Chapter 9 — Biomolecules

What are biomolecules? Biomolecules are organic and inorganic molecules produced by living organisms that perform structural roles, provide energy, act as catalysts, store genetic information and regulate cellular processes. Major classes: carbohydrates, lipids, proteins, nucleic acids, vitamins & minerals, and water.

• Classification
  • Organic biomolecules: carbohydrates, lipids, proteins, nucleic acids, vitamins.
  • Inorganic: water and minerals (ions).

Key features by class

• Carbohydrates
  • Monomers: monosaccharides (glucose, fructose). General formula often written Cn(H2O)m (e.g., glucose C6H12O6).
  • Oligo- & polysaccharides: sucrose (disaccharide), starch, glycogen, cellulose (polysaccharides).
  • Functions: immediate energy (glucose), energy storage (starch, glycogen), structural (cellulose).
  • Common test: Benedict's/Fehling's for reducing sugars; Iodine for starch.
• Lipids
  • Components: glycerol + fatty acids (triglycerides), phospholipids, sterols.
  • Saturated vs unsaturated fatty acids; phospholipids are amphipathic (form membranes).
  • Functions: long-term energy storage, membrane structure, insulation, hormones (steroid precursors).
  • Common test: Sudan III stain for lipids.
• Proteins
  • Monomers: amino acids (amino group, carboxyl group, R group). Peptide bond (condensation) links amino acids.
  • Structures: primary, secondary (α-helix, β-sheet), tertiary, quaternary.
  • Functions: enzymes (catalysts), transport (hemoglobin), structural (collagen), signalling (hormones), immunity (antibodies).
  • Common test: Biuret (peptide bonds).
• Nucleic acids
  • Monomers: nucleotides (phosphate + pentose sugar + nitrogenous base). DNA stores genetic information; RNA transfers and helps translate it.
  • Other nucleotide functions: ATP (energy currency), cAMP (signalling).
• Vitamins & Minerals
  • Micronutrients required in small amounts; vitamins act as coenzymes or antioxidants (water-soluble: B, C; fat-soluble: A, D, E, K). Minerals (Ca, Fe, Mg, Na, K) are structural or ionic cofactors.
• Water
  • Most abundant biomolecule: solvent, medium for reactions, temperature buffer (high specific heat), participates in hydrolysis and condensation reactions.

Important biochemical reactions (conceptual)

• Condensation (dehydration) reactions: join monomers by releasing water (e.g., formation of peptide or glycosidic bonds).
• Hydrolysis: break polymers by adding water (digestion of starch to glucose, proteins to amino acids).
• Enzyme-catalysed reactions: lower activation energy and increase reaction rates; sensitive to temperature, pH and substrate concentration.

Why biomolecules matter (big picture): Biomolecules form the chemical basis of life—providing structure (cell walls, membranes), storing and transferring genetic information (DNA/RNA), enabling metabolism (enzymes & ATP), and maintaining homeostasis (hormones, buffers, electrolytes).

Introduction: Water (H2O) is a small polar molecule essential for life. Its unique physical and chemical properties arise from the bent molecular geometry and strong hydrogen bonding between molecules. These properties make water an excellent solvent, temperature buffer, medium for biochemical reactions and a transport agent in organisms.

• Molecular structure and polarity: The oxygen atom is more electronegative than hydrogen, creating a polar molecule with a partial negative charge near oxygen and partial positive charges near hydrogens. This polarity promotes hydrogen bonding (H—O...H) between adjacent molecules.
• Hydrogen bonding: Each water molecule can form up to four hydrogen bonds (two via H and two via lone pairs on O). Hydrogen bonds are weaker than covalent bonds but strong enough collectively to determine many macroscopic properties.
• Cohesion and adhesion: Cohesion (water–water attraction) produces surface tension; adhesion (water–other surfaces) helps water climb and spread on surfaces. Combined, these properties enable capillary action important in plants (xylem transport).
• High specific heat (thermal stability): Water requires relatively large energy to change temperature, so it buffers temperature fluctuations in organisms and environments (oceans, lakes). This thermal inertia stabilizes climate and body temperature.
• High latent heat of vaporization: Evaporation of water absorbs considerable heat (cooling effect). Biological example: sweating and transpiration cool organisms.
• Excellent solvent: Water dissolves a wide range of ionic and polar substances because of its polarity and high dielectric constant. This property facilitates transport of nutrients, gases and wastes in blood and cell cytoplasm.
• Anomalous expansion (density maximum at ~4°C): Unlike most liquids, water reaches maximum density at about 4°C. Ice is less dense than liquid water and floats. This insulates aquatic life in winter and causes seasonal turnover in lakes.
• Surface tension and capillarity: High surface tension (due to cohesion) allows small insects to walk on water and helps form droplets; capillary rise moves water in narrow pores/tubes.
• Autoionization and role in pH: Water undergoes slight self-ionization: H2O ⇌ H+ + OH−. The equilibrium constant (Kw) and the [H+] determine pH, which affects enzyme activity and biochemical reactions.
• Chemical reagent in metabolism: Water participates directly in hydrolysis (breaking bonds by adding water) and condensation (removal of water to form bonds), central to digestion and biosynthesis of macromolecules.
• Optical clarity and transparency: Water is transparent to visible light, enabling photosynthesis in aquatic plants and organisms.

Key environmental and biological consequences: Ice floating protects aquatic ecosystems; high heat capacity moderates climate; solvent properties enable blood and cytoplasm to transport solutes; evaporative cooling (sweating/transpiration) regulates temperature; capillarity and cohesion aid water transport in plants.

Overview

pH, acids, bases and buffers are fundamental concepts that explain acidity, alkalinity and how biological systems maintain stable internal conditions. In aqueous solutions acidity is governed by the concentration of hydrogen ions (H+). pH is a logarithmic measure of [H+]; small pH changes correspond to large changes in [H+].

Definitions

• Arrhenius: Acid produces H+ in water; base produces OH–.
• Brønsted–Lowry: Acid = proton (H+) donor; base = proton acceptor. This is the most useful in biology.
• Lewis: Acid = electron-pair acceptor; base = electron-pair donor (broader chemical context).

pH and related quantities

• pH is defined as pH = −log10[H+]. Because of the negative log, lower pH = higher acidity.
• pOH = −log10[OH–]. For water at 25 °C, pH + pOH = 14 (Kw = [H+][OH–] = 1×10−14).
• Typical biological values: stomach fluid pH ~1.5–3.5, blood pH ≈ 7.35–7.45, cytosol pH ≈ 7.0–7.4.

Acid and base strength

Strength of a weak acid HA is measured by its acid dissociation constant Ka:

Ka = [H+][A–] / [HA]

and pKa = −log10 Ka. Small pKa → stronger acid. Relationship for conjugate acid–base pairs: Ka × Kb = Kw (at a given temperature).

Calculating pH for weak acids and bases (quick approximations)

• Weak acid HA, initial concentration C and Ka: if dissociation x is small, [H+] ≈ sqrt(Ka × C) and pH ≈ 0.5 (pKa − log10 C).
• Weak base calculations are analogous using Kb and conversion to [OH–], then pH = 14 − pOH (at 25 °C).

Buffers: what they are and how they work

A buffer is a solution containing a conjugate acid-base pair (e.g., HA and A–) that resists changes in pH when small amounts of acid or base are added. The key idea is Le Châtelier's principle: added H+ is consumed by the base (A–); added OH– is consumed by the acid (HA).

Henderson–Hasselbalch equation (practical buffer formula)

For the weak acid HA / conjugate base A– system:

pH = pKa + log10([A–] / [HA])

Use this to design buffers of desired pH by choosing appropriate pKa and ratio [A–]/[HA].

Buffer range and capacity

• A buffer is most effective within about ±1 pH unit of the pKa of the acid (i.e., pKa − 1 to pKa + 1).
• Maximum buffering capacity occurs when [A–] ≈ [HA] (so pH ≈ pKa).
• Buffer capacity increases with the total concentration of the buffering components.

Important biological buffers

• Bicarbonate buffer (CO2 / H2CO3 / HCO3–): central in blood. Physiological Henderson–Hasselbalch form often written as pH = pKa + log([HCO3–] / [CO2]) with effective pKa ≈ 6.1 (body temperature). Blood pH ≈ 7.4 is maintained by respiration (controls CO2) and kidneys (regulate HCO3–).
• Phosphate buffer (H2PO4– / HPO4 2–): important intracellular buffer; pKa ≈ 7.2 (useful near physiological intracellular pH).
• Protein buffers: amino acid side chains (e.g., histidine imidazole, pKa ≈ 6.0) and terminal groups can accept or donate protons; haemoglobin buffers blood pH.

Biological importance

Many enzymes and cellular processes are pH-sensitive. Even small deviations (e.g., blood pH 7.4 → 7.2) can impair enzyme activity, oxygen delivery, and metabolic reactions. Buffers plus physiological regulation (respiration, renal function) keep pH within narrow limits.

Measurement

pH is measured with pH paper (approximate) or pH meters (electrode producing voltage proportional to log[H+]).

Summary

• pH quantifies acidity: pH = −log[H+].
• Acids and bases can be described by Brønsted–Lowry concepts; strength measured by Ka / pKa.
• Buffers (conjugate acid–base pairs) resist pH changes; Henderson–Hasselbalch links pH, pKa and component ratio.
• Physiological buffering (bicarbonate, phosphate, proteins) is essential for life.

Overview
Biomolecules (carbohydrates, proteins, lipids, nucleic acids) are held together and react through specific chemical bonds. These bonds are of two broad types: covalent (strong, specific linkages like peptide, glycosidic, ester, phosphodiester, disulfide) and non-covalent (weaker interactions like hydrogen bonds, ionic interactions, hydrophobic effects, and van der Waals forces). Both bond types determine structure, stability and biological function. Enzymes catalyze most biomolecular reactions by lowering activation energy.

Covalent bonds & examples

• Peptide bond — linkage between amino acids formed by condensation (dehydration) between the carboxyl of one amino acid and amino of another; backbone of proteins.
• Glycosidic bond — covalent bond between monosaccharides (e.g., sucrose, starch) formed by condensation of OH groups.
• Ester bond — links fatty acids to glycerol in triglycerides (lipid formation) via condensation.
• Phosphodiester bond — links nucleotides in DNA/RNA between 3′-OH and 5′-phosphate groups.
• Disulfide bond — covalent S–S bond between cysteine residues stabilizing tertiary/quaternary protein structure (e.g., insulin).

Non-covalent interactions & roles

• Hydrogen bonds — critical for secondary structures (alpha helices, beta sheets), base-pairing in DNA.
• Ionic (electrostatic) interactions — salt bridges between charged side chains; important in enzyme active sites and protein stability.
• Hydrophobic interactions — drive folding by burying nonpolar residues away from water; key in membrane assembly.
• Van der Waals forces — weak, short-range attractions that contribute to tight molecular packing.

Common reactions in biomolecules

• Condensation (dehydration) reactions — two functional groups join with loss of water to form covalent bonds (peptide, glycosidic, ester, phosphodiester formation).
• Hydrolysis — water breaks covalent bonds (e.g., digestion of proteins, polysaccharides, lipids); usually enzyme-catalyzed.
• Redox reactions — electron transfer reactions; example: formation/cleavage of disulfide bonds during protein folding.
• Phosphorylation/dephosphorylation — addition/removal of phosphate groups (regulation of protein activity, signal transduction).
• Acid–base reactions — protonation/deprotonation affects ionization states of biomolecules, influencing reactivity and binding.

Biological significance
The pattern and combination of covalent and non-covalent bonds determine biomolecular shape, dynamics, recognition and catalysis. For example, DNA's double helix relies on covalent phosphodiester backbone plus hydrogen-bonded base pairs; enzymes form transient non-covalent interactions with substrates while forming or breaking covalent bonds during catalysis.

Role of enzymes and energetics
Although many bond-forming reactions (e.g., peptide bond formation) are thermodynamically unfavorable without input (ATP or activated intermediates), enzymes and coupling to high-energy molecules (ATP, activated sugars) make biosynthesis efficient. Hydrolysis of ATP (ATP + H2O → ADP + Pi + energy) is a common energy source for biosynthetic reactions.

Summary points

• Covalent bonds create molecular backbones; non-covalent bonds fine-tune structure/function.
• Condensation builds polymers; hydrolysis breaks them down.
• Enzymes regulate rates and specificity of bond-making/breaking.

Definition: Carbohydrates are organic biomolecules composed of carbon, hydrogen and oxygen, generally following the empirical formula (CH2O)_n. They serve as primary fuels and structural components in living organisms.

Classification:

• Monosaccharides – simplest sugars (not hydrolyzable). Example: glucose, fructose, galactose. Classified by carbon number (triose, tetrose, pentose, hexose) and by functional group (aldose, ketose).
• Oligosaccharides – 2–10 monosaccharide units joined by glycosidic bonds. Example: disaccharides: sucrose, lactose, maltose.
• Polysaccharides – many monosaccharide units (homopolysaccharides or heteropolysaccharides). Examples: starch (amylose + amylopectin), glycogen, cellulose, chitin.

Monosaccharide structure and isomerism: Monosaccharides have a backbone with multiple chiral centres. They can be drawn as Fischer projections (open-chain) or Haworth projections (cyclic). Ring formation (hemiacetal/hemi-ketal) creates anomeric carbon giving α and β anomers; interconversion in solution is called mutarotation.

Important structural details:

• Aldoses contain an aldehyde group (e.g., D-glucose); ketoses contain a ketone group (e.g., D-fructose).
• Anomeric carbon: the carbon formerly part of C=O in the open chain; glycosidic bonds form at this carbon.
• α vs β glycosidic bonds: Orientation of OH on anomeric carbon (α = trans to CH2OH in pyranoses, β = cis). Bond notation example: α(1→4), β(1→4), α(1→6) (branching).

Key examples & biological roles:

• Glucose: principal blood sugar, energy source; formula C₆H₁₂O₆.
• Starch: plant storage polysaccharide (amylose linear α(1→4), amylopectin branched α(1→6)).
• Glycogen: animal storage polysaccharide, highly branched (α(1→4) with α(1→6) branches).
• Cellulose: plant structural polymer of β(1→4)-linked glucose; provides rigidity (fibre).
• Disaccharides: sucrose (glucose–fructose, α(1→2), non-reducing), lactose (galactose–glucose, β(1→4), reducing), maltose (glucose–glucose, α(1→4), reducing).

Chemical reactions and tests:

• Glycosidic bond formation: condensation (dehydration): monosaccharide + monosaccharide → disaccharide + H₂O. Reverse is hydrolysis (enzymes: sucrase, lactase, amylase).
• Reducing sugars: sugars with free anomeric OH (or free aldehyde) reduce Benedict's or Fehling's reagent to yield a coloured precipitate.
• Iodine test: starch forms blue-black complex with iodine (tests for starch).

Functions in organisms: energy source and reserve (glucose, glycogen, starch), structural (cellulose, chitin), cell recognition and signalling (glycoproteins, glycolipids), dietary fibre (cellulose) aiding digestion.

Important concepts to remember: mutarotation (change in optical rotation as α and β interconvert), stereoisomer count ≈ 2ⁿ (n = number of chiral centres), and branching increases solubility and accessibility (e.g., glycogen vs cellulose).

Definition: Lipids are a diverse group of naturally occurring organic compounds that are largely hydrophobic (insoluble in water) but soluble in organic solvents. They include fats, oils, waxes, phospholipids, glycolipids and steroids.

Classification:

• Simple lipids – triacylglycerols (fats and oils), waxes.
• Compound lipids – phospholipids (e.g., phosphatidylcholine), glycolipids, lipoproteins.
• Derived lipids – steroids (e.g., cholesterol), terpenes, fat-soluble vitamins (A, D, E, K).

Building blocks & structure:

• Glycerol (a 3‑carbon alcohol) + fatty acids (long-chain carboxylic acids) form triacylglycerols (triglycerides) via ester bonds.
• Fatty acids vary by chain length (usually 4–24 C) and degree of unsaturation (single vs double bonds). Unsaturated fats may be cis or trans.
• Phospholipids contain a glycerol backbone, two fatty acids (hydrophobic tails) and a phosphate-linked polar head (hydrophilic) — basis of biological membranes (bilayer).
• Steroids have a characteristic four-ring fused structure; cholesterol is a key membrane steroid and precursor of steroid hormones.

Key properties:

• Insoluble in water, soluble in nonpolar organic solvents (ether, chloroform).
• Fats (saturated) are generally solid at room temperature; oils (unsaturated) are liquid.
• Melting point increases with chain length and decreases with number of double bonds.

Biological functions:

• High-energy storage: fats store more energy per gram than carbohydrates (~9 kcal/g).
• Structural: phospholipids and cholesterol are fundamental components of cell membranes.
• Insulation and protection: subcutaneous fat insulates and cushions organs.
• Signalling and hormones: steroid hormones (e.g., estrogen, testosterone) and lipid-derived messengers (prostoglandins).
• Vitamins: fat-soluble vitamins (A, D, E, K) require lipids for absorption and transport.

Important reactions:

• Esterification (formation of triglycerides): glycerol + 3 fatty acids → triacylglycerol + 3 H2O
• Hydrolysis (lipase action): triacylglycerol + 3 H2O → glycerol + 3 fatty acids
• Saponification: ester + NaOH → glycerol + sodium salt of fatty acid (soap)

Metabolism (overview): Fatty acids undergo β‑oxidation in mitochondria to yield acetyl‑CoA units that enter the citric acid cycle; this is a major source of ATP in many tissues. Complete oxidation of lipids releases more energy per carbon than carbohydrates.

Health & dietary notes: Excessive saturated and trans fats are associated with cardiovascular disease (raise LDL cholesterol). Unsaturated fats (monounsaturated and omega‑3 polyunsaturated) are considered healthier and have beneficial effects on heart health.

Recall points for Class 11:

• Know structures of glycerol, a saturated (palmitic) and unsaturated (oleic) fatty acid, a triglyceride, a phospholipid and cholesterol.
• Be able to state properties (hydrophobic, soluble in organic solvents) and major functions (energy reserve, membrane component, hormones).
• Write simple reactions: esterification and saponification.

What are proteins?
Proteins are large biological macromolecules made of one or more polypeptide chains of amino acids linked by peptide bonds. They perform structural, catalytic, transport, regulatory and defence functions in living organisms. The sequence of amino acids (primary structure) is dictated by the gene encoding the protein.

Amino acid basics

Each amino acid has a central carbon (alpha carbon) attached to an amino group (NH2), a carboxyl group (COOH), a hydrogen and an R group (side chain). General formula: NH2-CH(R)-COOH. At physiological pH most amino acids exist as zwitterions (NH3+ and COO-).

Peptide bond

Two amino acids join by a condensation reaction between the carboxyl of one and the amino of another to form a peptide bond (–CO–NH–) and release water. The peptide bond has partial double-bond character, making it planar and usually trans in configuration.

Levels of protein structure

• Primary structure: linear amino acid sequence; determines all higher structures.
• Secondary structure: local folding stabilized by hydrogen bonds between backbone C=O and N-H groups. Major types: alpha-helix and beta-pleated sheet.
• Tertiary structure: 3D folding of a single polypeptide stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges (S–S) and van der Waals forces.
• Quaternary structure: assembly of two or more polypeptide subunits (e.g., haemoglobin has four subunits).

Classification

• By shape: fibrous (structural, e.g., collagen, keratin) and globular (enzymes, transport proteins).
• By composition: simple (only amino acids) and conjugated (with prosthetic group, e.g., glycoproteins, metalloproteins).

Properties

• Amphoteric nature: can act as acid or base; has isoelectric point (pI) where net charge is zero.
• Solubility depends on pH and R groups; many proteins insoluble at their pI.
• Denaturation: loss of native structure and activity caused by heat, pH extremes, organic solvents, urea, detergents, heavy metal ions or reducing agents (which break disulfide bonds).
• Specificity: function depends on precise 3D shape; even small sequence changes can alter function.

Functions

• Enzymes: biological catalysts (e.g., digestive enzymes, DNA polymerase).
• Structural: collagen (connective tissue), keratin (hair, nails).
• Transport and storage: haemoglobin (oxygen transport), ferritin (iron storage), albumin.
• Movement: actin and myosin in muscles.
• Regulation: peptide hormones like insulin.
• Immune defence: antibodies (immunoglobulins).

Tests for proteins

• Biuret test: protein gives violet colour with Cu2+ in alkaline medium (positive for peptide bonds).
• Xanthoproteic test: nitration of aromatic residues gives yellow colour (turns orange on alkaline treatment).

Biological importance and examples

Proteins are essential for cell structure, metabolism, signalling and homeostasis. Disorders from defective proteins include sickle-cell anaemia (single amino acid substitution in haemoglobin) and many enzyme-deficiency diseases.

How sequence relates to function

The primary sequence determines folding and active site geometry. Protein engineering and mutations show how small sequence changes alter stability, activity or interaction with other molecules.

Practical measurements

Protein concentration is often measured by UV absorbance at 280 nm (due to aromatic residues) or by colorimetric assays (Bradford, Lowry, Biuret). SDS-PAGE separates proteins according to molecular weight for analysis.

Summary: Proteins are polymers of amino acids with hierarchical structures (primary to quaternary). Their chemical properties and side chains dictate folding, stability and diverse biological functions. Understanding proteins involves sequence, structure, function and methods to detect and quantify them.

Definition: Enzymes are biological catalysts—mostly globular proteins (some are RNA molecules called ribozymes)—that increase the rate of biochemical reactions without being consumed. They lower the activation energy and provide a specific active site for substrate binding.

Structure and terms:

• Active site: A specialised pocket where substrate(s) bind and reaction occurs.
• Substrate: The reactant molecule acted upon by an enzyme.
• Product: Molecule(s) produced after the reaction.
• Apoenzyme: Protein portion of an enzyme (inactive by itself).
• Cofactor: Non-protein component (metal ions) required for activity.
• Coenzyme: Organic cofactor (often derived from vitamins) that temporarily carries chemical groups or electrons.
• Holoenzyme: Apoenzyme + cofactor/coenzyme (active form).

Specificity: Enzymes show substrate specificity. Models:

• Lock and key: Active site is complementary to substrate shape.
• Induced fit: Active site changes conformation on substrate binding to improve fit and catalysis (more accepted).

Mechanism of action (brief): Binding of substrate to active site forms an enzyme-substrate (ES) complex. The enzyme stabilises the transition state, lowers activation energy, converts substrate to product, and releases product, regenerating the enzyme.

Factors affecting enzyme activity:

• Temperature: Activity increases with temperature up to an optimum (e.g., 37°C for many human enzymes), then denaturation causes activity to fall.
• pH: Each enzyme has an optimum pH (pepsin acidic, trypsin alkaline); deviations alter ionisation of active site residues and alter activity.
• Substrate concentration: Rate increases with [S] and approaches a maximum (saturation) when enzyme active sites are fully occupied.
• Enzyme concentration: Rate is proportional to enzyme concentration when substrate is in excess.
• Inhibitors: Substances that reduce enzyme activity (reversible: competitive/non-competitive; irreversible: covalent modification).

Enzyme kinetics (basic): The Michaelis–Menten model describes the initial rate (v) of many enzyme-catalysed reactions as a function of substrate concentration [S]:

ES formation: E + S <--> ES --> E + P

At steady state, v = (Vmax [S]) / (Km + [S]).

Where Vmax is the maximum rate when enzyme is saturated, and Km (Michaelis constant) is the [S] at which v = Vmax/2 and is a measure of enzyme affinity for substrate (lower Km = higher affinity).

Inhibition types (summary):

• Competitive: Inhibitor resembles substrate, competes for active site. Increases apparent Km, Vmax unchanged (at high [S] can overcome inhibition).
• Non-competitive (mixed): Inhibitor binds to enzyme or ES complex at a site other than active site. Vmax decreases, Km often unchanged (affinity unchanged).
• Irreversible: Inhibitor covalently modifies enzyme (e.g., nerve gas inhibit acetylcholinesterase).

Biological importance and applications: Enzymes are essential for metabolism (digestion, respiration, DNA replication, photosynthesis). Industrial uses include detergents (proteases), food industry (amylases, rennet in cheese), brewing (amylases), biotechnology (restriction enzymes, polymerases), diagnostics (enzyme markers AST/ALT), and medicine (enzyme replacement therapy).

Examples of important enzymes: amylase (starch digestion), pepsin (protein digestion in stomach), trypsin (small intestine), lipase (lipid digestion), catalase (breakdown of H2O2), urease (hydrolyses urea), lactase (breaks lactose), DNA polymerase, RNA polymerase, ATP synthase.

Key points to remember: Enzymes are highly specific, their activity is regulated, they are not consumed by reactions, and conditions (temperature, pH, substrate/enzyme concentrations, inhibitors, cofactors) determine their efficiency and rate.

What are Nucleic Acids?
Nucleic acids are long biomolecules that store and transmit genetic information. There are two main types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Basic building block: the nucleotide

• A nucleotide consists of three parts: a pentose sugar (ribose in RNA, deoxyribose in DNA), a nitrogenous base (purine: adenine, guanine; pyrimidine: cytosine, thymine in DNA, uracil in RNA), and one or more phosphate groups.
• A nucleoside = sugar + base (no phosphate).
• Nucleotides are linked by 3'-5' phosphodiester bonds to form a sugar–phosphate backbone; strands have directionality (5' end and 3' end).

Structure of DNA

• DNA is typically a double-stranded helix with two antiparallel polynucleotide chains wound around each other (right-handed B-form is common in cells).
• Base pairing is specific: A pairs with T (two hydrogen bonds), G pairs with C (three hydrogen bonds) — this is Chargaff’s rule (in double-stranded DNA A% ≈ T% and G% ≈ C%).
• Characteristic features: about 10 base pairs per turn, diameter ≈ 2 nm, major and minor grooves important for protein binding.

Structure and types of RNA

• RNA is usually single-stranded but folds into secondary structures (hairpins, loops) by intramolecular base pairing.
• Major types: mRNA (messenger RNA) carries coding information, tRNA (transfer RNA) brings amino acids in translation, rRNA (ribosomal RNA) is structural/enzymatic in ribosomes. Other classes: snRNA, miRNA, siRNA, long noncoding RNAs.

Functions

• DNA stores hereditary information and directs development, functioning and reproduction of organisms.
• RNA transfers and helps interpret the genetic code during protein synthesis (transcription, translation), and plays regulatory/enzymatic roles.

Key processes

• Replication — semi-conservative copying of DNA catalysed by DNA polymerases; requires a template and proceeds 5'→3'.
• Transcription — synthesis of RNA from DNA template by RNA polymerase.
• Translation — ribosome-mediated decoding of mRNA to make polypeptides with the help of tRNA.

Physical and chemical properties

• Nucleic acids absorb UV light strongly at ~260 nm (used to quantify them). Denaturation (strand separation) increases absorbance (hyperchromic effect).
• Melting temperature (Tm) is the temperature at which half of the DNA is single-stranded; Tm increases with higher GC content and salt concentration.

Biological significance & applications

• DNA fingerprinting, forensic identification, paternity testing, genetic engineering, PCR, sequencing, recombinant DNA technology.
• RNA-based technologies: mRNA vaccines, RNA interference (gene silencing), viral diagnostics.

Important experimental notes

• Measurement: absorbance at 260 nm (A260) is used to estimate concentration; purity by A260/A280 ratio (≈1.8 for pure DNA, ≈2.0 for pure RNA).
• Molecular weight estimate: one base pair of dsDNA ≈ 660 Da; use this to approximate MW of DNA fragments.

Overview

Vitamins and minerals are essential micronutrients required in small amounts for normal growth, metabolism and physiological functions. Unlike carbohydrates, proteins and fats, they do not provide energy but act as cofactors, coenzymes, antioxidants and regulators of biochemical processes.

Classification of Vitamins

• Fat-soluble vitamins: A, D, E, K. Soluble in lipids, absorbed with dietary fat and bile, stored in liver and adipose tissue; excess may cause toxicity.
• Water-soluble vitamins: B-complex (B1, B2, B3, B5, B6, B7, B9, B12) and C. Readily absorbed in gut, not stored (except B12); excess usually excreted in urine.

Key properties and roles

• Many B vitamins form parts of coenzymes (e.g., NAD+, FAD, coenzyme A) essential for energy metabolism.
• Vitamin C acts as an antioxidant and helps iron absorption; also required for collagen synthesis.
• Fat-soluble vitamins regulate vision (A), calcium-phosphate balance (D), protect membranes from oxidation (E), and enable blood clotting (K).
• Minerals are elements (Ca2+, Fe2+/Fe3+, I–, Zn2+, Mg2+, Na+, K+, etc.) that act as structural components (Ca in bone), cofactors for enzymes (Mg, Zn), electrolytes (Na, K) and trace regulators (I for thyroid hormones, Fe for haemoglobin).

Absorption, storage and interactions

• Fat-soluble vitamins require dietary fat and bile salts for absorption; disorders of fat absorption (e.g., cholestatic disease) lead to deficiency.
• Water-soluble vitamins are absorbed in intestine and excess is usually excreted; B12 requires intrinsic factor for absorption.
• Interactions: Vitamin C enhances non-heme iron absorption; excessive zinc can reduce copper absorption; vitamin D increases intestinal Ca2+ absorption.

Common deficiencies and consequences (summary)

• Vitamin A deficiency: night blindness, xerophthalmia.
• Vitamin D deficiency: rickets in children, osteomalacia in adults.
• Vitamin C deficiency: scurvy—bleeding gums, poor wound healing.
• Thiamine (B1) deficiency: beriberi, neurological and cardiac problems.
• Folate (B9) deficiency: macrocytic anemia, neural tube defects in fetuses.
• Vitamin B12 deficiency: pernicious anemia, neurological deficits.
• Iron deficiency: microcytic hypochromic anemia, fatigue.
• Iodine deficiency: goitre and hypothyroidism; cretinism in severe fetal deficiency.
• Calcium deficiency (long-term): reduced bone density, risk of osteoporosis.

Prevention and dietary sources

• Balanced diet with fruits, vegetables, dairy, meat, fish, pulses, nuts and fortified foods prevents most deficiencies.
• Supplementation or fortification (iodized salt, folic acid in pregnancy, vitamin D supplements in low sunlight) is used where deficiency risk is high.

Relevance to CBSE Class 11 Biology (Biomolecules)

This topic links biochemical roles (coenzymes, redox carriers) with physiology (growth, blood formation, bone health) and nutrition. Understanding specific vitamin and mineral functions, sources and deficiency signs is required for exam-level knowledge.

Overview
Tests and methods for biomolecules are qualitative and quantitative procedures used to detect, identify and measure carbohydrates, proteins, lipids and other biological molecules. Tests rely on chemical reactions (color changes, precipitate formation), staining, separation techniques (chromatography, electrophoresis) and instrument-based assays (spectrophotometry).

Carbohydrates — Principles and Common Tests

Principles: Many carbohydrate tests detect reducing groups (free aldehyde or ketone) or specific structural features (polysaccharide helical structure).

• Iodine test (starch): Iodine (I2/KI) fits into the helical amylose structure producing a blue‑black color. No color change for cellulose; glycogen gives reddish‑brown.
• Benedict’s / Fehling’s test (reducing sugars): Cu2+ (blue) is reduced to Cu2O (brick red/orange precipitate) in alkaline medium when reducing sugars are present. Used qualitatively; color change intensity correlates roughly with concentration.
• Barfoed’s test: Acidic Cu2+ reagent — monosaccharides react faster than disaccharides, so it distinguishes monosaccharides (positive quickly) from disaccharides.
• Seliwanoff’s test: Differentiates ketoses (fructose) from aldoses; ketoses give a deep cherry‑red rapidly due to dehydration and formation of hydroxymethylfurfural derivatives.
• Molisch’s test: General test for carbohydrates. Strong acid dehydrates sugars to furfural derivatives that react with α‑naphthol producing a violet ring.

Proteins and Amino Acids — Principles and Common Tests

Principles: Tests detect peptide bonds, free amino groups, aromatic residues, sulphur-containing amino acids, etc.

• Biuret test: Peptide bonds react with Cu2+ in alkaline medium to give violet/purple color. Requires at least dipeptides (best for proteins).
• Ninhydrin test: Free amino acids (and aminated amines) produce a deep purple (Ruhemann’s purple). Proline and hydroxyproline give yellow because they are secondary amines.
• Xanthoproteic and Millon’s tests: Xanthoproteic (conc. HNO3) nitrates aromatic rings (tyrosine, tryptophan) giving yellow → orange on alkalinization. Millon’s reagent gives red color with phenolic (tyrosine) groups.
• Lead acetate / Hydrogen sulfide tests: Detect sulphur‑containing amino acids (cysteine, cystine) via black precipitate of PbS or H2S reaction.
• Hopkins‑Cole test: Specific for tryptophan residues (violet ring with glyoxylic acid + conc. H2SO4).

Lipids — Principles and Common Tests

• Grease‑spot test: Lipid leaves a translucent, permanent spot on paper (water evaporates, grease remains).
• Emulsion test: Extract lipid in ethanol (or mix sample with ethanol), pour into water — a milky emulsion indicates lipids.
• Sudan III / Sudan IV / Sudan Black staining: Lipid‑soluble dyes stain fats orange/black in microscopy or slides.
• Acrolein test: Heating glycerol‑containing lipids with KHSO4 produces acrolein (sharp odor) — a classical but hazardous test.

Separation & Quantification Methods

• Paper chromatography / TLC: Separate sugars, amino acids, pigments. Rf value (ratio of distances) helps identify compounds. Spots can be visualized with ninhydrin (amino acids) or aniline phthalate (sugars).
• Electrophoresis: Separates proteins or nucleic acids by size/charge (gel electrophoresis) — produces band patterns used for identification or purity assessment.
• Spectrophotometry (UV‑Vis): Quantitative assays use Beer‑Lambert law (absorbance proportional to concentration). Examples: Bradford or Lowry assays for protein (measure at 595 nm or 750 nm); DNS or phenol‑sulfuric acid for sugars.
• Kjeldahl method: Determines nitrogen content of samples; used to estimate protein content (convert %N to protein using factor, commonly 6.25 for many foods).

Practical notes, sensitivity and limitations

• Some tests are qualitative and give only presence/absence or rough estimation (e.g., Benedict’s intensity). Instrumental methods (spectrophotometry) give accurate quantification when calibrated with standards.
• Certain reagents are hazardous (conc. acids, hot alkali, acrolein). Follow safety rules and proper disposal.
• Interfering substances can give false positives/negatives — choice of confirmatory tests and controls is important.

Summary
Understanding the chemical basis (reducing groups, peptide linkages, aromatic rings, hydrophobicity) helps select the right test or combination of tests. For quantitative work, separation (chromatography/electrophoresis) followed by spectrophotometric assay and standard calibration curves are standard practice.

Integration — Biomolecules (carbohydrates, lipids, proteins, nucleic acids, vitamins and coenzymes) do not act in isolation. Cellular metabolism integrates synthesis and degradation pathways so that building blocks, energy and signals are shared and balanced. Examples of integration: glycolysis and TCA link carbohydrate breakdown to ATP production and provide precursors for amino acid and lipid synthesis; transamination links amino acid metabolism to carbohydrate metabolism; β‑oxidation of fatty acids feeds acetyl‑CoA into the TCA cycle.

How integration works (key principles)

• Common intermediates: molecules such as glucose‑6‑phosphate, pyruvate and acetyl‑CoA connect different pathways.
• Enzyme regulation: feedback inhibition, allosteric control and covalent modification coordinate flux through pathways.
• Hormonal control: insulin, glucagon and others shift metabolism toward storage or mobilization of fuel.
• Compartmentation: pathways occur in specific cellular locations (cytosol, mitochondria), enabling separation and controlled integration.

Biological significance

• Energy currency: ATP produced by catabolism (glycolysis, TCA, oxidative phosphorylation) powers biosynthesis, transport and mechanical work.
• Structural roles: carbohydrates (cellulose) and proteins (collagen) provide support; phospholipids form membranes.
• Storage forms: glycogen (animals) and starch (plants) store glucose; triglycerides store long‑term energy.
• Transport and signalling: haemoglobin transports O2; hormones (peptide and steroid) and membrane receptors transmit signals.
• Catalysis and regulation: enzymes (proteins) accelerate reactions and control metabolic flux.
• Genetic information and inheritance: DNA stores genetic code; RNA translates it into proteins.
• Defense and recognition: antibodies (proteins) and glycoproteins on cell surfaces mediate immunity and cell recognition.
• Nutritional and ecological roles: essential amino acids, fatty acids and vitamins are required in diet; biomolecules determine organism interactions and energy flow in ecosystems.

Practical implications — Understanding integration explains clinical and everyday phenomena: blood glucose regulation after a meal (insulin promotes glycogen synthesis, fatty acid formation), muscle exercise (increased glycolysis and ATP turnover), starvation (gluconeogenesis, lipolysis, ketone formation).