(a) (i) How are carbohydrates stored in animal body? Mention any one organ where they are present. (ii) What i

Question

Q. (a)

(i) How are carbohydrates stored in animal body? Mention any one organ where they are present.

(ii) What is the basic structural difference between starch and cellulose?

(OR)

(b) Differentiate between:

(i) Peptide linkage and Glycosidic linkage

(ii) Nucleoside and Nucleotide

CBSECBSE Class XII Board 2023Subjective· 2mImportance★★★★★

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Part (a)Concept understanding — Carbohydrate Functions Plants

Carbohydrates in Plants: Why They Matter

Think of a plant as a factory that builds itself out of thin air and sunlight. The raw material is carbon dioxide from the air, the energy comes from sunlight, and the first product it makes is glucose — a simple sugar. That glucose is the starting point for everything else.

Now, glucose is useful, but it's also fragile and reactive. A plant can't just leave piles of glucose lying around. It needs to store that energy for later (like a battery), and it needs to build strong structures (like a skeleton). This is where carbohydrates come in — they are glucose molecules linked together in different ways to serve different purposes.

The Two Big Jobs of Carbohydrates in Plants

1. Energy Storage (The Battery)

Plants make glucose during the day when the sun shines. But they need energy at night too, and during cloudy days, and when they're growing new leaves or making seeds. So they pack glucose molecules into a long, branched chain called starch.

Note

Starch is to plants what glycogen is to animals — a compact, insoluble way to store glucose. It's stored in roots (potatoes), seeds (rice, wheat), and stems (sugarcane).

When the plant needs energy, it breaks starch back down into glucose, which it then burns (respires) to get ATP — the energy currency of cells.

2. Structural Support (The Skeleton)

Plants don't have bones. Instead, they build rigid cell walls from cellulose — a completely different arrangement of glucose molecules. Cellulose chains lie flat and hydrogen-bond to each other, forming incredibly strong, rope-like fibres.

Watch out

Humans cannot digest cellulose. We lack the enzyme (cellulase) to break the bonds between its glucose units. That's why grass and wood pass right through us — they're structural, not food for us.

Cellulose gives plant cells their shape, allows trees to grow tall, and makes leaves stiff enough to catch sunlight.

The Precise Statement

\alpha

There's also a third, less famous role: sucrose (table sugar) is the main form in which plants transport glucose from leaves to other parts. It's a disaccharide — two glucose-like units stuck together — that dissolves easily in sap and doesn't react as readily as pure glucose.

A Quick Comparison Table

Why this formula?

Carbohydrate Functions in Plants: Why They Matter

Carbohydrates are not just energy sources — they are the structural backbone and chemical currency of plant life. Let’s break down why each key function works the way it does.

1. Photosynthesis: The Source of All Carbohydrates

Key equation:

6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2

Why this holds:

Carbon fixation: Plants use light energy to split water ( $H_2O$ ) into protons, electrons, and oxygen. The electrons reduce $CO_2$ to form glucose ( $C_6H_{12}O_6$ ).
Energy storage: Glucose is the first stable carbohydrate — it stores chemical energy in its C–H bonds. The 6-carbon skeleton is ideal because it can be easily polymerised into starch or cellulose.
Oxygen as byproduct: The oxygen comes from water, not $CO_2$ — proven by isotope labelling ( $^{18}O$ in water appears in $O_2$ ).

Exam tip: Remember — the light reaction produces ATP and NADPH; the Calvin cycle uses them to reduce $CO_2$ to sugar.

2. Starch: Energy Reserve (Why Glucose is Stored as Starch)

Key formula:

\text{Glucose} \xrightarrow{\text{condensation}} \text{Amylose} + \text{Amylopectin} \ (\text{Starch})

Why starch, not free glucose?

Osmotic problem: Free glucose would draw water into cells via osmosis, causing swelling or bursting. Starch is insoluble — it doesn’t affect water potential.
Compact storage: Starch granules pack many glucose units in a small volume. Amylose is helical (tight), amylopectin is branched (even denser).
Quick mobilisation: Enzymes (amylases) can rapidly break starch back to glucose when energy is needed (e.g., at night, during germination).

Derivation insight: The $\alpha$ -(1→4) and $\alpha$ -(1→6) glycosidic bonds in starch are hydrolysable — this is why starch is a reserve, not a structural material.

3. Cellulose: Structural Support (Why Glucose is Polymerised Differently)

Key formula:

\text{Glucose} \xrightarrow{\beta\text{-(1→4) bonds}} \text{Cellulose} \ (\text{linear chains})

Why $\beta$ bonds instead of $\alpha$ ?

$\beta$ -(1→4) linkage flips every alternate glucose molecule 180°. This allows hydrogen bonding between parallel chains, forming strong microfibrils.
Rigidity: Cellulose is crystalline — it resists tensile stress. This is why plant cell walls can withstand turgor pressure.
Indigestibility: Most animals (including humans) lack cellulase enzymes. Only ruminants and termites (with microbial symbionts) can break $\beta$ bonds.

Key contrast: Starch = $\alpha$ bonds (flexible, digestible). Cellulose = $\beta$ bonds (rigid, indigestible). This is a classic exam comparison.

4. Sucrose: Transport Sugar (Why Not Glucose?)

Key formula:

\text{Glucose} + \text{Fructose} \xrightarrow{\text{glycosidic bond}} \text{Sucrose} + H_2O

Why sucrose for transport?

Non-reducing sugar: Sucrose has no free aldehyde/ketone group — it doesn’t react with proteins or other molecules during transport. Glucose would.
Energy efficiency: Sucrose carries two hexoses per molecule — twice the energy per transport event.
Phloem loading: Sucrose is actively loaded into sieve tubes via SUT transporters (sucrose uptake transporters). This creates osmotic flow (pressure flow hypothesis). …

Part (b)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. …

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Animals store carbohydrate as glycogen, chiefly in the liver (and muscle); starch uses $\alpha$ -1,4 glycosidic linkages (helical, digestible) while cellulose uses $\beta$ -1,4 linkages (linear, indigestible).
A peptide linkage is an amide bond between amino acids and a glycosidic linkage is an ether/acetal bond between sugars; a nucleoside is base + sugar, a nucleotide is base + sugar + phosphate.

Part (a)

(i) How carbohydrates are stored in animals

Free glucose would cause osmotic problems, so animals polymerise excess glucose into glycogen, a highly branched polysaccharide ("animal starch"). The branching allows rapid mobilisation of glucose. It is stored mainly in the liver (to regulate blood glucose) and in skeletal muscle.

(ii) Basic structural difference between starch and cellulose

Feature	Starch	Cellulose
Monomer	$\alpha$ -D-glucose	$\beta$ -D-glucose
Linkage	$\alpha$ -1,4 (+ $\alpha$ -1,6 branches in amylopectin)	$\beta$ -1,4 only
Shape	coiled / helical	straight, linear fibres
Digestibility (humans)	digestible ( $\alpha$ -amylase)	indigestible (no cellulase)

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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 26 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 2026Set ANNUAL1 mark
$Q. Write the name of two polysaccharides found in plants.$
›Reveal solutionSolution
The two important polysaccharides found in plants are starch and cellulose.

Concept. Polysaccharides are long condensation polymers of monosaccharide units joined by glycosidic linkages. In plants:

Starch — the food-storage polysaccharide, a polymer of $\alpha$ -D-glucose consisting of amylose (linear) and amylopectin (branched). …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. Which of the following is a polysaccharide ?$
(a) maltose
(b) sucrose
(c) fructose
(d) cellulose
›Reveal solutionSolution
Cellulose is a polymer of thousands of glucose units (a polysaccharide); maltose and sucrose are disaccharides and fructose is a monosaccharide. Answer: (d).

Maltose = glucose + glucose (disaccharide).

Sucrose = glucose + fructose (disaccharide). …
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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 2025Set ANNUAL1 mark
$Q. What is the basic structural difference between starch and cellulose?$
›Reveal solutionSolution
Both are glucose polymers, but they differ in the type of glycosidic linkage joining the glucose units — $\alpha$ in starch, $\beta$ in cellulose — which changes the overall shape and digestibility of the polymer.

Structural comparison

Starch is a polymer of $\alpha$ -D-glucose units. It consists of two components: amylose (a long unbranched chain of glucose units joined by $\alpha(1\rightarrow4)$ -glycosidic linkages, which coils into a helical structure) and amylopectin (a branched-chain polysaccharide of glucose units joined mainly by $\alpha(1\rightarrow4)$ linkages, with branching through $\alpha(1\rightarrow6)$ -glycosidic linkages roughly every 25 units).

Cellulose is a polymer made up only of $\beta$ -D-glucose units, joined exclusively by $\beta(1\rightarrow4)$ -glycosidic linkages, forming a long, straight, unbranched chain. These linear chains run parallel and are extensively hydrogen-bonded to each other, giving cellulose fibres great mechanical strength (as in the plant cell wall). …
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CBSE 2024Set A11 markMCQ
$Q. The carbohydrate which is also known as animal starch and stored in animal body is :$
(a) Starch
(b) Sucrose
(c) Glycogen
(d) Cellulose
›Reveal solutionSolution
Glycogen is the reserve carbohydrate stored in the animal body, hence "animal starch" — option (c).

Carbohydrates that act as food-storage (reserve) polysaccharides differ between plants and animals:

In plants the reserve carbohydrate is starch. …
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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
$Q. Name two components of starch.$
›Reveal solutionSolution
Starch is a mixture of two glucose polymers — the linear, helical amylose and the highly branched amylopectin.

Starch, the storage polysaccharide of plants, is composed of two distinct components, both built from $\alpha$ -D-glucose units:

Amylose (≈15–20% of starch): a linear, unbranched chain of $\alpha$ -D-glucose units joined by $\alpha(1\rightarrow4)$ glycosidic linkages. It coils into a helical shape and is responsible for the blue-black colour starch gives with iodine (iodine molecules slot inside the helix). …
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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 F1 markMCQ
$Q. Which of the following carbohydrates is the most abundant in nature?$
(a) Glucose
(b) Fructose
(c) Starch
(d) Cellulose
›Reveal solutionSolution

Cellulose, the structural polysaccharide of plant cell walls, is the most abundant carbohydrate (and organic compound) on Earth.

Cellulose is a straight-chain polymer of beta-D-glucose units joined by beta-1,4-glycosidic linkages. It forms the structural framework of plant cell walls and, since plants dominate the biosphere, it is the single most abundant carbohydrate in nature. Glucos …

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CBSE 2023Set ANNUAL1 mark
$Q. A mixture of amylose and amylopectin is called ______.$
›Reveal solutionSolution
Starch, the storage polysaccharide in plants, is a mixture of two glucose polymers: amylose and amylopectin.

Starch is made of two components:

Amylose (~15-20%): a long unbranched chain of alpha-D-glucose units joined by C1-C4 glycosidic linkages; it is water-soluble and responsible for the blue colour with iodine.

Amylopectin (~80-85%): a branched-chain polymer of alpha-D-glucose, with branching occurring through C1-C6 glycosidic linkages; it is insoluble in water. …
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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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Q.(a)

Part (a)

(i) How carbohydrates are stored in animals

(ii) Basic structural difference between starch and cellulose

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