Biomolecules — Class 12 Chemistry Notes & Practice

Take a breath. If the word biomolecules makes you picture a scary wall of ring structures, you are not alone — almost every Class 12 student feels that on day one. Here is the good news: this chapter is one of the most scoring units in the whole paper. It is worth 7 marks in the CBSE 2026–27 Chemistry syllabus (Unit 10), and almost every mark comes from clear definitions, neat comparisons and a handful of reactions you can genuinely memorise in a weekend. There is very little heavy calculation. If you can describe things carefully, you can score here.

Think of your body as a busy kitchen. Carbohydrates are the fuel on the stove, proteins are the utensils and workers, enzymes are the specialised chefs who only cook one dish each, vitamins are the tiny pinches of spice you cannot live without, nucleic acids are the recipe book locked in the safe, and hormones are the notes the manager slips under the door telling everyone to speed up or slow down. Every heading in this chapter is just one of those characters. Once you meet them properly, they stop being scary.

This page is a full biomolecules class 12 notes with solved examples guide. We build every idea from zero, then practise it. You will find a compact biomolecules class 12 formula and reaction list woven into the sections, seventeen worked model answers written the way an examiner wants to see them, three hand-drawn schematic diagrams, and a worksheet of biomolecules class 12 chemistry important questions with hidden answers so you can test yourself honestly. Work through it slowly. You do not need to finish it in one sitting.

What You’ll Learn

Your Game Plan

Do not read this chapter front to back in one go. Follow this order instead — it is designed so each step makes the next one easier.

1. Day 1 — Carbohydrates. Learn the classification tree first, then glucose, then the disaccharides and polysaccharides. This is roughly half the unit.
2. Day 2 — Proteins. Amino acids → zwitterion → peptide bond → the four structural levels → denaturation. Each idea sits on the one before it.
3. Day 3 — Enzymes and Vitamins. Short, definition-heavy and very examinable. Make a deficiency-disease table in your own handwriting.
4. Day 4 — Nucleic acids and hormones. Learn DNA vs RNA as a table, not as paragraphs.
5. Day 5 — Practice only. Close the notes, attempt the worksheet at the bottom of this page, then check. Rewrite any answer you got wrong, in full.

Study Notes

Carbohydrates — Classification and Structure

Start with the name. Old chemists noticed that many sugars had the formula C_x(H₂O)_y — carbon plus what looked like water — so they called them carbohydrates, meaning “hydrates of carbon”. The name stuck even though it is not really accurate (rhamnose and deoxyribose break the pattern, and acetic acid fits the pattern without being a sugar). What actually defines a carbohydrate chemically is much tidier: a carbohydrate is a polyhydroxy aldehyde or a polyhydroxy ketone, or a compound that gives one on hydrolysis. That single sentence is worth writing on the inside cover of your notebook.

From there, classification is just a question of “how many sugar units do you get when you boil it with dilute acid?”. Get none extra (it will not break down) → monosaccharide. Get two to about ten → oligosaccharide, of which the two-unit ones, the disaccharides, are the only ones the board cares about. Get very many → polysaccharide. That is the entire tree in the diagram above.

There is a second, cross-cutting classification you must also know, and it is the one students most often confuse with the first. Based on reducing power, sugars are either reducing or non-reducing. A reducing sugar has a free aldehyde or free ketone group (a free hemiacetal carbon), so it can reduce Tollens’ reagent to a silver mirror and Fehling’s solution to a red-brown Cu₂O precipitate. All monosaccharides are reducing. Among disaccharides, maltose and lactose are reducing; sucrose is not, because both of its anomeric carbons are tied up in the linkage. If you have already met these tests in the oxidation and reduction language used in Electrochemistry, the logic will feel familiar — the sugar is the reducing agent, the metal ion is reduced.

Naming, too, is systematic and free marks. Count the carbons: triose (3), tetrose (4), pentose (5), hexose (6). Look at the carbonyl: aldehyde → aldo-, ketone → keto-. Glue them together. Glucose has six carbons and an aldehyde, so it is an aldohexose. Fructose has six carbons and a ketone, so it is a ketohexose. Ribose has five carbons and an aldehyde: aldopentose. That is all there is to it.

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Monosaccharides: Glucose and Fructose

Glucose is the single most important molecule in this chapter. It is the sugar your blood carries, the sugar plants make in photosynthesis, and the building block of starch, cellulose, glycogen, sucrose, maltose and lactose. Learn glucose properly and half the unit falls into place.

How it is prepared. Two routes are in the syllabus. From sucrose: boil cane sugar with dilute HCl or H₂SO₄ in alcohol and you get equal amounts of glucose and fructose — C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose). Glucose is less soluble in alcohol, so it crystallises out first. From starch: hydrolyse starch with dilute H₂SO₄ at about 393 K under roughly 2–3 atm pressure — (C₆H₁₀O₅)_n + n H₂O → n C₆H₁₂O₆. Remember the conditions; they are frequently asked as a one-mark fill-in.

How chemists proved the open-chain structure. This is a lovely piece of detective work and it is very examinable, because each clue is a separate mark. Glucose adds one molecule of hydroxylamine to give a monoxime and one molecule of HCN to give a cyanohydrin → there is exactly one carbonyl group. It gives a silver mirror with Tollens’ reagent and a red precipitate with Fehling’s solution → that carbonyl is an aldehyde. Acetylation with acetic anhydride gives a pentaacetate → there are five –OH groups, and since the molecule is stable they must be on five different carbons. Prolonged heating with HI gives n-hexane → the six carbons are in a straight, unbranched chain. Mild oxidation with bromine water gives gluconic acid (six carbons, one –COOH); strong oxidation with nitric acid gives saccharic acid (six carbons, two –COOH, one at each end), confirming the aldehyde sits at C–1 and a primary alcohol at C–6.

Where the open chain fails. Three stubborn facts refuse to fit: glucose does not give the 2,4-DNP (Schiff’s) test, does not form a hydrogensulphite addition product with NaHSO₃, and the pentaacetate does not react with hydroxylamine — all three suggest no free –CHO. Worse, glucose exists as two crystalline forms, α-D-glucose (m.p. 419 K, specific rotation +111°) and β-D-glucose (m.p. 423 K, +19.2°), whose solutions both drift to +52.5° on standing. That drift is called mutarotation. The resolution is the cyclic hemiacetal: the –OH on C–5 attacks the C–1 aldehyde, closing a six-membered pyranose ring and creating a new stereocentre at C–1 called the anomeric carbon. The two ring forms differing only at that carbon are anomers.

Fructose is glucose’s partner: same molecular formula C₆H₁₂O₆, same molar mass, but a ketone at C–2 instead of an aldehyde at C–1. It cyclises to a five-membered furanose ring. Here is the fact that trips everyone up: fructose still reduces Tollens’ and Fehling’s reagents even though it is a ketose. Why? Because those reagents are alkaline, and in base fructose isomerises (via an enediol) into glucose and mannose, which then reduce the reagent. So “reducing sugar” does not mean “must be an aldose”.

Because glucose and fructose have identical molar masses, any technique that measures molar mass — freezing-point depression, osmotic pressure and the other colligative properties you practised in the Class 12 Chemistry chapter on Solutions — cannot tell them apart. Only chemistry can.

Diagram: Fischer and Haworth projections of α- and β-D-glucopyranose with full stereochemistry — to be added by illustrator.

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Disaccharides: Sucrose, Maltose and Lactose

Join two monosaccharides by squeezing out one molecule of water between their –OH groups and you get a disaccharide. The oxygen bridge left behind is called a glycosidic linkage. Three of them are in your syllabus, and the cleanest way to remember them is a single line each: sucrose = glucose + fructose, maltose = glucose + glucose, lactose = glucose + galactose. All three share the formula C₁₂H₂₂O₁₁ and the molar mass 342.30 g mol^–1.

Sucrose (table sugar) is the odd one out and the board knows it. Its linkage runs C–1 of α-glucose to C–2 of β-fructose, which are the anomeric carbons of both units. With no free anomeric carbon left, neither ring can open, so sucrose is non-reducing. It also shows the beautiful phenomenon of inversion: sucrose itself is dextrorotatory (+66.5°), but on hydrolysis it gives glucose (+52.5°) and fructose (–92.4°), and because fructose rotates further to the left than glucose does to the right, the mixture comes out laevorotatory (–39.9°). The sign of rotation literally flips, which is why the product is called invert sugar.

Maltose (from starch digestion) joins C–1 of one α-D-glucose to C–4 of the next: an α-1,4-glycosidic linkage. The second unit’s anomeric carbon is free, so maltose is reducing. Lactose (milk sugar) joins C–1 of β-D-galactose to C–4 of D-glucose: a β-1,4-glycosidic linkage. Again a free anomeric carbon on the glucose unit, so lactose is reducing too.

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Polysaccharides: Starch, Cellulose and Glycogen

Now string hundreds or thousands of glucose units together. All three polysaccharides in your syllabus are made from the same monomer — glucose — and yet one is soft food, one is wood and cotton, and one is your body’s emergency fuel tank. The whole difference is in the type of linkage and the amount of branching. That is a genuinely beautiful chemistry lesson: same bricks, different mortar, completely different building.

Starch is the plants’ store cupboard, found in rice, wheat, potato and maize. It is a mixture of two fractions. Amylose is about 15–20 % of starch, is water-soluble, and is a long unbranched chain of 200–1000 α-D-glucose units linked C–1 to C–4. Amylopectin is the other 80–85 %, is water-insoluble, and is branched: mainly α-1,4 chains with an α-1,6 branch roughly every 20–25 units. Amylose is the fraction that gives the deep blue-black colour with iodine, because its helix traps I₂ molecules inside.

Cellulose is the most abundant organic polymer on Earth — cotton is about 90 % cellulose, wood roughly half. It is a straight, unbranched chain of β-D-glucose units joined C–1 to C–4. Those β-linkages make flat ribbons that lie side by side and hydrogen-bond to one another, which is why cellulose is tough, insoluble and fibrous. Humans have no cellulase enzyme, so we cannot digest it — it passes through as dietary fibre. Cows and termites manage only because gut microbes do the job for them.

Glycogen is “animal starch”, stored in liver and muscle. Structurally it is like amylopectin but much more highly branched. That matters practically: more branches means more free chain ends, and enzymes chew glucose off the ends, so glycogen can be mobilised into blood glucose very quickly when you sprint or when blood sugar falls.

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Amino Acids: Structure, Zwitterion and Classification

Proteins are polymers, and their monomers are α-amino acids. The general formula is short enough to sketch in three seconds: a central carbon carrying –NH₂, –COOH, –H and a side chain R, written R–CH(NH₂)–COOH. “α” simply means the amino group sits on the carbon immediately next to the carboxyl group. Only the R group changes from one amino acid to the next, and there are about twenty R groups in nature. Twenty letters; every protein you own is a word spelled with them.

Classification by R group. If R is a plain hydrocarbon or otherwise neutral, the amino acid is neutral (glycine, alanine, valine). If R carries an extra –COOH, it is acidic (aspartic acid, glutamic acid). If R carries an extra basic nitrogen, it is basic (lysine, arginine). Count the acidic and basic groups in the whole molecule — whichever wins decides the label.

Essential versus non-essential. Ten of the twenty cannot be made by the human body at all and must come from the diet — these are the essential amino acids (valine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, histidine, arginine). The other ten the body synthesises for itself and they are called non-essential. Note carefully: “non-essential” does not mean unimportant, it means “you do not have to eat it”. That distinction is a favourite one-mark trap.

The zwitterion — the single most examined idea here. An amino acid contains an acid (–COOH) and a base (–NH₂) in the same molecule, so the proton simply hops across internally. The result is the zwitterion (also called the dipolar or internal salt form): R–CH(NH₃⁺)–COO^–. Because the molecule is really a salt, amino acids behave like ionic solids — they are crystalline, have unexpectedly high melting points (they usually decompose above 470 K rather than melt cleanly), are soluble in water but poorly soluble in organic solvents, and are amphoteric: they neutralise both acids and bases. If you have seen how a substance can react with both an acid and a base in the Class 10 Science chapter on Acids, Bases and Salts, the amphoteric behaviour here is exactly the same idea, just carried by one molecule instead of two.

The pH at which the amino acid exists entirely as the zwitterion — net charge zero, so it does not migrate to either electrode in an electric field — is called the isoelectric point (pI). Below pI the molecule picks up a proton and becomes a cation, so it migrates to the cathode; above pI it loses a proton and becomes an anion, so it migrates to the anode.

One more structural point: except for glycine (R = H), the α-carbon of every amino acid carries four different groups, so every amino acid except glycine is chiral and optically active. Naturally occurring amino acids are almost all of the L-configuration.

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Proteins: Peptide Bond and Primary to Quaternary Structure

Two amino acids can join by losing a molecule of water between the –COOH of one and the –NH₂ of the other. The –CO–NH– link created is a peptide bond, and it is nothing exotic — it is simply an amide bond. Two units give a dipeptide, three a tripeptide, many a polypeptide. When a polypeptide reaches a molar mass of roughly 10 000 g mol^–1 or more and takes up a definite biological shape, we call it a protein.

Proteins split into two families by shape and behaviour. Fibrous proteins have chains lying parallel and held by hydrogen and disulphide bonds; they are tough, insoluble in water and structural — keratin in hair and nails, myosin in muscle, collagen in tendons. Globular proteins have chains coiled into compact balls, are soluble in water, and do the chemistry — insulin, haemoglobin, albumin and all enzymes.

The four structural levels in the diagram are the classic five-mark question, so learn them as a story rather than a list. Primary structure is the order of the amino acids — the spelling of the word. Change one letter and you can change everything; sickle-cell haemoglobin differs from normal haemoglobin by a single amino acid. Secondary structure is how short stretches of that chain curl up locally, into a right-handed α-helix (hydrogen bonds run along the coil, as in wool keratin) or a β-pleated sheet (chains stretched side by side and hydrogen-bonded across, as in silk fibroin). Tertiary structure is the overall three-dimensional fold of the whole chain, stabilised by disulphide bridges, ionic (salt) bridges, hydrogen bonds, van der Waals forces and hydrophobic interactions. Quaternary structure appears only when two or more folded chains club together into one working unit — haemoglobin is four subunits, which is why it can carry four oxygen molecules.

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Denaturation of Proteins

Crack an egg into a hot pan. The clear, runny white turns opaque, white and solid, and no amount of cooling will turn it back. You have just watched denaturation, and the kitchen is the best laboratory you have for this topic.

Definition to memorise word for word: denaturation is the process in which a protein loses its secondary, tertiary and quaternary structure — its biological activity along with it — while the primary structure remains intact, when it is subjected to physical or chemical stress such as heat, strong acid or alkali, heavy-metal salts, alcohol, urea or ultraviolet radiation.

The mechanism follows straight from the Key Rule you just read. The higher levels are held by weak forces; heat supplies enough energy to shake those hydrogen bonds and salt bridges apart, and acid or alkali changes the charges on the side chains so the ionic bridges no longer attract. The globular protein uncoils into a random tangle, exposes its water-hating interior, and clumps together — which is exactly why the coagulated protein looks cloudy and becomes insoluble. But the peptide bonds are covalent and far too strong to be affected, so the sequence survives.

Everyday examples worth quoting in the exam: boiling an egg (soluble globular albumin becomes insoluble solid), curdling of milk when lactic acid is produced, using alcohol as a disinfectant (it denatures the proteins of bacteria), and the medical use of an egg-white or milk drink as a first-aid antidote in heavy-metal poisoning — the metal ion binds the food protein rather than the patient’s own enzymes. In most laboratory cases denaturation is irreversible.

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Enzymes: Nature, Specificity and Mechanism

An enzyme is a biocatalyst — a substance produced by a living cell that speeds up a biochemical reaction without being used up. Chemically, almost all enzymes are globular proteins. They are the reason your body can do at 310 K and pH 7 what a chemistry laboratory would need high temperature and strong acid to achieve.

Naming. Take the substrate or the reaction and add -ase. Maltase hydrolyses maltose, urease hydrolyses urea, oxidase carries out oxidation, invertase inverts sucrose. If a question says “name the enzyme that hydrolyses lactose”, the answer is almost always sitting inside the question.

Specificity is the headline property. Enzymes are extraordinarily fussy. Urease hydrolyses urea and nothing else; maltase attacks the α-glycosidic bond of maltose but ignores the β-bond of cellulose. The standard picture is the lock-and-key model: the substrate fits a specially shaped cavity on the enzyme surface called the active site, much as one key fits one lock. Substrate binds to form an enzyme–substrate complex, the strained bonds react, and the products leave, freeing the enzyme to go round again. In modern language the fit is a little flexible (induced fit), but for CBSE the lock-and-key picture is what is wanted.

How they work, in one line: an enzyme lowers the activation energy of the reaction by providing an alternative path through the enzyme–substrate complex — it does not change the position of equilibrium or the enthalpy of reaction, only the speed. Enzymes are also astonishingly efficient, raising rates by factors of 10⁶ to 10²⁰, and they need only tiny amounts. Being proteins, they work best in a narrow band of temperature (around 310 K in humans) and pH (usually 5–7), and outside that band they denature and stop working — which is why a high fever is dangerous.

Diagram: realistic enzyme active-site and induced-fit rendering — to be added by illustrator.

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Vitamins: Classification and Deficiency Diseases

Vitamins are organic compounds required in very small amounts in the diet, which the body cannot synthesise (or cannot make in sufficient quantity), and whose absence causes a specific deficiency disease. They are not fuel and they are not building material — think of them as the tiny screws that hold a large machine together. A few grams a year can be the difference between health and blindness.

Two honest exceptions are worth knowing because examiners like them: vitamin D can be made in the skin under sunlight, and vitamin K and some B vitamins are produced by intestinal bacteria. So “the body cannot make them at all” is a slight over-statement — write “cannot synthesise them in adequate amounts” instead.

The classification is based on solubility, and this single idea explains almost every difference between the two groups. Fat-soluble vitamins A, D, E and K dissolve in fats and oils, get stored in the liver and adipose tissue, need not be eaten daily — and, because they accumulate, can actually be toxic in excess. Water-soluble vitamins — the B group and C — are not stored (vitamin B₁₂ being the exception), are excreted in urine, and must therefore be supplied regularly in the diet.

Now the deficiency table. Do not try to understand it — just drill it. Two marks in the paper are almost guaranteed and they are the easiest two marks in the unit.

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Nucleic Acids: DNA and RNA Structure

Nucleic acids are the instruction manuals of life, and the chemistry is more orderly than the biology makes it sound. Build one from the bottom up in three steps and you will never be confused again.

Step 1 — the nucleoside. Take a pentose sugar and attach a nitrogen base to its C–1 by a β-glycosidic linkage. Sugar + base = nucleoside. Step 2 — the nucleotide. Now attach a phosphoric acid unit to the C–5 –OH of that sugar. Sugar + base + phosphate = nucleotide. Step 3 — the polymer. Link nucleotides through phosphodiester bridges running from C–3 of one sugar to C–5 of the next, and you have a polynucleotide, which is a nucleic acid.

The bases. There are two structural families. Purines have a fused two-ring system: adenine (A) and guanine (G). Pyrimidines have a single ring: cytosine (C), thymine (T) and uracil (U). DNA uses A, G, C and T. RNA uses A, G, C and U — uracil takes thymine’s place.

The Watson–Crick double helix. DNA is two polynucleotide strands wound round a common axis, running in opposite directions, with the bases turned inwards and paired by hydrogen bonds. The pairing is strict: adenine always pairs with thymine through two hydrogen bonds, and guanine always pairs with cytosine through three hydrogen bonds. Notice the design logic — a big purine always pairs with a small pyrimidine, so the ladder keeps a constant width all the way down. This complementarity is what makes replication possible: unzip the ladder and each half tells you exactly how to rebuild the other. That is, at the chemical level, the same faithful copying of information you studied as inheritance in the Class 10 Science chapter on Heredity.

What each does. DNA stores the genetic information and passes it to daughter cells; RNA carries it out. Three RNAs are worth naming: messenger RNA (m-RNA) carries the message from DNA to the ribosome, ribosomal RNA (r-RNA) forms the ribosome where the protein is assembled, and transfer RNA (t-RNA) ferries individual amino acids into place. The overall flow — DNA makes RNA makes protein — means the proteins section you read earlier and this section are really one story.

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Hormones: Types and Functions

If enzymes are the workers, hormones are the management memos. A hormone is a molecule produced in an endocrine (ductless) gland and released straight into the bloodstream, which carries it to a distant target organ where it triggers a response. They act as intercellular messengers, work at extremely low concentration, and are continuously destroyed and replaced — which is exactly why the body can turn a signal off as well as on.

Chemical classification — three families, and the board asks for these names:

• Steroid hormones — built on the four-ring steroid skeleton. Examples: testosterone (male sex hormone), estradiol and other estrogens (female sex hormones), cortisone and the glucocorticoids.
• Polypeptide and protein hormones — chains of amino acids. Examples: insulin, glucagon, oxytocin, vasopressin and growth hormone.
• Amino-acid-derived hormones — small molecules made by modifying a single amino acid. Examples: adrenaline (epinephrine), noradrenaline and thyroxine.

Functions you should be able to quote. Insulin, from the pancreas, lowers blood glucose by promoting its uptake and storage as glycogen; its deficiency causes diabetes mellitus. Glucagon, also from the pancreas, does the opposite and raises blood glucose — the pair works like an accelerator and a brake, keeping the level roughly steady. Thyroxine, from the thyroid, contains iodine and controls the basal metabolic rate; too little iodine in the diet causes hypothyroidism and goitre, while an excess of thyroxine causes hyperthyroidism. Adrenaline and noradrenaline, from the adrenal medulla, are the stress hormones — they raise heart rate, blood pressure and blood glucose in the classic “fight or flight” response. Testosterone and estradiol control the development of male and female secondary sexual characters respectively.

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Quick Revision Table for Last-Minute Study

On the morning of the paper you will not have time to re-read this page. Read only this table. It is the whole unit compressed into the answers examiners actually award marks for.

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Practice Worksheet with Answers

Close the notes now. Attempt all ten questions on paper first, in full sentences, with the marks in mind — then open the answers. Marking your own work honestly is worth more than reading the chapter a second time. These are original questions written in the style of biomolecules class 12 chemistry important questions, covering the whole 7-mark unit.

Score yourself out of 28. Anything above 20 on a first honest attempt means this unit is already safe for you.

One last thought before you close this page. Biomolecules is not a chapter you conquer in a single heroic night; it is a chapter you win by small, boring, daily improvement. Learn one more deficiency disease than you knew yesterday. Write one more model answer than you wrote yesterday. Aim for one more correct answer than yesterday — do that for two weeks and the 7 marks stop being a worry and start being a certainty. That is kaizen, and it works.

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