Q.102 Botany ⭐ 360° Option Elimination Active

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NEET 2023 Botany Question 102 Solution

Topic: Biomolecules · Official NTA Booklet Code G5

Question

Cellulose does not form blue colour with Iodine because

✓

Verified Against NTA Final Answer Key

Option (4) : It does not contain complex helices and hence cannot hold iodine molecules.

📚 Biomolecules Easy

📋 Given Parameters & Problem Setup

Macromolecule Cellulose (unbranched β-1,4-glucan polymer)

Reagent Iodine solution (

I_2 / KI

)

Test Result No blue color formation

⚡

The 30-Second Exam Hall Shortcut: ⚡ Helical Starch vs. Linear Cellulose Trap

⏱️ Save ~20 seconds

Starch forms secondary helical structures that physically trap $I_2$ molecules in their interior coils to give an intense blue color. Cellulose is an unbranched, rigid, linear polymer lacking secondary helices, so it cannot hold iodine!

1 Starch gives blue color with

I_2

because amylose forms helical coils accommodating polyiodide ions.

2 Cellulose consists of unbranched chains held by hydrogen bonds into linear microfibrils without helical coils.

3 Without helical coils, no iodine entrapment can occur.

4 Option (4) gives this exact conformational explanation.

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Biomolecules structural polysaccharide principle.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Mechanism of Starch-Iodine Complex

Amylose in starch forms a helical secondary structure with internal hydrophobic cavities:

\text{Amylose (\alpha-1,4-helical)} + I_2 \longrightarrow [\text{Starch}\cdot I_5^-]_{(\text{inclusion complex})} \implies \text{\textbf{Deep Blue Colour}}

Step 2

Structural Architecture of Cellulose

Cellulose is a linear, unbranched homopolymer of $\beta$ -D-glucose linked via $\beta$ -1,4-glycosidic bonds. Extensive inter- and intramolecular hydrogen bonding holds adjacent chains parallel in planar sheets, preventing helical coil formation:

\text{Cellulose (\beta-1,4-linear)} \implies \text{No helical interior} \implies \text{\textbf{Cannot hold }} I_2

Step 3

Chemical Inertness to Iodine

Because cellulose does not possess the prerequisite helical secondary structure to trap polyiodide ions, it produces a negative result (no blue colouration) upon addition of iodine solution.

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 1

✗ Incorrect

Verdict & Scientific Reason:

Cellulose does not undergo chemical hydrolysis or breakdown upon exposure to dilute iodine.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming a negative visual test implies destruction of the substrate.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Cellulose is a high-molecular-weight homopolysaccharide composed of thousands of glucose units, not a disaccharide.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing cellobiose (disaccharide) with cellulose (polysaccharide).

3 3

✗ Incorrect

Verdict & Scientific Reason:

Cellulose is linear, not helical. Claim contradicts known polysaccharide biophysics.

⚠️

Student Fallacy (Why candidates pick this trap): Misremembering starch characteristics as cellulose characteristics.

4 4

✓ 100% Correct

Verdict & Scientific Reason:

Accurate NCERT biophysical rationale: absence of secondary helical coils prevents entrapment of iodine molecules.

💡

Why this is airtight: None; verbatim textbook statement.

⚠️

NTA Trap Alert: Chemical Reaction vs. Physical Entrapment Trap

Students frequently believe iodine chemically reacts with starch covalently, rather than being physically entrapped inside helical coils.

🛡️ How to avoid the -1 mark: Remember that starch-iodine color is a physical coordination inclusion complex, fully dependent on helical secondary geometry.

📖

Core NEET Syllabus Standard

NCERT Biology · Biomolecules Standard Benchmark

Core Scientific Principle Tested:

"Starch forms helical secondary structures and can hold I2 molecules in the helical portion. Cellulose does not contain complex helices and hence cannot hold I2."

Syllabus Focus: Polysaccharides

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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Classification & Functions of Biomolecules (Proteins, Polysaccharides & Secondary Metabolites)

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NEET 2023 Botany Question 102 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Helical Starch vs. Linear Cellulose Trap

Step-by-Step Rigorous Workflow & Mathematical Proof

Mechanism of Starch-Iodine Complex

Structural Architecture of Cellulose

Chemical Inertness to Iodine

360° Option Elimination Engine Key Feature

NTA Trap Alert: Chemical Reaction vs. Physical Entrapment Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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