Biomolecules

Biomolecules is one of those chapters where NEET rewards memory more than derivation — a handful of structures, classifications, and characteristic reactions generate 1–2 questions almost every year. The trick is to organise the information into small comparison tables in your head rather than trying to read it as continuous prose.

What Biomolecules Are, and How Carbohydrates Are Classified

Living cells are chemical factories, and the molecules they build and break down — sugars, proteins, nucleic acids, lipids, vitamins, hormones — are collectively called biomolecules. Chemically they are ordinary organic compounds obeying ordinary organic rules; what makes them special is their size, their stereochemical precision, and the fact that biological function collapses the moment shape is lost.

Carbohydrates were originally named as "hydrates of carbon" because most fit the formula C_x(H₂O)_y. That definition leaks badly (rhamnose and deoxyribose don't fit; acetic acid does fit but isn't a sugar), so the modern definition is chemical: carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or compounds that yield these on hydrolysis. A sugar with an aldehyde group at C-1 is an aldose; one with a keto group (usually at C-2) is a ketose.

Three classification schemes are examinable:

1. By hydrolysis behaviour

• Monosaccharides — cannot be hydrolysed further: glucose, fructose, galactose, ribose. About 20 occur in nature.
• Oligosaccharides — give 2–10 monosaccharide units: sucrose, maltose, lactose (all disaccharides).
• Polysaccharides — give a large number of units: starch, cellulose, glycogen.

2. By taste — sugars (sweet, crystalline, water-soluble: glucose, sucrose) versus non-sugars (tasteless, insoluble: starch, cellulose).
3. By reducing behaviour — reducing sugars give positive Tollens' and Fehling's tests because they possess a free aldehydic or ketonic group in solution (all monosaccharides, plus maltose and lactose). Non-reducing sugars do not (sucrose, and the polysaccharides).

Glucose: Structure, Reactions, and Anomers

Glucose (dextrose, grape sugar) is the reference monosaccharide. It is prepared industrially by acid hydrolysis of starch (starch + water, dilute H₂SO₄, 393 K, 2–3 atm) and in the laboratory from sucrose using dilute HCl or H₂SO₄, which gives an equimolar mixture of glucose and fructose.

The open-chain structure was deduced from these observations, which are frequently asked directly:

• Molecular formula C₆H₁₂O₆; prolonged heating with HI gives n-hexane, so the six carbons are in an unbranched chain.
• Reaction with hydroxylamine gives a monoxime, and with HCN a cyanohydrin — one carbonyl group.
• Acetylation with acetic anhydride gives a penta-acetate — five –OH groups, each on a different carbon.
• Mild oxidation with bromine water gives the six-carbon monocarboxylic acid gluconic acid, so the carbonyl is an aldehyde.
• Oxidation with nitric acid gives the dicarboxylic saccharic (glucaric) acid, showing a primary alcohol at the other end.

The specific stereochemistry (D-(+)-glucose) was established by Fischer. In the D/L convention, the reference is the configuration of the lowest chiral carbon (C-5 in glucose) compared with (+)-glyceraldehyde — this letter has nothing to do with the sign of optical rotation, which is written separately as (+) or (–).

Two facts do not fit the open-chain picture: glucose does not restore the colour of Schiff's reagent, does not react with NaHSO₃, and its pentaacetate does not react with hydroxylamine. This is because glucose exists mainly as a cyclic hemiacetal — the C-5 hydroxyl adds across the C-1 aldehyde to form a six-membered pyranose ring. C-1 becomes a new chiral centre, giving two diastereomers called anomers: α-D-glucose (C-1 OH on the same side as the C-5 reference in the Fischer projection; drawn down in the Haworth form) and β-D-glucose (drawn up). The two differ in melting point and specific rotation (+111° and +19.2°) and interconvert in solution through the trace open-chain form, so a fresh solution's rotation drifts to an equilibrium value of about +52.7° — this is mutarotation.

Fructose is a ketohexose, also D-(–)-fructose (note: D configuration but laevorotatory). Its C-2 keto group is attacked by the C-5 –OH, so fructose commonly forms a five-membered furanose ring. Despite being a ketose, fructose reduces Tollens' and Fehling's reagents, because in the alkaline medium of those reagents it tautomerises to an aldose.

Disaccharides and Polysaccharides

A disaccharide forms when the anomeric –OH of one unit condenses with an –OH of another, giving a glycosidic linkage (an oxide bridge).

• Sucrose = α-D-glucose + β-D-fructose, joined C-1 → C-2. Both anomeric carbons are used up, so sucrose is non-reducing. Sucrose is dextrorotatory (+66.5°); on hydrolysis it yields glucose (+52.5°) and fructose (–92.4°), and since fructose rotates more strongly, the mixture is laevorotatory (net ≈ –39.9°). This sign reversal is why the product is called invert sugar and the process inversion of cane sugar.
• Maltose = two α-D-glucose units, C-1 → C-4. One anomeric carbon remains free, so maltose is reducing.
• Lactose (milk sugar) = β-D-galactose + β-D-glucose, C-1 → C-4; reducing.

Polysaccharides are the storage and structural carbohydrates:

• Starch — the reserve carbohydrate of plants; a mixture of amylose (≈15–20%, water-soluble, linear, α-1,4 linked glucose) and amylopectin (≈80–85%, insoluble, branched, α-1,4 chains with α-1,6 branch points).
• Cellulose — the structural material of plant cell walls; a straight chain of D-glucose units joined by β-1,4 linkages. The β linkage is the reason humans cannot digest cellulose.
• Glycogen — "animal starch", stored in liver and muscle; structurally like amylopectin but more highly branched.

Carbohydrates also matter functionally: they are the primary fuel of respiration, they are structural (cellulose in plants, chitin in insects), and they appear in cell-surface glycoproteins and glycolipids involved in recognition.

Amino Acids, Peptides, and Protein Structure

Proteins are polymers of α-amino acids — a carboxyl group and an amino group on the same carbon. Twenty amino acids build all natural proteins. Key classifications:

• Neutral, acidic, basic — depending on whether the numbers of –COOH and –NH₂ groups are equal, or COOH-rich (glutamic, aspartic acid), or NH₂-rich (lysine, arginine).
• Essential vs non-essential — essential amino acids (e.g. valine, leucine, lysine) cannot be synthesised in the body and must come from diet; non-essential ones (e.g. glycine, alanine) can be made in the body.

Amino acids exist as internal salts or zwitterions (⁺NH₃–CHR–COO⁻), which explains their high melting points, water solubility, and negligible solubility in non-polar solvents. In acid they behave as cations, in base as anions — hence they are amphoteric. The pH at which the amino acid carries no net charge and does not migrate in an electric field is its isoelectric point. All natural α-amino acids except glycine are chiral and optically active; the natural ones are L-amino acids.

Amino acids link through peptide bonds (–CO–NH–), formed by loss of water between the –COOH of one and the –NH₂ of the next. Two residues give a dipeptide, many give a polypeptide; when the chain is long and has a definite biological function it is called a protein. Proteins are further divided into fibrous (chains held side by side by hydrogen bonds and disulphide bridges; insoluble in water; keratin, myosin, collagen — usually structural) and globular (chains coiled into compact spheres; water-soluble; insulin, albumin, haemoglobin — usually functional).

Four levels of structure:

1. Primary — the exact sequence and linkage order of amino acids. Changing it changes the protein entirely.
2. Secondary — the local regular shape adopted by the backbone, stabilised by hydrogen bonds between the C=O and N–H groups: the α-helix (intramolecular H-bonding, right-handed coil) and the β-pleated sheet (chains laid side by side, intermolecular H-bonding, stretched and "pleated").
3. Tertiary — the overall three-dimensional folding of the whole chain into fibrous or globular form; stabilised by hydrogen bonds, disulphide bridges, van der Waals forces and electrostatic interactions.
4. Quaternary — the way two or more separate polypeptide subunits assemble (haemoglobin's four chains being the standard example).

Denaturation is the loss of secondary and tertiary structure — and hence of biological activity — when a protein is heated, or exposed to acid, alkali, alcohol or heavy metal ions. The primary structure (the sequence and its peptide bonds) survives. Coagulation of egg white on boiling and curdling of milk are everyday examples. Denaturation is usually irreversible.

Enzymes, Vitamins, and Nucleic Acids

Enzymes are biocatalysts, almost all of them globular proteins, that make biological reactions occur at body temperature and near-neutral pH. Two features define them: they are extraordinarily specific (urease hydrolyses only urea; maltase acts only on maltose) and extraordinarily efficient, increasing rates by factors of 10⁸–10²⁰ by lowering activation energy. Enzymes are typically named after the substrate or reaction with the suffix -ase — sucrase, oxidase, lipase. They work by binding the substrate at an active site whose shape and functional groups complement the substrate.

Vitamins are organic compounds needed in small amounts that the body cannot synthesise (or cannot synthesise sufficiently) and which must be supplied by diet; their absence causes specific deficiency diseases. They are classified by solubility:

• Fat-soluble vitamins — A, D, E, K; stored in the liver and adipose tissue, absorbed along with dietary fats, and capable of accumulating to toxic levels if oversupplied. Vitamin A deficiency causes night blindness; D deficiency causes rickets in children and osteomalacia in adults; K deficiency impairs blood clotting.
• Water-soluble vitamins — B-complex and C; not stored in significant amounts (with the partial exception of B₁₂) and must be supplied regularly, but excess is simply excreted in urine rather than accumulating to toxic levels. Vitamin C deficiency causes scurvy (bleeding gums, poor wound healing); B₁ (thiamine) deficiency causes beriberi; B₂ (riboflavin) deficiency causes cracked lips and an inflamed tongue; niacin deficiency causes pellagra.

Nucleic acids are the polymeric information-carriers of the cell, built of repeating nucleotide units. Each nucleotide has three parts: a nitrogenous base, a pentose sugar, and a phosphate group. The base attaches to C-1′ of the sugar (giving a nucleoside, without phosphate), and the phosphate esterifies the sugar's 5′-OH (giving the complete nucleotide). Two sugar types distinguish the two nucleic acids: deoxyribose (lacking the 2′-OH) in DNA, and ribose in RNA.

Nitrogenous bases fall into two chemical classes:

• Purines — double-ringed: adenine (A) and guanine (G), common to both DNA and RNA.
• Pyrimidines — single-ringed: cytosine (C), common to both; thymine (T), found only in DNA; uracil (U), found only in RNA (replacing thymine).

Successive nucleotides are joined by 3′–5′ phosphodiester bonds, giving a sugar–phosphate backbone with the bases projecting outward. In the DNA double helix, two antiparallel strands are held together by hydrogen bonding between complementary base pairs: A pairs with T via two hydrogen bonds, and G pairs with C via three hydrogen bonds — the extra hydrogen bond is why GC-rich DNA is more thermally stable. This strict complementarity is the chemical basis of DNA replication and of the genetic code. RNA is usually single-stranded and comes in three main functional forms — messenger RNA (carries the genetic message), ribosomal RNA (structural/catalytic component of ribosomes) and transfer RNA (adaptor molecule during translation).

Common Mistakes and Exam Traps

1. Reversing which sugars are reducing. Sucrose is non-reducing because both anomeric carbons (of glucose and fructose) are tied up in the glycosidic bond; maltose and lactose retain one free anomeric carbon and are reducing — students often assume all disaccharides behave alike.
2. Confusing D/L configuration with the sign of optical rotation. D and L refer only to spatial configuration at the reference carbon (relative to glyceraldehyde), while (+) and (−) refer purely to the observed direction of rotation — D-fructose is laevorotatory, i.e. D-(−)-fructose, which surprises students expecting D to always mean dextrorotatory.
3. Losing track of what denaturation destroys. Denaturation disrupts secondary, tertiary and quaternary structure (and hence biological activity) but leaves the primary structure — the peptide-bonded amino acid sequence — intact; claiming denaturation "breaks peptide bonds" is a common but incorrect exam answer.
4. Mixing up which base is missing from which nucleic acid. DNA contains thymine but not uracil; RNA contains uracil but not thymine. Both contain adenine, guanine and cytosine, so the exam trap is specifically about the T/U pair.