Biomolecules

Every mechanism met across this entire thread, nucleophilic substitution, nucleophilic addition, condensation, esterification, is quietly running right now inside every living cell, built from nothing more exotic than the carbon, hydrogen, oxygen and nitrogen already met a hundred times over. Carbohydrates fit a deceptively simple-looking definition, optically active polyhydroxy aldehydes or ketones, or compounds that yield such units on hydrolysis, but the definition has to be a functional one rather than a formula-matching one: acetic acid, CH3COOH, happens to fit the general carbohydrate formula Cx(H2O)y just as neatly as glucose does, yet it is not a carbohydrate at all, while rhamnose, C6H12O5, is a genuine carbohydrate despite not fitting the formula. This chapter follows five distinct biomolecule families in turn, carbohydrates, proteins, enzymes, vitamins and nucleic acids, plus hormones as intercellular messengers built from several of those same families, and the recurring discovery across all five is that the chemistry is never actually new, only newly applied.

Carbohydrates split into three groups purely by their own behaviour on hydrolysis: a monosaccharide cannot be hydrolysed into anything simpler, roughly twenty occurring in nature, glucose, fructose and ribose among the most common; an oligosaccharide yields two to ten monosaccharide units, disaccharides, yielding exactly two, being by far the most common case, and those two units may be identical or different, sucrose splitting into glucose plus fructose while maltose splits into two glucose molecules alone; and a polysaccharide yields a genuinely large number of monosaccharide units, starch, cellulose and glycogen among the common examples, none of them sweet, which is why polysaccharides are also called non-sugars. A separate, functionally useful classification cuts across all three groups: a reducing sugar carries a free aldehyde or ketone group able to reduce Fehling's solution and Tollens' reagent, every monosaccharide qualifying regardless of whether it is an aldose or a ketose, while a sugar whose reactive groups are tied up in a linkage to another sugar cannot reduce either reagent and counts as non-reducing instead.

Glucose's own open-chain structure was pieced together entirely from how it reacts, six separate lines of evidence converging on one answer. Its molecular formula, C6H12O6, was established first; prolonged heating with hydrogen iodide reduced it all the way to n-hexane, proving all six carbons sit in one straight, unbranched chain; reaction with hydroxylamine gave an oxime, and addition of hydrogen cyanide gave a cyanohydrin, both confirming a genuine carbonyl group somewhere in the molecule; mild oxidation with bromine water converted glucose specifically to a six-carbon carboxylic acid, gluconic acid, and since bromine water oxidises only an aldehyde and not a ketone, this pinned the carbonyl down as an aldehyde group exactly; acetylation with acetic anhydride gave a stable glucose pentaacetate, confirming five distinct -OH groups, each on its own separate carbon since the compound was stable; and stronger oxidation with nitric acid converted both glucose and gluconic acid to the same dicarboxylic acid, saccharic acid, proving a primary alcohol group sat at the chain's far end, since only a primary -OH oxidises all the way to a second -COOH. Fischer worked out the exact spatial arrangement of every -OH group from there, assigning glucose its full stereochemical structure.

Glucose is correctly named D-(+)-glucose, and the two labels mean genuinely different things: (+) records the molecule's own dextrorotatory optical activity, while D describes its configuration relative to D-(+)-glyceraldehyde, the simplest possible sugar with an asymmetric carbon, comparing specifically the -OH group on glucose's lowest asymmetric carbon against the same position in glyceraldehyde, D and L carrying no necessary relationship to optical rotation direction at all. Yet the open-chain structure this evidence built, however well it explained glucose's reactions with HI, bromine water, and nitric acid, ran straight into a genuine puzzle: glucose fails Schiff's test, fails to form an addition product with sodium hydrogensulphite, and its pentaacetate does not react with hydroxylamine at all, three separate signs the free aldehyde group is somehow simply not there when it should be; and stranger still, glucose crystallises in two genuinely distinct forms, alpha, melting at 419 K, and beta, melting at 423 K, depending only on the crystallisation conditions used, a fact no open-chain structure could ever explain on its own.

Both puzzles resolve the same way: one of glucose's own -OH groups, at C5, folds back and adds to the aldehyde carbon at C1, forming a genuine cyclic hemiacetal, no free -CHO left over at all, which is exactly why Schiff's test, sodium hydrogensulphite addition, and hydroxylamine on the pentaacetate all come back negative, and this six-membered ring, by analogy with the oxygen-containing ring compound pyran, is called a pyranose. C1, the former aldehyde carbon and now a genuinely new stereocentre, is called the anomeric carbon, and the -OH group forming there can point either of two ways, giving alpha-glucose and beta-glucose, anomers, distinguishable only at that one carbon, matching exactly the two crystalline forms already observed, with the open-chain form persisting alongside both in a genuine equilibrium. Fructose, a ketohexose rather than an aldohexose, cyclises the same way but through its own C5-OH adding to the C2 ketone instead, forming a smaller, five-membered ring named furanose after furan; fructose belongs to the D-series structurally, yet rotates light the opposite way, correctly written D-(-)-fructose, D and optical rotation direction once again shown to be genuinely independent facts about the same molecule.

Two monosaccharides join through a glycosidic linkage, an oxide bridge formed by losing one water molecule, and whether the reducing groups of both monosaccharides end up tied into that linkage decides whether the disaccharide itself is reducing or non-reducing. Sucrose, hydrolysing to equimolar D-(+)-glucose and D-(-)-fructose, links glucose's own C1 directly to fructose's C2, the exact two carbons that would otherwise be each sugar's own reducing group, so both are consumed in forming the bond and sucrose itself is non-reducing; and since sucrose's own dextrorotation gets replaced, after hydrolysis, by a net laevorotation, fructose's own strong laevorotation, -92.4 degrees, simply outweighing glucose's dextrorotation, +52.5 degrees, the hydrolysis product is called invert sugar, the sign of rotation genuinely inverting partway through the reaction. Maltose, by contrast, links two alpha-D-glucose units through C1 of one and C4 of the other, leaving the second glucose's own C1 free to reopen into an aldehyde in solution, so maltose remains a reducing sugar; and lactose, milk sugar, links beta-D-galactose's C1 to beta-D-glucose's C4 the same way, its own free glucose C1 likewise making lactose a reducing sugar too.

Starch, the main plant storage polysaccharide and humanity's principal dietary carbohydrate source, is built entirely from alpha-glucose in two distinct forms: amylose, water-soluble and roughly 15 to 20 percent of starch, an unbranched chain of 200 to 1000 units joined purely by C1-C4 linkages, and amylopectin, water-insoluble and the remaining 80 to 85 percent, a branched chain still built from C1-C4 links along the backbone but adding C1-C6 links wherever a branch point occurs. Cellulose, exclusively a plant product and the most abundant organic substance in the entire plant kingdom, uses the very same C1-C4 linkage pattern as amylose, but built from beta-glucose rather than alpha-glucose, that single stereochemical difference alone turning a soluble, digestible energy store into cell-wall structural material tough enough to build wood and cotton from. Glycogen, the animal equivalent, storing glucose in liver, muscle and brain until enzymes break it back down on demand, resembles amylopectin's own branching pattern closely, only more heavily branched still, and the same monomer, glucose, ends up serving completely different biological roles, energy storage, structural support, quick-release reserve, decided entirely by which stereochemistry and which linkage pattern joins it together.

An amino acid carries both an amino group and a carboxyl group at once, and its own name depends on exactly how far apart they sit, alpha, beta, gamma, delta and onward marking the amino group's position relative to the carboxyl carbon, though protein hydrolysis yields exclusively alpha-amino acids, the amino group always on the carbon directly next to -COOH. Most of the twenty naturally occurring amino acids carry trivial names tracing back to their own discovery, glycine named for its own sweet taste, glykos in Greek, tyrosine first isolated from cheese, tyros, and each is conventionally abbreviated to a three-letter or single-letter code for compact reference. A further classification sorts amino acids as acidic, basic, or neutral, purely by the relative count of amino versus carboxyl groups in the side chain, while a separate, dietarily crucial classification splits them into non-essential, the body able to synthesise these on its own, and essential, which the body cannot make at all and must obtain entirely through diet, ten of the twenty falling into this essential category.

An amino acid is colourless, crystalline, water-soluble and unusually high-melting, physical behaviour that reads far more like a salt than like a simple amine or a simple carboxylic acid, and the reason is that both functional groups are present at once, in the very same molecule: in aqueous solution, the carboxyl group loses its own proton while the amino group accepts one, producing a dipolar zwitterion, net neutral overall yet carrying both a genuine positive and a genuine negative charge simultaneously. This dual charge is exactly why amino acids behave amphoterically, reacting readily with both acids and bases, the positively charged ammonium end accepting further base, the negatively charged carboxylate end accepting further acid. Every naturally occurring alpha-amino acid except glycine is optically active, since the alpha-carbon carries four different groups whenever the side chain R is not itself a plain hydrogen, and exists in both D and L forms, though nature overwhelmingly favours the L-configuration, the -NH2 group written on the left in the conventional Fischer projection.

Two amino acids join when one molecule's carboxyl group combines with a second's amino group, losing a water molecule and forming a genuine amide, the peptide bond, -CO-NH-, and the resulting two-amino-acid product is a dipeptide, three a tripeptide, and so on up through polypeptide once the count passes ten; a polypeptide is generally called a protein once it exceeds roughly a hundred amino acid residues and a molecular mass above 10,000 units, though the boundary is genuinely fuzzy, insulin, with only 51 residues, still counting as a protein since it already carries a protein's own well-defined, folded conformation. Proteins sort into two broad shapes: fibrous proteins, their polypeptide chains running parallel and held together by hydrogen and disulphide bonds into an insoluble, fibre-like structure, keratin in hair and wool and myosin in muscle among the examples, and globular proteins, their chains instead coiling into a compact, roughly spherical, water-soluble shape, insulin and the albumins among the examples.

A protein's own structure and shape can be studied at four progressively more complex levels. Primary structure is simply the exact sequence of amino acids along the chain, and changing even one amino acid in that sequence produces a genuinely different protein entirely. Secondary structure describes the shape a long chain folds into locally, most commonly either an alpha-helix, a right-handed screw held together by hydrogen bonds between each amino acid's own -NH and the C=O of the turn just above it, or a beta-pleated sheet, chains stretched nearly flat and laid side by side, held together by hydrogen bonds between neighbouring chains instead, resembling pleated fabric closely enough to earn the name. Tertiary structure is the chain's own overall three-dimensional folding beyond the secondary level, producing the fibrous or globular shape already met, stabilised by hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic attraction all working together; and quaternary structure applies only to proteins built from more than one separate polypeptide chain, describing how those distinct subunits arrange themselves relative to each other, haemoglobin, with its four subunits, a well-known example.

A protein sitting in its native, biologically active, uniquely folded three-dimensional shape can lose that shape entirely under physical stress like a temperature change or chemical stress like a pH shift, either disturbing the hydrogen bonds holding the folded structure together, globular proteins unfolding, helices uncoiling, and biological activity disappearing along with the shape, denaturation. What survives denaturation is worth noting precisely because it reveals something about how the four structural levels actually relate: secondary and tertiary structure are both destroyed, but primary structure, the underlying amino acid sequence itself, remains completely intact throughout, since no peptide bonds actually break, only the weaker hydrogen bonds and other folding forces holding the higher-level shape together. Egg white coagulating into a solid mass on boiling is a familiar, everyday example of exactly this process, and milk curdling, caused by lactic acid produced by bacteria naturally present in milk, is a second, equally ordinary one.

Life depends on a genuine sequence of chemical reactions, digestion, absorption, energy production among them, all proceeding under remarkably mild conditions inside the body, made possible by biocatalysts called enzymes, almost every one of them itself a globular protein. An enzyme is specific in two distinct senses at once: specific to the particular substrate it acts on, and specific to the particular reaction it carries out, and naming reflects both, an enzyme named after its own substrate, maltase catalysing maltose's hydrolysis, or after its own reaction type, an oxidoreductase catalysing coupled oxidation and reduction, the ending -ase marking an enzyme either way. Enzymes work the same fundamental way any chemical catalyst does, by lowering the reaction's own activation energy, though the scale of that reduction can be dramatic: acid-catalysed hydrolysis of sucrose needs an activation energy of 6.22 kilojoules per mole, while the enzyme sucrase manages the identical hydrolysis at only 2.15 kilojoules per mole, a striking illustration of exactly how much more efficient a well-matched biological catalyst can be than an ordinary chemical one.

Vitamins are organic compounds needed only in small dietary amounts, yet their absence causes specific, well-defined diseases; most cannot be synthesised in the body at all, though plants generally can make them and gut bacteria contribute some as well, and even an excess of vitamins carries real risk, which is why supplement use is best guided by medical advice rather than assumption. The name itself has a small history worth knowing: coined from vital plus amine, since the earliest vitamins identified genuinely contained amino groups, the final e was later dropped once further vitamins turned out not to contain amino groups at all. Vitamins sort into two groups purely by solubility: fat-soluble vitamins, A, D, E and K, store in the liver and in adipose tissue, while water-soluble vitamins, the B-group and C, cannot be stored this way at all, vitamin B12 the sole exception, and must instead be supplied regularly through diet since the body simply excretes any surplus in urine. Each vitamin's own deficiency disease is specific and traceable: vitamin A's absence causes night blindness, vitamin C's causes scurvy, vitamin D's causes rickets in children, and vitamin K's absence lengthens blood clotting time measurably.

Complete hydrolysis of either nucleic acid yields the same three ingredients in different specific forms: a pentose sugar, phosphoric acid, and nitrogen-containing heterocyclic bases, DNA's own sugar being beta-D-2-deoxyribose and RNA's being beta-D-ribose, one oxygen atom the entire difference between them. Both nucleic acids share three of their four bases outright, adenine, guanine and cytosine, but diverge on the fourth: DNA carries thymine, RNA carries uracil instead. A base attached to a sugar's own 1' position makes a nucleoside; a nucleoside further linked to phosphoric acid at the sugar's 5' position makes a nucleotide, and nucleotides link to each other through a phosphodiester bond bridging the 5' and 3' carbons of successive sugars, building the full chain, base sequence along that chain constituting the molecule's own primary structure. Watson and Crick's own secondary-structure discovery, the double helix, describes two such chains wound around each other, held together by hydrogen bonds forming only between specific base pairs, adenine exclusively with thymine and cytosine exclusively with guanine, making the two strands genuinely complementary rather than identical; RNA, by contrast, exists as a single strand, sometimes folding back on itself, and comes in three functionally distinct types, messenger RNA, ribosomal RNA and transfer RNA, each carrying out a different role in the cell's own protein-building machinery.

Hormones act as intercellular messengers, produced by endocrine glands and poured directly into the bloodstream, which carries them to wherever they act, and they draw on every biomolecule family this chapter has covered: some are steroids, estrogens and androgens among them; some are polypeptides, insulin and endorphins among them; and some are amino acid derivatives, epinephrine and norepinephrine among them. Insulin and glucagon work as a genuine matched pair, insulin released when blood glucose rises to bring the level back down and glucagon pulling the level back up when it falls too far, together holding blood glucose within a narrow, healthy range; thyroxine, an iodinated derivative of the amino acid tyrosine, keeps metabolism running at its proper pace, too little causing hypothyroidism, lethargy and weight gain, too much causing hyperthyroidism instead, and iodine deficiency specifically causing hypothyroidism alongside a visibly enlarged thyroid gland, a public-health problem addressed directly by adding sodium iodide to ordinary table salt. Steroid hormones from the adrenal cortex and the gonads round out the picture: glucocorticoids manage carbohydrate metabolism and the body's own response to stress, mineralocorticoids manage water and salt excretion by the kidney, and testosterone, estradiol and progesterone govern male and female secondary characteristics and the reproductive cycle respectively, every one of these signals ultimately just another small molecule, built from the same carbon chemistry this entire thread has followed from Class 10 all the way through to here.