Biomolecules Class 12 Notes for CBSE Board Revision

These biomolecules class 12 notes cover the complete CBSE Unit 10 — carbohydrates, proteins, enzymes, vitamins, nucleic acids and hormones — in the teaching order a teacher would follow. Every definition, table and reaction here is grounded in the rationalised NCERT Class 12 Chemistry Part II textbook.

Revise in one pass: skim the overview, drill the disaccharide, vitamin and DNA-versus-RNA tables, then attempt the two solved examples before checking the common-mistakes table. The exam notes near the end tell you which details earn marks in board answers.

For the rest of the syllabus, pair this page with the Class 12 Chemistry notes hub and the Class 12 notes index.

Reference: NCERT Class 12 Chemistry textbook, chapter Biomolecules.

Biomolecules Class 12 Notes: The Six Families to Master

Biochemistry is the study of what goes on chemically inside a living system. This unit surveys the six families of biomolecules that make up the molecular logic of life. Each family below is developed in the same four steps: definition → classification → structure → function.

• Carbohydrates — optically active polyhydroxy aldehydes or ketones; the body’s main energy food.
• Proteins — polymers of alpha-amino acids joined by peptide bonds; the basis of body structure and function.
• Enzymes — biocatalysts, almost all globular proteins, that speed up reactions under mild conditions.
• Vitamins — essential organic food factors required in small amounts.
• Nucleic acids — DNA and RNA; carry heredity and direct protein synthesis.
• Hormones — intercellular messengers produced by endocrine glands.

Carbohydrates: Definition and the Three-Way Classification

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis (NCERT, p. 281). Sweet ones are called sugars; the common home sugar is sucrose and the sugar in milk is lactose.

The old name “hydrate of carbon” came from the general formula \( \mathrm{C}_x(\mathrm{H}_2\mathrm{O})_y \), but the formula is only a hint, not the definition.

The formula trap: acetic acid and rhamnose

Two examples break the hydrate formula as a definition. Acetic acid \( \mathrm{CH}_3\mathrm{COOH} \) fits \( \mathrm{C}_2(\mathrm{H}_2\mathrm{O})_2 \), yet it is not a carbohydrate — it has no polyhydroxy aldehyde or ketone structure. Rhamnose \( \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_5 \) is a genuine carbohydrate but does not fit the formula at all.

The lesson: always test the chemical definition, never just the empirical formula. This is the chapter’s favourite trick question.

On the basis of behaviour on hydrolysis, carbohydrates fall into three groups (NCERT, p. 282):

• Monosaccharides — cannot be hydrolysed further (glucose, fructose, ribose). About 20 occur in nature.
• Oligosaccharides — yield two to ten monosaccharide units on hydrolysis (disaccharides, trisaccharides, and so on).
• Polysaccharides — yield a large number of monosaccharide units (starch, cellulose, glycogen, gums); not sweet, hence called non-sugars.

A monosaccharide with an aldehyde group is an aldose; one with a keto group is a ketose. The carbon count also enters the name, as Table 10.1 shows.

All monosaccharides, whether aldose or ketose, are reducing sugars — they reduce Fehling’s solution and Tollens’ reagent. The full reducing versus non-reducing picture comes with disaccharides.

Glucose: Preparation and Evidence for the Open-Chain Structure

Glucose occurs free in sweet fruits and honey, and in large amounts in ripe grapes. Its formula is \( \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \); it is an aldohexose also called dextrose, and it is the monomer of starch and cellulose (NCERT, p. 283). Two commercial preparations matter for exams:

1. From sucrose: boiling cane sugar with dilute HCl or \( \mathrm{H}_2\mathrm{SO}_4 \) in alcoholic solution gives equal amounts of glucose and fructose (NCERT, p. 282).
2. From starch: hydrolysis of starch with dilute \( \mathrm{H}_2\mathrm{SO}_4 \) at 393 K under 2–3 atm pressure gives glucose.

The open-chain structure was fixed by six pieces of evidence. The product name in each case is the exam-winning detail:

The name is D-(+)-glucose. ‘D’ is a configuration: the molecule is correlated to D-(+)-glyceraldehyde by comparing the lowest asymmetric carbon, whose –OH lies on the right. ‘(+)’ is optical rotation: the molecule turns plane-polarised light to the right. ‘D’ and ‘L’ have no relation to optical activity — D-fructose, for example, is laevorotatory (NCERT, p. 284).

Cyclic Glucose and Fructose: Anomers, Pyranose and Furanose

The open-chain structure could not explain three facts about glucose (NCERT, p. 285):

• Glucose does not give Schiff’s test and forms no NaHSO\(_3\) addition product, despite having an aldehyde group.
• The pentaacetate of glucose does not react with hydroxylamine — no free –CHO group.
• Glucose exists as two crystalline forms: alpha (m.p. 419 K) and beta (m.p. 423 K).

Explanation: one –OH group at C5 adds across the –CHO, forming a cyclic hemiacetal. The ring has six members, so glucose gets the pyranose name, in analogy with the parent compound pyran. The figure below shows this ring formation.

The anomeric carbon is C1 — the aldehyde carbon before cyclisation. The alpha and beta forms differ only in the –OH configuration at C1, so they are called anomers, and they exist in equilibrium with the open chain. The accurate way to draw these rings is the Haworth structure.

Fructose is a ketohexose with the same formula as glucose, obtained with glucose on hydrolysis of sucrose. Its ketone group sits at C2; the –OH at C5 adds to it, forming a five-membered ring called furanose, named after furan. Fructose belongs to the D-series but is laevorotatory, so it is written D-(-)-fructose; its anomers differ at C2 (NCERT, p. 286).

Disaccharides: Glycosidic Linkages and Reducing Behaviour

A glycosidic linkage is an oxide linkage formed by the loss of a water molecule between two monosaccharide units (NCERT, p. 287). If the reducing groups — aldehydic or ketonic — are bonded in the linkage, the sugar is non-reducing; if they remain free, or can become free, it is reducing.

Invert sugar: sucrose is dextrorotatory, but hydrolysis gives glucose \( (+52.5^{\circ}) \) and fructose \( (-92.4^{\circ}) \). The laevorotation of fructose outweighs the dextrorotation of glucose, so the mixture turns laevorotatory — the sign of rotation inverts, hence the name (NCERT, p. 287).

The table below links reducing behaviour to the two classic reagent tests. “Reduces” means the sugar reduces the reagent, giving the characteristic colour change or precipitate.

Why this pattern? All monosaccharides reduce both reagents. Sucrose is the odd one out because both carbonyl groups are trapped in the glycosidic linkage; maltose and lactose can regenerate a free aldehyde at C1 of the second unit.

Polysaccharides: Starch, Cellulose and Glycogen Compared

Polysaccharides hold a large number of monosaccharide units joined by glycosidic linkages and act mainly as food storage or structural material (NCERT, p. 288).

The alpha versus beta difference has a real consequence: starch (alpha) is our most important dietary source, while cellulose (beta) works as structural material in wood and cotton. Glycogen is the animal reserve — when the body needs glucose, enzymes break glycogen down to glucose.

Real-life application — carbo-loading: because muscles store glycogen as a quick glucose reserve, endurance athletes “carbohydrate-load” before long events to pack their liver and muscles with glycogen. During the event, enzymes release glucose from glycogen on demand — the textbook fact is the athlete’s strategy.

Importance of carbohydrates (NCERT, p. 289):

• Storage molecules — starch in plants, glycogen in animals.
• Cellulose builds the cell wall of plants and bacteria.
• Wood (furniture) and cotton fibre (clothing) are cellulose; it feeds the textiles, paper, lacquer and brewery industries.
• Honey has long been used in Ayurveda as an instant source of energy.

Key Definitions: Carbohydrates to Hormones in One Table

Here is the complete glossary for the chapter, in words you can use directly in short-answer definitions.

Amino Acids: Essential or Non-Essential, and the Zwitter Ion

Proteins are polymers of alpha-amino acids. The general skeleton is \( \mathrm{R}-\mathrm{CH}(\mathrm{NH}_2)-\mathrm{COOH} \), where R is the side chain. Depending on the position of \( -\mathrm{NH}_2 \) relative to \( -\mathrm{COOH} \), amino acids are classified as alpha, beta, gamma and so on — but only alpha-amino acids are obtained on hydrolysis of proteins (NCERT, p. 290).

Amino acids are classified as acidic, basic or neutral by counting groups (NCERT, p. 291):

Essential amino acids cannot be synthesised in the body and must come from the diet — there are ten of them, marked with an asterisk in Table 10.2 (valine, leucine, isoleucine, lysine, and others). Non-essential amino acids can be synthesised in the body.

Amino acids are colourless, crystalline, water-soluble solids with high melting points — they behave like salts, not like simple amines or acids. In water the \( -\mathrm{COOH} \) group loses a proton and the \( -\mathrm{NH}_2 \) group gains one, giving a zwitter ion: a neutral, dipolar ion carrying both charges.

Because of this form, amino acids show amphoteric behaviour, reacting with both acids and bases.

Except glycine, all naturally occurring alpha-amino acids are optically active because the alpha-carbon is asymmetric. Most natural amino acids have the L-configuration, drawn with \( -\mathrm{NH}_2 \) on the left. This is the same functional-group logic you met in the Amines chapter.

Trivial names often tell a story: glycine comes from the Greek glykos (sweet), and tyrosine was first obtained from cheese (tyros).

Peptide Bonds and Protein Structure at Four Levels

A peptide bond is an amide linkage \( -\mathrm{CO}-\mathrm{NH}- \) formed between the \( -\mathrm{COOH} \) group of one amino acid and the \( -\mathrm{NH}_2 \) group of another, with elimination of a water molecule. Combining glycine with alanine gives the dipeptide glycylalanine (NCERT, p. 292).

• Dipeptide (2 amino acids), tripeptide (3), tetrapeptide (4), pentapeptide (5), hexapeptide (6).
• More than ten residues: polypeptide.
• A polypeptide with more than 100 residues and molecular mass above 10,000 u is called a protein. Insulin (51 residues) counts because it has a well-defined protein conformation.

Protein structure is studied at four levels, each more complex than the last (NCERT, p. 293):

1. Primary: the specific sequence of amino acids in the polypeptide chain. Changing the sequence creates a different protein.
2. Secondary: regular folding of the backbone into an alpha-helix or beta-pleated sheet, stabilised by hydrogen bonds between the \( -\mathrm{NH} \) and carbonyl (C=O) groups of the peptide bond.
3. Tertiary: overall folding of the chain, giving fibrous or globular shapes.
4. Quaternary: spatial arrangement of two or more polypeptide subunits.

The alpha-helix figure shows the chain twisted into a right-handed screw; every \( -\mathrm{NH} \) group of one residue is hydrogen-bonded to the C=O of an adjacent turn. The four-level diagram below shows how primary structure folds into secondary, tertiary and quaternary levels — each coloured ball is an amino acid.

The main forces that stabilise secondary and tertiary structures are hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic forces of attraction.

Denaturation: secondary and tertiary gone, primary intact

A protein in its native form has a unique three-dimensional structure and biological activity. When temperature or pH changes disturb its hydrogen bonds, globules unfold, helices uncoil and the protein loses its activity — this is denaturation. Secondary and tertiary structures are destroyed while the primary sequence stays intact.

Boiling coagulates egg white; milk curdles because bacteria form lactic acid (NCERT, p. 294).

Worked Examples: Hydrolysis Products and Reducing Behaviour

Two application problems, solved step by step, to fix the chapter’s most examinable logic.

Example A: Identify a reducing disaccharide from its hydrolysis

Example B: Classify amino acid side chains

Enzymes: Biocatalysts That Lower Activation Energy

Enzymes are biocatalysts, and almost all of them are globular proteins. They work under the mild conditions of the body and are highly specific — for a particular reaction and a particular substrate (NCERT, p. 295).

• Named after the substrate: maltase catalyses the hydrolysis of maltose into glucose.
• Named after the reaction type: oxidoreductase enzymes catalyse oxidation of one substrate with simultaneous reduction of another.
• The ending -ase marks an enzyme.

\[ \mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} \xrightarrow{\text{Maltase}} 2\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6} \]

Enzymes, like chemical catalysts, reduce the magnitude of activation energy. Acid hydrolysis of sucrose needs \( 6.22\ \text{kJ mol}^{-1} \); the enzyme sucrase brings it down to \( 2.15\ \text{kJ mol}^{-1} \). That comparison is the quantitative point to quote.

Vitamins: Sources and Deficiency Diseases in One Table

Vitamins are organic compounds required in small amounts in the diet to perform specific biological functions for normal growth and health (NCERT, p. 295). The name came from vital + amine; the ‘e’ was dropped when later work showed most vitamins lack amino groups.

Most cannot be synthesised in the body, so they are essential food factors — and excess of vitamins is harmful.

• Fat-soluble: vitamins A, D, E and K — soluble in fats and oils, stored in the liver and adipose (fat-storing) tissue.
• Water-soluble: B-group vitamins and vitamin C — readily excreted in urine, cannot be stored (except \( \mathrm{B}_{12} \)), so they must be supplied regularly.

Memory device: think of the phrase “KADE is kept in the liver” — the four fat-soluble vitamins (K, A, D, E) are the ones stored in the liver and fat tissue; B and C are washed out in urine.

Nucleic Acids: DNA versus RNA

Complete hydrolysis of DNA or RNA yields a pentose sugar, phosphoric acid and nitrogen-containing bases. DNA has beta-D-2-deoxyribose; RNA has beta-D-ribose. DNA carries adenine (A), guanine (G), cytosine (C) and thymine (T); RNA has uracil (U) in place of thymine (NCERT, p. 297).

A nucleoside is a base attached to the 1′ position of the sugar. Sugar carbons are numbered 1′, 2′, 3′ and so on to distinguish them from base atoms. When the nucleoside is linked to phosphoric acid at the 5′ position, it becomes a nucleotide (NCERT, p. 298). The figure below shows the base-sugar unit of a nucleoside.

Nucleotides are joined together by a phosphodiester linkage between the 5′ carbon of one sugar and the 3′ carbon of the next. The dinucleotide figure below shows exactly this connection.

Watson and Crick proposed a double-strand helix for DNA: two chains wound about each other, held by hydrogen bonds between specific base pairs — A with T, C with G. The strands are therefore complementary, which explains how identical copies of parental DNA pass to daughter cells.

RNA, in its secondary structure, is a single-stranded helix that sometimes folds back on itself (NCERT, p. 299).

• DNA is the chemical basis of heredity; it holds the coded message for protein synthesis and self-duplicates during cell division.
• RNA carries out the message. There are three types: messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).

Because a person’s DNA base sequence is unique and cannot be altered by surgery or treatment, DNA fingerprinting is used (NCERT, p. 300):

• in forensic laboratories, to identify criminals;
• to determine the paternity of an individual;
• to identify dead bodies in accidents by comparing the DNA of parents or children;
• to identify racial groups and rewrite biological evolution.

Hormones: Steroid, Peptide and Amino Acid Messengers

Hormones act as intercellular messengers. They are produced by endocrine glands, poured directly into the bloodstream, and transported to the site of action (NCERT, p. 300).

• Thyroxine is an iodinated derivative of tyrosine. Low levels cause hypothyroidism (lethargy, obesity); high levels cause hyperthyroidism. Low dietary iodine enlarges the thyroid, which is why commercial table salt is “iodised” with sodium iodide.
• Glucocorticoids control carbohydrate metabolism and modulate inflammatory and stress reactions; mineralocorticoids control the excretion of water and salt by the kidney.
• Failure of the adrenal cortex can cause Addison’s disease — hypoglycemia, weakness, increased susceptibility to stress — which is fatal unless treated with glucocorticoids and mineralocorticoids.
• Gonadal hormones: testosterone (male secondary characters), estradiol (female characters and the menstrual cycle), and progesterone (prepares the uterus for implantation of the fertilised egg).

Common Mistakes Students Make in Biomolecules

These seven errors are the classic marks-losing slips. Each row shows the wrong idea and the rule that fixes it.

Exam Notes: What Earns the Marks

These points come from reading the chapter’s end-of-unit exercises (10.1–10.25) with an examiner’s eye. They are observations of how the content is examined, not predictions.

Definition questions (glycosidic linkage, peptide bond, denaturation, nucleoside, nucleotide) are short-answer staples: write the definition, then one structural detail. For linkage questions, always state the carbons — sucrose C1–C2; maltose and lactose C1–C4.

Key Reactions of Biomolecules to Memorise

The reaction list every answer needs. Conditions matter — dilute H\(_2\)SO\(_4\) at 393 K under 2–3 atm for starch, H\(^+\) for sucrose. Verify each equation against the official rationalised NCERT Chemistry Part II text (Unit 10) at ncert.nic.in.

Biomolecules Revision Summary in One Table

The whole chapter on one screen — family, building block, and the single fact most likely to be asked.

If you remember only one fact per family, this table is it — everything above is the proof and the drill. For every chapter in the syllabus, browse the complete set of CBSE notes.

Frequently Asked Questions

Why is sucrose a non-reducing sugar even though glucose and fructose are reducing sugars?

Because in sucrose the reducing groups of both units are used up in the glycosidic linkage — C1 of alpha-D-glucose bonds to C2 of beta-D-fructose. No free aldehyde or ketone group remains to reduce Fehling’s solution or Tollens’ reagent.

What is the difference between a nucleoside and a nucleotide?

A nucleoside is a base attached to the 1′ position of a pentose sugar. A nucleotide is a nucleoside linked to phosphoric acid at the 5′ position. Nucleic acids are polynucleotides joined by phosphodiester linkages.

Why can vitamin C not be stored in the body while vitamin A can?

Vitamin C is water-soluble, so it is readily excreted in urine and must be supplied regularly. Vitamin A is fat-soluble and is stored in the liver and adipose tissue, so the body keeps a reserve.

How is glycogen different from starch in structure and function?

Both are polymers of glucose. Glycogen is the animal storage form found in liver, muscle and brain, and its structure resembles amylopectin but is more highly branched. Starch is the plant storage form: unbranched amylose (15–20%) plus branched amylopectin (80–85%).

Why does glucose not give Schiff’s test despite having an aldehyde group?

Because in solution glucose exists mainly as the cyclic hemiacetal, in which the aldehyde carbon is tied up in the ring. The open-chain form is present only in a tiny equilibrium amount, too small to react with Schiff’s reagent or NaHSO\(_3\).

What is the difference between D-glucose and L-glucose?

They are mirror-image stereoisomers. D and L describe configuration relative to glyceraldehyde, judged by the lowest asymmetric carbon — not the direction of rotation. D-glucose is dextrorotatory (+), but D-fructose is laevorotatory (−), which shows the two labels are independent.

Reference: NCERT Class 12 Chemistry textbook, chapter Biomolecules.