Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc. Proteins and carbohydrates are essential constituents of our food. In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms. All proteins are polymers of $\alpha$-amino acids. Proteins can be classified into two types on the basis of their molecular shape — Fibrous and Globular proteins. Vitamins are accessory food factors required in the diet. They are classified as fat-soluble and water-soluble. Deficiency of vitamins leads to many diseases. Nucleic acids are the polymers of nucleotides which in turn consist of a base, a pentose sugar and phosphate moiety. There are two types of nucleic acids — DNA and RNA. Nucleic acids are responsible for the transfer of characters from parents to offsprings. (a) Write the name of basic building units of proteins and nucleic acids. How can you differentiate between Fibrous and Globular proteins on the basis of their structures ? (b) (i) What products would be formed when a nucleotide from DNA containing thymine is hydrolyzed ? OR (b) (ii) Write one structural difference between DNA and RNA. (c) Give one example each of a fat-soluble vitamin and a water-soluble vitamin.

> [!TLDR] > (a) Proteins are polymers of $\alpha$-amino acids; nucleic acids of nucleotides. Fibrous proteins are long thread-like insoluble chains; globular proteins are compact soluble spheres. Hydrolysing a thymine DNA nucleotide gives thymine + 2-deoxyribose + phosphoric acid. > (b) DNA has deoxyribose and is double-stranded, RNA has ribose and is single-stranded; a fat-soluble vitamin is A and a water-soluble vitamin is C. ## Part (a) **Basic building units.** A polymer is a chain of repeating monomers. - Proteins are polymers of **$\alpha$-amino acids** joined by peptide (amide) bonds. - Nucleic acids (DNA/RNA) are polymers of **nucleotides**; each nucleotide = a nitrogenous base + a pentose sugar + a phosphate group, joined into the chain by phosphodiester bonds. **Fibrous vs globular proteins (by structure).** - **Fibrous proteins:** polypeptide chains run *parallel* to one another in long strands or sheets, cross-linked by hydrogen bonds and disulfide bridges. This makes them thread-like, mechanically strong and insoluble in water; their role is structural. Examples: keratin (hair, nails), collagen (tendons), myosin. - **Globular proteins:** the chain folds back on itself into a compact, roughly spherical tertiary structure with the hydrophobic residues buried inside. They are generally soluble in water and carry out dynamic functions. Examples: insulin, haemoglobin, enzymes. **(b)(i) Hydrolysis of a DNA nucleotide containing thymine.** A nucleotide has three parts held by two hydrolysable linkages — the *N*-glycosidic bond (base–sugar) and the phosphoester bond (sugar–phosphate). Complete hydrolysis cleaves both: …

Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids

Question

\alpha

(a) Write the name of basic building units of proteins and nucleic acids. How can you differentiate between Fibrous and Globular proteins on the basis of their structures ?

(b)

(i) What products would be formed when a nucleotide from DNA containing thymine is hydrolyzed ?

(OR)

(b)

(ii) Write one structural difference between DNA and RNA.

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Part (a)Concept understanding — Biochemical Bonds

Biochemical Bonds: The Glue That Holds Life Together

Imagine you're building with LEGO bricks. Some bricks click together tightly and never come apart unless you really yank them. Others snap together lightly and can be pulled apart with a gentle tug. Some bricks don't even click — they just stick because of static cling or magnetism.

Biochemical bonds are exactly like that. They are the forces that hold atoms together inside the molecules of your body — your DNA, proteins, fats, and carbohydrates. Without these bonds, you'd literally fall apart into a pile of individual atoms.

The Core Idea

Atoms bond because being bonded is more stable (lower energy) than being alone. Think of it like this: a single atom is like a person standing alone in a cold room. Bonding is like huddling together for warmth — you lose some freedom of movement, but you gain stability.

In biochemistry, we care about four main types of bonds. They differ in strength, how they form, and what they do in living systems.

1. Covalent Bonds — The Strong, Permanent LEGO Clicks

This is the strongest bond in biology. Two atoms share electrons — like two people holding the same umbrella. Each atom contributes one or more electrons, and they both "own" the pair.

Key properties:

Very strong (100–400 kJ/mol)
Forms the backbone of all biomolecules
Takes a lot of energy (or enzymes) to break

Where you find it:

The carbon-carbon bonds in your DNA's sugar-phosphate backbone
The peptide bonds linking amino acids into proteins
The bonds within a glucose molecule

Note

A single covalent bond shares 2 electrons. A double bond shares 4. Triple bonds are rare in biology but exist (e.g., in cyanide).

2. Ionic Bonds — The Static Cling of Opposites

Some atoms steal electrons from others. When that happens, one atom becomes positively charged (lost an electron) and the other becomes negatively charged (gained one). Opposite charges attract — that's an ionic bond.

Key properties:

Moderate strength (5–100 kJ/mol in dry conditions)
Very weak in water (because water molecules get in between)
Easily broken by changes in pH or salt concentration

Where you find it:

In salt bridges that help proteins fold into their correct shape
Between the phosphate groups of DNA and positively charged proteins (histones)

Watch out

Ionic bonds are often called "bonds" but in water they behave more like attractions. Don't confuse them with covalent bonds — they're much weaker in biological fluids.

3. Hydrogen Bonds — The Gentle, Reversible Magnets

This is the most important weak bond in biology. A hydrogen atom that's already covalently bonded to an electronegative atom (like oxygen or nitrogen) gets a slight positive charge. It then gets attracted to another electronegative atom nearby.

Think of it like a weak magnet — it holds things together but can be easily undone.

Key properties:

Weak individually (5–30 kJ/mol)
But many together can be very strong
Easily broken by heat or changes in pH
Directional — they only work when atoms are properly aligned

Where you find it:

Between the two strands of DNA (this is what holds the double helix together)
In protein folding (between amino acids in the backbone)
Between water molecules (giving water its unique properties)

Important

Hydrogen bonds are the reason DNA can unzip for replication. If DNA used covalent bonds between strands, it would be impossible to separate without destroying the molecule.

4. Van der Waals Interactions — The Fleeting, Accidental Touches

Even neutral atoms have temporary, uneven distributions of electrons. These create tiny, momentary charges that attract nearby atoms. It's like two people accidentally brushing shoulders in a crowd — brief, weak, but real.

Key properties:

Extremely weak (0.5–5 kJ/mol per interaction)
Only work when atoms are very close (within 0.3–0.4 nm)
Add up significantly when many atoms are packed together

Where you find it:

In the hydrophobic core of proteins (where oily amino acids pack tightly)
Between lipid tails in cell membranes
In enzyme-substrate binding (helps "grip" the substrate)

Putting It All Together: A Biological Example

Consider a protein in your body. It's a long chain of amino acids held together by covalent peptide bonds. That chain then folds into a specific shape. The folding is guided by:

Hydrogen bonds between backbone atoms (forming alpha helices and beta sheets)
Ionic bonds between charged side chains …

Why this formula?

Biochemical Bonds: Why the Key Formulas Hold

Biochemical bonds are the forces that hold atoms together in biomolecules. The key formulas come from electrostatics and quantum mechanics — not from biology itself. Let's break down the why behind the most important ones.

1. Ionic Bond Energy: Coulomb's Law

Formula:

E = \frac{k \cdot q_1 \cdot q_2}{r}

Why it holds:

Opposite charges attract — this is a fundamental law of physics (Coulomb's law).
In a biochemical context, consider a sodium ion ( $\text{Na}^+$ ) and a chloride ion ( $\text{Cl}^-$ ). The energy released when they come together is directly proportional to the product of their charges ( $q_1 q_2$ ) and inversely proportional to the distance ( $r$ ) between them.
The constant $k$ accounts for the medium (water vs. vacuum). In water, the effective force is weaker because water molecules partially shield the charges — this is why ionic bonds in biology are often weaker in aqueous environments.

Key insight: The formula is not arbitrary — it's derived from the inverse-square law of electrostatics, integrated over the distance the charges move toward each other.

2. Covalent Bond Energy: The Morse Potential (Approximation)

Formula (simplified):

E = D_e \left(1 - e^{-a(r - r_0)}\right)^2

Why it holds:

Covalent bonds arise from shared electrons between atoms. The energy is not a simple inverse-square law because electrons are delocalized.
The Morse potential is an empirical formula that captures two key observations:
1. At equilibrium distance ( $r_0$ ): Energy is minimum ( $E = 0$ in this form).
2. If atoms are pulled apart ( $r \to \infty$ ): Energy approaches $D_e$ (the bond dissociation energy).
3. If atoms are pushed too close ( $r \to 0$ ): Energy skyrockets due to Pauli repulsion (electrons can't occupy the same space).
The exponential term $e^{-a(r - r_0)}$ models the rapid drop in attractive force as distance increases — this comes from quantum mechanical overlap of electron clouds.

Key insight: The formula is a curve fit to quantum mechanical calculations, not a first-principles derivation. But it works because it respects the physics: attraction at long range, repulsion at short range, and a stable minimum.

3. Hydrogen Bond Energy: Dipole-Dipole Interaction

Formula (approximate):

E \approx -\frac{2 \mu_1 \mu_2}{4 \pi \epsilon_0 r^3} \cdot \cos\theta

Why it holds:

A hydrogen bond (e.g., between water molecules) is not a true bond — it's a strong dipole-dipole interaction.
The dipole moment ( $\mu$ ) arises because oxygen is more electronegative than hydrogen, creating partial charges ( $\delta^+$ and $\delta^-$ ).
The energy depends on:
- Strength of dipoles ( $\mu_1 \mu_2$ )
- Distance ( $r$ ) — falls off as $1/r^3$ , much faster than ionic bonds ( $1/r$ )
- Orientation ( $\cos\theta$ ) — strongest when dipoles are aligned head-to-tail

Key insight: The $1/r^3$ dependence comes from the derivative of the dipole field. Unlike point charges, dipoles have a field that decays faster — this is why hydrogen bonds are directional and weaker than covalent bonds.

4. Van der Waals Interaction: Lennard-Jones Potential

Formula:

E = 4\epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right]

Why it holds:

Van der Waals forces arise from temporary fluctuations in electron distribution — even nonpolar molecules have instantaneous dipoles. …

Part (b)Concept understanding — RNA Single Stranded

What Does "Single Stranded" Mean for RNA?

Think of DNA as a double‑stranded zipper — two complementary strands held together by base‑pairing (A‑T, G‑C). RNA, by contrast, is usually single‑stranded: it’s just one chain of nucleotides, not paired with a second strand.

But here’s the key intuition: single‑stranded does not mean unstructured. That single strand can fold back on itself, forming short double‑helical regions wherever complementary bases happen to be nearby. The result is a complex 3‑D shape — like a tangled piece of string that occasionally sticks to itself.

Note

The “single strand” is the backbone — a continuous chain of sugar‑phosphate units. The bases (A, U, G, C) stick out from this backbone and can pair with bases on the same strand if the sequence allows.

The Precise Statement

RNA is single‑stranded means that its molecular structure consists of one polynucleotide chain, not two antiparallel chains as in DNA. This single chain is the primary structure.

However, the secondary structure of RNA can contain short double‑helical segments (stems) formed by intramolecular base‑pairing (A‑U, G‑C, and sometimes G‑U). These stems are separated by loops (unpaired regions), giving rise to characteristic shapes like hairpins, bulges, and pseudoknots.

5'\text{-} \text{...AGCU...} \text{-}3'

Why Does This Matter?

Flexibility — A single strand can adopt many shapes, allowing RNA to perform diverse roles: messenger (mRNA), adapter (tRNA), catalyst (ribozyme), regulator (miRNA, siRNA).
Self‑assembly — The folding is driven by base‑pairing and stacking, just like in DNA, but only within the same molecule.
Viral genomes — Many viruses (e.g., influenza, HIV, SARS‑CoV‑2) store their genetic information as single‑stranded RNA. This makes them more prone to mutations (no second strand for error‑checking) and a target for antiviral drugs.

Watch out

Do not confuse “single‑stranded” with “unstable.” RNA is chemically less stable than DNA (the 2′‑OH group makes it prone to hydrolysis), but its single‑stranded nature is a structural feature, not a measure of stability.

A Quick Comparison

| Feature | DNA (usually) | RNA (usually) | …

Why this formula?

RNA Single Stranded: Understanding the Key Formula(e)

Let’s start with a fundamental question: Why does a single-stranded RNA molecule have a specific length-to-mass relationship? The answer lies in the molecular structure and the way nucleotides are linked.

The Core Formula

For a single-stranded RNA, the number of nucleotides ( $N$ ) is related to the molecular mass ( $M$ ) by:

N = \frac{M}{M_{\text{nucleotide}}}

where $M_{\text{nucleotide}}$ is the average mass of one RNA nucleotide.

Why This Holds

RNA is a linear polymer — each nucleotide is linked to the next by a phosphodiester bond. There are no branches or cross-links in a typical single-stranded RNA.
Each nucleotide contributes equally to the total mass. The mass of a single RNA nucleotide (ribonucleotide monophosphate) is approximately ~340 Da (daltons). This includes:
- A ribose sugar (~150 Da)
- A phosphate group (~80 Da)
- A nitrogenous base (A, U, G, or C) — average ~110 Da
No correction for ends is needed in most exam contexts because the mass difference between the 5' and 3' ends is negligible compared to the total mass of hundreds or thousands of nucleotides.

Example Application

If an RNA has a molecular mass of 68,000 Da, the number of nucleotides is:

N = \frac{68,000}{340} \approx 200 \text{ nucleotides}

This is not a coincidence — it’s a direct consequence of the repeating unit structure.

The "Why" Behind the Approximation

You might wonder: Why 340 Da and not exactly 340? Because:

The four bases (A, U, G, C) have slightly different masses (A=135, U=112, G=151, C=111 Da).
In exams, 340 Da is the accepted average for a ribonucleotide monophosphate in single-stranded RNA.

Key insight: The formula works because RNA is a homopolymer in terms of backbone structure — every nucleotide is identical in its sugar-phosphate backbone, only the base varies.

What About Double-Stranded RNA? …

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Proteins are polymers of $\alpha$ -amino acids; nucleic acids of nucleotides. Fibrous proteins are long thread-like insoluble chains; globular proteins are compact soluble spheres. Hydrolysing a thymine DNA nucleotide gives thymine + 2-deoxyribose + phosphoric acid.
DNA has deoxyribose and is double-stranded, RNA has ribose and is single-stranded; a fat-soluble vitamin is A and a water-soluble vitamin is C.

Part (a)

Basic building units. A polymer is a chain of repeating monomers.

Proteins are polymers of $\alpha$ -amino acids joined by peptide (amide) bonds.
Nucleic acids (DNA/RNA) are polymers of nucleotides; each nucleotide = a nitrogenous base + a pentose sugar + a phosphate group, joined into the chain by phosphodiester bonds.

Fibrous vs globular proteins (by structure).

Fibrous proteins: polypeptide chains run parallel to one another in long strands or sheets, cross-linked by hydrogen bonds and disulfide bridges. This makes them thread-like, mechanically strong and insoluble in water; their role is structural. Examples: keratin (hair, nails), collagen (tendons), myosin.
Globular proteins: the chain folds back on itself into a compact, roughly spherical tertiary structure with the hydrophobic residues buried inside. They are generally soluble in water and carry out dynamic functions. Examples: insulin, haemoglobin, enzymes.

(b)(i) Hydrolysis of a DNA nucleotide containing thymine. A nucleotide has three parts held by two hydrolysable linkages — the N-glycosidic bond (base–sugar) and the phosphoester bond (sugar–phosphate). Complete hydrolysis cleaves both: …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 17 on this concept.

CBSE 2026Set 56/2/11 markMCQ
$\alpha$
›Reveal solutionSolution
Proteins are linear chains of $\alpha$ -amino acids linked by peptide bonds — a specific type of covalent bond formed between the carboxyl group of one amino acid and the amino group of the next, with the elimination of water. The correct answer is (B) Peptide Bond.

Why This Question Tests a Core Biochemical Idea

The question is about the primary structure of proteins — the simplest level of protein organization. A protein is a polymer, and like any polymer, it has repeating units (monomers) held together by a specific chemical linkage. The monomers here are $\alpha$ -amino acids, and the linkage that joins them is not just any covalent bond — it has a special name because of how it forms and its unique properties.

Many students get confused because all the options are types of bonds found in biomolecules. The trick is to match the bond to the specific monomers and the reaction that creates it.

Step-by-Step Reasoning

Identify the monomers. Proteins are built from $\alpha$ -amino acids. Each amino acid has a central carbon ( $\alpha$ -carbon) bonded to an amino group ( $-\mathrm{NH}_2$ ), a carboxyl group ( $-\mathrm{COOH}$ ), a hydrogen atom, and a variable side chain ( $R$ ). The key functional groups for linking are the $-\mathrm{NH}_2$ and $-\mathrm{COOH}$ groups.

Understand how two amino acids join. When two amino acids link, the carboxyl group of the first amino acid reacts with the amino group of the second. This is a condensation reaction (also called dehydration synthesis) — a molecule of water ( $\mathrm{H}_2\mathrm{O}$ ) is removed. The oxygen from the carboxyl group and two hydrogens from the amino group form the water.

Name the resulting bond. The chemical bond that forms between the carbon of the first amino acid's carboxyl group and the nitrogen of the second amino acid's amino group is called an amide bond. In biochemistry, this specific amide bond between amino acids is universally known as a peptide bond. The structure is $-\mathrm{CO}-\mathrm{NH}-$ .

Eliminate the other options.

(A) Covalent Bond — This is too broad. A peptide bond is a covalent bond, but the question asks for the specific name of the bond joining amino acids in proteins. "Covalent bond" is the general category, not the precise answer.

(C) Glycosidic Bond — This bond joins monosaccharides to form carbohydrates (e.g., starch, cellulose). It involves a sugar's anomeric carbon and an alcohol or another sugar. It has nothing to do with amino acids.

(D) Coordinate Bond — This is a special type of covalent bond where both shared electrons come from the same atom. It is found in coordination complexes (e.g., metal ions with ligands) and is not the standard linkage in protein backbones. …
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CBSE 2025Set A1 mark
$Q. Fill in the blank: If third amino acid combines to a dipeptide, the product is called a ______.$
›Reveal solutionSolution

A peptide is named by how many amino acid residues it contains: two = dipeptide, three = tripeptide, and so on.

When two amino acids join through a peptide (amide) bond, formed by condensation between the –COOH of one amino acid and the –NH2 of another (with loss of a water molecule), the product is a dipeptide. If a third amino acid molecule now condenses onto this dipeptide (forming another peptide bond), the r …

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CBSE 2024Set ANNUAL1 mark
$Q. Name the linkage between two monosaccharide units in a disaccharide.$
›Reveal solutionSolution

Two monosaccharide units are joined together by a glycosidic linkage, formed between the anomeric carbon of one sugar and a hydroxyl group of the other, with loss of a water molecule.

When two monosaccharides combine, an -OH group of one monosaccharide unit condenses with an -OH group of the anomeric carbon of the other unit, eliminating a molecule of water and forming a C–O–C bridge. This bridging oxygen linkage is called a glycosidic linkage.

…

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CBSE 2024Set ANNUAL1 mark
$Q. What do you mean by glycosidic linkage?$
›Reveal solutionSolution

A glycosidic linkage is the acetal-type C–O–C bond formed when the anomeric $-OH$ of one sugar condenses with an $-OH$ of another molecule, releasing water; it is the bond that links monosaccharides into disaccharides and polysaccharides.

When two monosaccharide molecules combine, the anomeric carbon (the carbon bearing the hemiacetal $-OH$ , formed from the original aldehyde/ketone carbon) of one sugar reacts with a hydroxyl group of the second sugar. A molecule of water is eliminated, and a new C–O–C bond is formed between the two sugar rings — this bond is called a glycosidic linkage:

$\text{Sugar}_1-OH + HO-\text{Sugar}_2 \longrightarrow \text{Sugar}_1-O-\text{Sugar}_2 + H_2O$ …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. Proteins are polymers of amino acids, that are connected to each other by$
(a) glycosidic linkage
(b) phosphodiester linkage
(c) peptide bond
(d) none of these
›Reveal solutionSolution

Proteins are condensation polymers of amino acids joined by peptide (amide) linkages.

A peptide bond forms when the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another, releasing a water molecule and forming an amide linkage (-CO-NH-). Chains of amino acids linked this way are called polypeptides, and proteins are (typically large) po …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. The linkage which holds various amino acid units in the primary structure of protein is$
(a) glycosidic linkage
(b) hydrogen bond
(c) peptide linkage
(d) ionic bond
›Reveal solutionSolution

The primary structure of a protein is the linear sequence of amino acids joined by peptide (amide, $-CO-NH-$ ) bonds; the other bond types listed stabilize higher-order structure, not the primary sequence.

Peptide linkage: A peptide (amide) bond forms when the carboxyl group ( $-COOH$ ) of one $\alpha$ -amino acid condenses with the amino group ( $-NH_2$ ) of another, eliminating a molecule of water:

$R\text{-CH(NH}_2\text{)-COOH} + H_2N\text{-CH(R')-COOH} \rightarrow R\text{-CH(NH}_2\text{)-CO-NH-CH(R')-COOH} + H_2O$

Repeating this condensation along a chain of amino-acid residues builds a polypeptide; the specific, covalently fixed sequence of residues linked this way is the primary structure of the protein.

…

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CBSE 2022Set ANNUAL1 markMCQ
$Q. Identify the peptide bond among the following:$
(a) –C(–)(–)–O–C(–)(–)– (a chain where two carbon atoms, each carrying two additional vertical bonds, are linked through a bridging oxygen)
(b) –O–P(=O)(–O–)–O–C(–)(H)– (a phosphorus atom double-bonded to one O above and singly bonded to another O below, flanked by –O– and –C(H)– groups — a phosphodiester-type linkage)
(c) –CO–NH–
(d) –COO–
›Reveal solutionSolution

The peptide bond is the amide linkage -CO-NH-, formed between the -COOH group of one amino acid and the -NH2 group of the next, with loss of water.

Proteins are polymers of alpha-amino acids joined together by peptide bonds. A peptide bond forms when the carboxyl (-COOH) group of one amino acid condenses with the amino (-NH2) group of the next amino acid, eliminating a molecule of water:

-COOH + H2N- -> -CO-NH- + H2O

…

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CBSE 2022Set TERM21 mark
$Q. In proteins α-amino acids are linked to each other by ................. linkage.$
›Reveal solutionSolution

Amino acids join into proteins through peptide bonds, formed by condensation between the -COOH of one amino acid and the -NH2 of the next.

Proteins are polymers of α-amino acids. Two amino acids combine with the loss of a water molecule, forming an amide (-CO-NH-) bond between the carboxyl group of one amino acid and the amino group of the next:

$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-CO-NH-CHR'-COOH + H_2O$

…

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CBSE 2022Set ANNUAL1 mark
$Q. Write the name of monomer of polymer, Nylon-6.$
›Reveal solutionSolution

Nylon-6 is obtained from one monomer, caprolactam.

Nylon-6 is a synthetic condensation (step-growth) polymer of the polyamide family. It is prepared by heating caprolactam (a cyclic amide with a 6-membered ring, epsilon-caprolactam) with a trace of water, which opens the ring and polymerises it.

…

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CBSE 2021Set NC1 mark
$H_2N\text{--}CH_2\text{--COOH}$
›Reveal solutionSolution

The bond joining the $-COOH$ of one amino acid to the $-NH_2$ of another, with loss of a water molecule, is called a peptide linkage.

When the carboxyl group ( $-COOH$ ) of glycine condenses with the amino group ( $-NH_2$ ) of alanine, a molecule of water is eliminated and an amide-type bond ( $-CO\text{--}NH-$ ) forms between the two amino-acid residues:

$H_2N\text{--}CH_2\text{--COOH} + H_2N\text{--CH(CH}_3\text{)--COOH} \xrightarrow{-H_2O} H_2N\text{--}CH_2\text{--CO--NH--CH(CH}_3\text{)--COOH}$

…

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CBSE 2020Set 56/1/11 markMCQ
$Q. Assertion (A) : Sucrose is a non-reducing sugar. Reason (R) : Sucrose has glycosidic linkage. (A) Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is correct, but Reason (R) is incorrect statement. (D) Assertion (A) is incorrect, but Reason (R) is correct statement.$
›Reveal solutionSolution
Sucrose is indeed a non-reducing sugar, and it does have a glycosidic linkage — but every disaccharide has a glycosidic linkage, so (R) doesn't explain why sucrose specifically is non-reducing. The answer is (B).

Why some sugars reduce and others don't

A reducing sugar is one that can act as a reducing agent, which requires a free or potentially free anomeric carbon — the carbon involved in the hemiacetal/hemiketal group. When this carbon is free (or can open up from a cyclic form), the sugar can exist in equilibrium with its open-chain aldehyde or ketone form, which readily reduces reagents like Fehling's or Benedict's solution.

The key question: is the anomeric carbon available, or is it locked away?

What makes sucrose special

Sucrose's structure: Sucrose is a disaccharide made of glucose (a six-membered pyranose ring) and fructose (a five-membered furanose ring). The glycosidic bond connects C1 of glucose (its anomeric carbon) to C2 of fructose (its anomeric carbon). This is an $\alpha(1 \to 2)$ linkage.

Both anomeric carbons are involved: Unlike maltose or lactose — where one monosaccharide unit retains a free anomeric carbon that can open and close — sucrose uses both anomeric carbons in the glycosidic bond. Neither glucose nor fructose unit can revert to its open-chain form.

No free aldehyde or ketone: Without the ability to open up, sucrose cannot expose a carbonyl group. No carbonyl means no reducing behavior.

Watch out

A common mistake is thinking "glycosidic linkage = non-reducing." That's false. Maltose and lactose both have glycosidic linkages ( $\alpha(1 \to 4)$ and $\beta(1 \to 4)$ respectively), yet they are reducing sugars because one anomeric carbon remains free.

Evaluating the Assertion and Reason

Assertion (A): "Sucrose is a non-reducing sugar."

This is correct. Sucrose does not reduce Fehling's or Benedict's reagent because both anomeric carbons are tied up in the glycosidic bond.

Reason (R): "Sucrose has glycosidic linkage." …
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CBSE 2020Set 56/2/11 mark
$Q. Name the linkage formed when carboxyl end of one amino acid condenses with amino end of other amino acid.$
›Reveal solutionSolution
The linkage formed is a peptide bond — a covalent amide bond ( $-\text{CO}-\text{NH}-$ ) created by a condensation reaction between the carboxyl group ( $-\text{COOH}$ ) of one amino acid and the amino group ( $-\text{NH}_2$ ) of another, releasing a water molecule.

Why This Linkage Matters

Amino acids are the building blocks of proteins. Each amino acid has a central carbon bonded to an amino group ( $-\text{NH}_2$ ), a carboxyl group ( $-\text{COOH}$ ), a hydrogen atom, and a variable side chain (R group). To build a protein, these monomers must join end-to-end in a specific, repeating pattern.

The key chemical trick is condensation (also called dehydration synthesis). When the carboxyl end of one amino acid reacts with the amino end of another, a water molecule is eliminated — the $-\text{OH}$ from the carboxyl and one $-\text{H}$ from the amino group combine to form $\text{H}_2\text{O}$ . What remains is a covalent bond linking the two amino acids.

This bond is not just any bond — it is an amide bond in organic chemistry terms, but in biochemistry it has a special name.

Step-by-Step Reasoning

Identify the reacting groups.

The first amino acid contributes its carboxyl group ( $-\text{COOH}$ ). The second amino acid contributes its amino group ( $-\text{NH}_2$ ). These are the functional groups that must interact.

Recognize the reaction type.

This is a condensation reaction (dehydration synthesis). The carboxyl loses an $-\text{OH}$ group, and the amino loses one $-\text{H}$ atom. Together they form $\text{H}_2\text{O}$ , which is released.

Write the resulting linkage.

After water is removed, the remaining atoms form a bond:

$-\text{CO} - \text{NH} -$
This is a carbon-nitrogen single bond where the carbon is doubly bonded to an oxygen (a carbonyl group) and singly bonded to the nitrogen. Chemically, this is an amide bond.

Give the biochemical name.

In the context of proteins and amino acids, this specific amide bond is called a peptide bond. It is the fundamental linkage that holds amino acids together in a polypeptide chain.

Confirm the directionality. …
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Part (a)

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