility, allows interactions with cations and proteins, and defines migration behaviour in electrophoresis. Directionality is critical for enzymatic polymerisation by polymerases and for reading genetic information.

DNA double helix and stabilising interactions
DNA typically forms a right-handed double helix with two antiparallel strands held together by complementary base pairing: A–T pairs via two hydrogen bonds, G–C pairs via three hydrogen bonds. In addition to hydrogen bonds, base stacking interactions between adjacent aromatic bases stabilise the helix through van der Waals forces and hydrophobic effects. The helix exhibits major and minor grooves that provide access points for proteins such as transcription factors and polymerases to recognise specific sequences without unwinding the duplex.

RNA structure and functional versatility
RNA is usually single-stranded but forms complex secondary and tertiary structures through intramolecular base pairing: hairpins, internal loops and pseudoknots. These structures enable diverse functions beyond information transfer: tRNA provides adaptor function with folded cloverleaf structure; rRNA contributes catalytic peptidyl transferase activity in ribosomes; small RNAs regulate gene expression. The 2′-OH present on ribose grants RNA chemical reactivity useful in catalysis but also makes RNA more susceptible to hydrolytic cleavage compared to DNA.

Biological roles and chemical modifications
DNA stores genetic information and provides template for replication; RNA translates and regulates that information. Chemical modifications occur in both DNA (methylation) and RNA (methylation, pseudouridylation) and affect stability, structure and recognition. Mutations—chemical changes in bases—can alter genetic messages and protein sequences, underscoring the chemical basis of heredity and variation.

Chemistry of templated polymerisation
Replication and transcription share the chemical principle of templated polymerisation: incoming nucleotide triphosphates (dNTPs for DNA, NTPs for RNA) form phosphodiester bonds with the 3′-OH of the growing chain, releasing pyrophosphate (PPi). The polymerase active site orchestrates nucleophilic attack of the 3′-OH on the α-phosphate of the incoming nucleotide, and subsequent hydrolysis of PPi drives the reaction forward by removing product and making the step effectively irreversible. Base pairing enforces complementarity so that information is copied faithfully under normal conditions.

Replication fidelity and proofreading
Accuracy in replication relies on base-pairing chemistry and enzyme proofreading. DNA polymerases often possess 3′→5′ exonuclease activity that removes misincorporated nucleotides by cleaving phosphodiester bonds, allowing replacement with correct bases. Fidelity is critical to prevent mutations; chemical insults and errors can introduce mismatches that, if not repaired, may become permanent changes in sequence.

Transcription chemistry and differences from replication
RNA polymerases synthesise RNA complementary to a DNA template using NTP substrates. Transcription is selective and regulated; RNA strands are typically single-stranded and may fold into functional structures. Unlike DNA polymerases, most RNA polymerases do not have extensive proofreading, making transcription more error-prone but acceptable because transcripts are transient and not inherited as template DNA is.

Translation and peptide bond formation
Translation occurs at the ribosome where mRNA codons are decoded by tRNA adaptors carrying specific amino acids. Peptide bond formation is catalysed by the ribosomal peptidyl transferase centre (a ribozyme) and proceeds by nucleophilic attack of the α-amino group of the aminoacyl-tRNA at the A-site on the ester linkage of the peptidyl-tRNA at the P-site, producing a new peptide bond and peptidyl-tRNA at the A-site. Energy for polypeptide elongation comes indirectly from aminoacyl-tRNA synthetase activation (ATP hydrolysis) and GTP-driven translocation steps.

Chemical inhibition and applications
Many antibiotics target specific chemical steps of replication, transcription or translation by binding polymerases or ribosomal sites, inhibiting phosphodiester bond formation or translocation. Antiviral drugs often target viral polymerases or reverse transcriptase. Chemical understanding of these mechanisms informs therapeutic design and explains selectivity and resistance.

Definition and classification
Vitamins are organic molecules required in minute amounts for normal physiological and metabolic functions. They are classified as water-soluble (B-complex group and vitamin C) or fat-soluble (A, D, E, K) based on solubility and storage behaviour. Many vitamins function as precursors to coenzymes — organic cofactors that bind to enzymes and participate directly in catalytic transformations by carrying chemical groups, electrons or atoms between substrates and active sites.

Roles of water-soluble vitamins as coenzymes
B-complex vitamins are central to metabolism: niacin (vitamin B3) is the precursor of NAD+ and NADP+, which shuttle electrons in oxidative and reductive metabolism; riboflavin (B2) forms FMN and FAD used in redox reactions; pantothenic acid (B5) is a component of coenzyme A that carries acyl groups; pyridoxal phosphate (B6) assists in amino group transfer and decarboxylation reactions; folate (B9) as tetrahydrofolate transfers one-carbon units in nucleotide biosynthesis; biotin (B7) carries CO2 in carboxylation reactions. Vitamin C serves as a reducing agent and is required for hydroxylation reactions in collagen biosynthesis.

Fat-soluble vitamins and regulatory roles
Fat-soluble vitamins are stored in tissues and have diverse roles: vitamin A (retinol) is involved in vision and gene regulation; vitamin D regulates calcium and phosphate homeostasis by acting as a hormone; vitamin E acts as an antioxidant protecting membranes from lipid peroxidation; vitamin K is essential for γ-carboxylation of glutamate residues in blood-clotting factors. Their hydrophobic chemistry allows association with membranes and lipoproteins for transport and storage.

Chemical mechanisms of coenzyme action
Coenzymes facilitate reactions that amino acid side chains alone cannot perform. NAD+ accepts hydride ions (H–) and is reduced to NADH, enabling dehydrogenase-catalysed oxidations. Coenzyme A forms thioesters (R–C(=O)–S–CoA) that activate acyl groups for transfer and condensation reactions. Tetrahydrofolate stabilises and transfers single-carbon units in different oxidation states. These chemical transformations are central to metabolism and biosynthesis.

Clinical relevance and deficiency
Vitamin deficiencies disrupt coenzyme-dependent pathways leading to disease: B1 deficiency impairs pyruvate dehydrogenase activity causing beri-beri or Wernicke–Korsakoff symptoms; vitamin C deficiency causes scurvy due to defective collagen hydroxylation; vitamin D deficiency leads to rickets via impaired calcium metabolism. Understanding chemical roles of vitamins informs nutritional guidance, therapeutic supplementation and design of vitamin-derived drugs.

Composition and amphipathic chemistry
Biological membranes are composed principally of amphipathic lipids—phospholipids, glycolipids and cholesterol—with embedded and associated proteins and carbohydrates. Amphipathic molecules contain both hydrophilic (polar head) and hydrophobic (nonpolar tail) regions; in aqueous environments they self-assemble such that hydrophobic tails are shielded from water and hydrophilic heads face the aqueous phases. This self-assembly is thermodynamically driven by the hydrophobic effect and minimisation of free energy.

Bilayer structure and properties
The phospholipid bilayer forms the basic barrier separating cellular compartments. The bilayer is fluid: lipids and many proteins diffuse laterally within the plane, allowing dynamic reorganisation. Fluidity depends on fatty acyl saturation and length (unsaturated and shorter chains increase fluidity) and cholesterol, which has a condensing effect at high temperature and prevents tight packing at low temperature. Bilayer asymmetry is maintained by specific enzymes (flippases, floppases) and ensures different lipid compositions in inner and outer leaflets for functional specialisation.

Membrane proteins and functions
Membrane proteins are integral (spanning the bilayer) or peripheral (loosely associated). Integral proteins include channels, transporters, receptors and enzymes; their transmembrane segments are typically hydrophobic helices or beta-barrels. Peripheral proteins attach to membrane surfaces or to other proteins and mediate signalling, cytoskeletal attachment and enzymatic activities. Glycoproteins and glycolipids with carbohydrate chains on the extracellular face participate in cell recognition, adhesion and immune interactions.

Transport mechanisms
Molecules cross membranes via passive diffusion (small nonpolar molecules), facilitated diffusion through channels and carriers (down electrochemical gradients), and active transport (pumps using energy to move substrates against gradients). Chemical energy from ATP hydrolysis or coupled ion gradients (secondary active transport) drives uphill transport. Selectivity arises from size, charge and specific binding interactions within transport proteins.

Membrane dynamics and cellular processes
Membranes participate in signalling (receptor–ligand interactions triggering cascades), endocytosis and exocytosis (vesicle formation and fusion), and compartmentalisation of metabolic pathways. Lipid microdomains (rafts) enriched in cholesterol and sphingolipids concentrate certain proteins and affect signalling. Understanding membrane chemistry explains drug uptake, viral entry, and physiological regulation of transport and communication.

Principles of separation
Chromatography and electrophoresis are essential chemical techniques to separate, identify and quantify biomolecules based on differences in size, charge, polarity and affinity. Chromatography separates analytes between a stationary phase and a mobile phase; each compound’s partitioning or adsorption behaviour determines retention and elution order. Electrophoresis uses an electric field to move charged molecules through a medium; mobility depends on charge-to-mass ratio and shape, allowing size and charge-based separation.

Paper, TLC and column chromatography
Paper chromatography utilises absorbent paper as stationary phase and solvent migration by capillary action; it is suitable for small, polar molecules like amino acids and sugars. Thin-layer chromatography (TLC) uses a thin adsorbent layer (silica gel/alumina) on a plate; separation is rapid and Rf values provide relative identification. Column chromatography packs silica or other resins in a column; samples elute with solvent gradients, collecting fractions for analysis. High-performance liquid chromatography (HPLC) applies high pressure to speed separation and increase resolution, with detectors (UV, fluorescence) for quantification.

Electrophoresis methods
Agarose gel electrophoresis separates nucleic acids by size: DNA migrates towards the anode proportional to length. Polyacrylamide gel electrophoresis (PAGE) provides higher resolution for proteins and small nucleic acids. SDS-PAGE denatures proteins and coats them with negative charge proportional to length, producing separation primarily by molecular weight. Native PAGE preserves protein conformation and separates by charge and size. Isoelectric focusing separates proteins by their isoelectric points in a pH gradient, allowing precise fractionation by pI.

Detection and downstream analysis
Separated molecules are visualised by staining (e.g., Coomassie Brilliant Blue for proteins, ethidium bromide or safer dyes for nucleic acids) or by autoradiography/fluorescence. Chromatography fractions and gel bands can be analysed by mass spectrometry for sequence and mass determination, or by enzymatic assays and immunodetection (Western blotting) to confirm identity. Coupling separation techniques to detectors enables quantitative and qualitative characterisation critical for research and diagnostics.

Applications and considerations
Choice of technique depends on sample type, resolution required and downstream analysis. Parameters such as solvent polarity, pH, matrix pore size and applied voltage must be optimised. Mastery of these chemical separation principles allows purification of biomolecules, analysis of metabolic profiles, and development of diagnostic assays and biotech workflows.

Chemical organisation of metabolism
Metabolism is the network of chemical reactions in living organisms that transform nutrients into energy and building blocks. It is divided into catabolic pathways that break down substrates to release energy and anabolic pathways that build complex molecules using energy. Key chemical reactions include oxidations/reductions, phosphorylations, hydrolyses, decarboxylations and group transfers. Energetic coupling allows unfavourable synthetic steps to proceed by linking them to exergonic reactions such as ATP hydrolysis.

Energy carriers and activated intermediates
ATP is the principal energy currency; hydrolysis of its phosphoanhydride bonds releases free energy used to drive endergonic reactions. Other activated intermediates include NADH and FADH2 (reducing equivalents), and acetyl-CoA (activated acyl group). These carriers shuttle electrons and chemical groups between pathways and provide thermodynamic and kinetic control points for metabolism.

Central metabolic pathways
Glycolysis converts glucose to pyruvate with net production of ATP and NADH and occurs in the cytosol. Pyruvate is further oxidised to acetyl-CoA entering the citric acid cycle (TCA cycle) where oxidation yields NADH, FADH2 and GTP, and releases CO2. NADH and FADH2 donate electrons to the electron transport chain in oxidative phosphorylation, generating a proton gradient that drives ATP synthesis. Lipid metabolism yields acetyl-CoA via β-oxidation and supplies substantial energy; amino acid catabolism feeds into central intermediates after deamination.

Regulation and chemical control
Metabolic flux is tightly regulated at key enzymatic steps by allosteric effectors, covalent modification (e.g., phosphorylation), substrate availability and hormonal control. Rate-limiting steps often have large free-energy changes and are points of control. Enzyme isoforms, compartmentalisation (mitochondria vs cytosol) and reversible/irreversible steps provide chemical logic for efficient pathway regulation.

Physiological and clinical connections
Understanding chemical steps of metabolism explains responses to fasting, exercise, disease and drugs. Disorders such as diabetes, inborn errors of metabolism and mitochondrial diseases reflect specific enzyme defects with predictable chemical consequences. Nutritional chemistry determines fuel choice and metabolic adaptations, and pharmacological agents often target metabolic enzymes to treat disease.

DNA as chemical repository of information
Hereditary information is encoded chemically in the linear sequence of nucleotide bases in DNA. The chemical specificity of complementary base pairing (A–T, G–C) and the stability of the phosphodiester backbone enable reliable storage and templated copying of genetic information. The directionality of the backbone (5′→3′) and sequence context determine how polymerases replicate and transcribe genes, and the chemical properties of bases determine mutational susceptibilities.

Chemical causes of mutations
Mutations arise from chemical alterations to bases and the backbone or from errors during replication. Spontaneous reactions include deamination (cytosine → uracil), depurination (loss of purine bases), and tautomeric shifts that temporarily change base pairing preferences leading to misincorporation. External agents such as alkylating chemicals add methyl or ethyl groups to bases causing mispairing; UV radiation induces covalent dimer formation between adjacent pyrimidines (e.g., thymine dimers) that distort the helix; ionising radiation causes strand breaks and base modifications. Chemical adducts and bulky lesions obstruct polymerases and can result in mutations if misrepaired.

Repair mechanisms and chemical steps
Cells deploy multiple chemical repair pathways to correct lesions: base excision repair recognises and removes damaged bases via glycosylases, leaving an abasic site that is processed and filled in; nucleotide excision repair removes bulky helix-distorting lesions as oligonucleotide fragments and fills the gap by polymerases; mismatch repair recognises and excises replication errors. Enzymatic recognition, excision and resynthesis involve precise chemical steps—hydrolytic cleavage of glycosidic bonds, endonuclease cutting of phosphodiester bonds and DNA polymerase-mediated phosphodiester bond formation—preserving sequence fidelity.

Chemical detection and applications
Detection of mutations and DNA modifications uses chemical and enzymatic methods: sequencing techniques (Sanger, next-generation) read base order; PCR amplifies targeted regions for analysis; bisulfite sequencing chemically distinguishes methylated cytosines. Understanding the chemical basis of mutagenesis informs cancer biology, genetic counselling and the development of targeted therapies that exploit repair defects. In biotechnology, deliberate chemical mutagens are used to induce variation for selection and study of gene function.