NCERT Solutions for Class 12 Chemistry Chapter 10 – Biomolecules (English Medium)

• “Mono” means one and “saccharide” means sugar.

• So a monosaccharide is a single carbohydrate unit.

• They usually have the general formula (CH₂O)_n (commonly n = 3 to 7).

• Monosaccharides contain a carbonyl group (aldehyde or ketone) and multiple –OH groups, which is why they are sweet and water soluble.

A sugar is called “reducing” if it can donate electrons and reduce:

• Tollens’ reagent: Ag⁺ → Ag (silver mirror)
• Fehling’s solution: Cu²⁺ → Cu₂O (red precipitate)

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✅ Reducing sugars have a free anomeric carbon (the carbonyl carbon in open-chain form).

Aldoses (like glucose) have –CHO directly.

Some ketoses (like fructose) still act reducing because in basic medium they tautomerise to an aldehyde form.

✅ If the anomeric carbon is tied up in a glycosidic bond (like in sucrose), the sugar can’t open up to give a free carbonyl group → non-reducing.

Starch is the plant’s “food bank”. During growth, respiration, or when sunlight is not available, plants break starch into glucose to release energy.

Cellulose is the main material of plant cell walls. It acts like a strong framework that helps plants stay upright and protects cells.

• Monosaccharides are the simplest sugars that cannot be broken into smaller sugars (example: ribose, fructose).

• Disaccharides are made by joining two monosaccharides through a glycosidic bond (example: maltose, lactose).

• It is formed by a condensation reaction between the –OH groups of two monosaccharides.

• During bond formation, one molecule of water (H₂O) is released.

• The resulting bond is an O-glycosidic bond that joins the sugars into a disaccharide or polysaccharide.

✅ Example (sucrose):
Glucose + Fructose → Sucrose (joined by a glycosidic linkage) + H₂O

Glycogen: stored in animals (liver, muscles)

Starch: stored in plants (seeds, roots, tubers like potato)

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Starch is a mixture of:

✅ Amylose (15–20%): mostly unbranched, relatively more soluble
✅ Amylopectin (80–85%): branched, less soluble

Glycogen is a branched polymer of α-D-glucose (single main type, but highly branched).

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✅ Both glycogen and amylopectin are branched, but:

Glycogen is more highly branched than amylopectin.

Amylopectin has longer branch segments (about 20–25 glucose units)
Glycogen has shorter branch segments (about 10–14 glucose units)

• Hydrolysis breaks the glycosidic linkage of a disaccharide using water (often with acid or enzymes).

• So each disaccharide splits into its two monosaccharide units:

✅ Sucrose + H₂O → 𝛼– D-(+)- glucose + β-D- (-) -fructose

✅ Lactose + H₂O → β-D-(+)- galactose + β-D-(+)-glucose

Both starch and cellulose are polymers of D-glucose, but the orientation of the linkage is different.

Starch

✅ α-1,4 linkage in amylose (mostly straight/helix)
✅ α-1,6 branching in amylopectin (branched)

Cellulose

✅ β-1,4 linkage throughout (straight, rigid chains)

👉 These straight chains align and form strong hydrogen bonding, making cellulose tough.

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👉 If glucose were purely open-chain aldehyde, it should show typical aldehyde reactions (2,4-DNP, Schiff, NaHSO₃ addition) and should form oxime from pentaacetate.

👉 But these reactions fail, and glucose instead forms α/β glucosides, which strongly suggests that glucose exists mainly as a hemiacetal ring in solution (so the free –CHO is not readily available).

Our body needs amino acids to build proteins, enzymes, and tissues.

Some amino acids cannot be made in the body (essential), so we must eat them.

Some amino acids can be made by the body from other nutrients (non-essential).

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✅ Secondary structure refers to the shape of the polypeptide chain formed mainly due to hydrogen bonding between groups in the peptide backbone (not between side chains).

1) α-Helix

• The polypeptide chain coils into a right-handed helix.
• ✅ A hydrogen bond forms between:
👉 C=O of one amino acid and N–H of the amino acid four units ahead in the chain.
• The R-groups (side chains) project outward, so they can interact easily.

2) β-Pleated Sheet

• The polypeptide chains are stretched out and arranged side by side.
• ✅ They are held together by intermolecular hydrogen bonds between:
👉 C=O and N–H groups of adjacent chains/segments.
• The sheet looks like folds (pleats), and the R-groups extend above and below the sheet.

👉 In an α-helix, the polypeptide chain coils like a spring.

✅ A hydrogen bond forms between:

👉 the C=O (carbonyl oxygen) of one amino acid residue and the N–H (amide hydrogen) of the fourth amino acid residue ahead in the same chain.

👉 Since these H-bonds are within the same chain, they are called intramolecular H-bonds.

✅ These repeated H-bonds hold the helix tightly and make it stable.

• Globular: compact, rounded/spherical
• Fibrous: elongated, rod-like or thread-like

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• Globular: generally soluble in water (or aqueous solutions)
• Fibrous: generally insoluble in water

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• Globular: functional proteins (do active jobs in the body)
✅ Examples: enzymes, hemoglobin, insulin, antibodies

• Fibrous: structural proteins (give strength and support)
✅ Examples: keratin (hair, nails), collagen (connective tissue), fibroin (silk)

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• Globular: usually have strong tertiary (3°) structure (often also quaternary)
• Fibrous: mainly have secondary (2°) structure (repeating pattern), less complex folding

In aqueous solution, the –COOH group donates H⁺ and the –NH₂ group accepts H⁺:

👉 H₂N–CH(R)–COOH ⇌ H₃N⁺–CH(R)–COO⁻

✅ This is called a zwitterion (overall neutral but has both + and − charges).

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✅ In acidic medium:
The –COO⁻ part accepts H⁺ → acts as a base.

✅ In basic medium:
The –NH₃⁺ part donates H⁺ → acts as an acid.

👉 इसलिए amino acids amphoteric होते हैं (both acidic + basic nature).

Enzymes are highly specific, meaning one enzyme usually works on a particular substrate and catalyses a specific reaction.

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They are needed in very small amounts because they can be used again and again.

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Enzymes work best at an optimum temperature (about 310 K in the human body) and an optimum pH (around 7.4 for many blood enzymes).

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✅ Example:
👉 Maltase catalyses the hydrolysis of maltose into glucose.

✅ Secondary (2°), tertiary (3°) and quaternary (4°) structures are destroyed because denaturation breaks:

👉 hydrogen bonds
👉 ionic interactions
👉 hydrophobic interactions

✅ Primary (1°) structure remains intact because peptide bonds are not broken (amino acid sequence stays the same).

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👉 Globular proteins often unfold into a more random, thread-like form.

👉 Many proteins become less soluble and may coagulate/precipitate.

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✅ Since shape controls function, denaturation causes proteins (especially enzymes) to lose their biological activity.

👉 Fat-soluble vitamins (A, D, E, K) dissolve in fats and can be stored in the liver and body fat, so they don’t need to be taken daily in large amounts.

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👉 Water-soluble vitamins (B-complex, C) dissolve in water and are not stored much in the body, so they are usually needed regularly through diet.

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👉 Vitamin K helps in the formation of important clotting factors in the liver, so it plays a key role in stopping bleeding.

🤔 Why essential:
👉 Helps in vision, especially night vision
👉 Maintains healthy skin and mucous membranes
👉 Supports immunity

Deficiency disease:
👉 Night blindness (and severe deficiency can cause eye problems like xerophthalmia)

Important sources:
✅ Fish liver oil, carrots, butter, milk, eggs, green leafy vegetables (spinach)

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Why essential:
👉 Needed for collagen synthesis (skin, gums, wound healing)
👉 Helps immune defence
👉 Prevents bleeding gums and weakness

Deficiency disease:
👉 Scurvy

Important sources:
✅ Citrus fruits (orange, lemon), amla, green leafy vegetables, strawberries, capsicum/pepper

✅ DNA stores hereditary information and passes it from one generation to the next (and from one cell to daughter cells).

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✅ RNA helps in protein formation:

👉 mRNA carries the message from DNA
👉 tRNA brings amino acids
👉 rRNA forms ribosomes where proteins are made

📌 A nucleoside is formed when a base attaches to a sugar.

👉 Example: adenosine = adenine + ribose

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• A nucleotide is formed when one or more phosphate groups attach to the nucleoside.

👉 Example: AMP = adenosine + phosphate

✅ So, the main difference is the phosphate group:

• No phosphate → nucleoside
• Phosphate present → nucleotide

DNA has four bases: A, T, G, C.

They pair only like this:

• A (adenine) with T (thymine) via two hydrogen bonds
• G (guanine) with C (cytosine) via three hydrogen bonds

✅ So, if one strand has A at a position, the other must have T there, and if one has G, the other must have C.

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If one strand is:
👉 5′–A T G C–3′

Then the other must be:
👉 3′–T A C G–5′

✅ The sequences are different, so strands are not identical.
✅ But each base is the correct partner, so they are complementary.

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✅ Complementarity allows each strand to act as a template during DNA replication, helping make an exact copy of genetic information.

Category	Point	DNA	RNA
A) Structural differences	Sugar	Has deoxyribose sugar	Has ribose sugar
	Strands	Usually double-stranded (double helix)	Usually single-stranded
	Nitrogen bases	A, G, C, T (thymine)	A, G, C, U (uracil) (U replaces T)
	Stability	More stable (good for long-term storage)	Less stable and more reactive (short-lived)
B) Functional differences	Main role	Stores and transmits genetic information; can replicate	Helps in making proteins (expressing genetic information)
	Types	One main type (genetic store)	Three major functional types: ✅ mRNA (message) ✅ tRNA (brings amino acids) ✅ rRNA (forms ribosomes)
	Location (general)	Mainly in nucleus (also mitochondria)	Made in nucleus but works mostly in cytoplasm (ribosomes)

Carries the genetic message from DNA to the ribosomes.

Works like a blueprint for making proteins.

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Brings specific amino acids to the ribosome.

Matches amino acids according to the codons on mRNA.

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Forms the main part of ribosomes.

Helps in proper protein assembly (structural + catalytic role).