Q.102 Botany ⭐ 360° Option Elimination Active

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NEET 2026 Botany Question 102 Solution

Topic: Photosynthesis in Higher Plants · Official NTA Booklet Code 50

Question

Which pigment has absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700)?

(1)Chlorophyll b
(2)Chlorophyll a
(3)Xanthophylls
(4)Carotenoids

✓

Verified Against NTA Final Answer Key

Option (2) : Chlorophyll a

📚 Photosynthesis in Higher Plants Easy

📋 Given Parameters & Problem Setup

Photosystem PS I (Photosystem I / P700)

Absorption Peak 700 nm in the red region

Target Pigment acting as the reaction centre

⚡

The 30-Second Exam Hall Shortcut: ⚡ The Chief Photosynthetic Pigment Rule

⏱️ Save ~40 seconds

Reaction centres of BOTH PS I (P700) and PS II (P680) are ALWAYS Chlorophyll a!

1 In green plants, the reaction centre is formed by a single molecule of CHLOROPHYLL A.

2 In PS I: Chlorophyll a has an absorption peak at 700 nm (P700).

3 In PS II: Chlorophyll a has an absorption peak at 680 nm (P680).

4 Chlorophyll b, xanthophylls, and carotenoids are merely accessory antenna pigments that transfer energy to Chlorophyll a.

5 Option (2) is Chlorophyll a! Pick Option (2) in 5 seconds!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Photosynthesis verbatim: The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Structure of the Light Harvesting Complex (LHC)

Photosystems are composed of two functional parts: 1. Antenna complex (LHC): Hundreds of accessory pigment molecules (Chlorophyll b, xanthophylls, carotenoids) bound to proteins that harvest light of various wavelengths and transfer energy by resonance. 2. Reaction Centre: A unique single molecule of Chlorophyll a that traps excitation energy and ejects high-energy electrons into the electron transport chain.

\text{Reaction Centre} \equiv \text{Single molecule of Chlorophyll } a

• Chlorophyll a is the essential primary photosynthetic pigment.

Step 2

Contrast PS I (P700) and PS II (P680)

The protein environment alters the absorption maxima of the reaction centre chlorophyll a in the two photosystems: - In PS I: Reaction centre chlorophyll a has an absorption maximum at 700 nm (designated P700). - In PS II: Reaction centre chlorophyll a has an absorption maximum at 680 nm (designated P680).

\text{PS I Reaction Centre} = \text{Chlorophyll } a \; (\lambda_{\text{max}} = 700\text{ nm})

• Chlorophyll a is the unique pigment forming P700.

Step 3

Conclusion

Chlorophyll a has the absorption peak at 700 nm in the reaction centre PS I (P700).

\text{Matches Option (2)}

• Matches Option (2).

⭐

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 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Chlorophyll b is an accessory pigment with absorption maxima in blue (~450 nm) and red-orange (~640 nm); it never acts as a reaction centre.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing accessory chlorophyll b with reaction centre chlorophyll a.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

Exact match with NCERT: Chlorophyll a is the universal reaction centre of PS I (P700) and PS II (P680).

💡

Why this is airtight: None; textbook mastery of photosynthetic systems.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Xanthophylls are yellow accessory pigments that protect chlorophyll a from photo-oxidation.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming accessory pigments act as reaction centres.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Carotenoids are shield pigments that transfer absorbed photons to chlorophyll a.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking carotenoids absorb at 700 nm.

⚠️

NTA Trap Alert: Thinking Chlorophyll b is the Reaction Centre for One of the Photosystems

Students sometimes guess that since there are two photosystems (PS I and PS II), one must use Chlorophyll a and the other Chlorophyll b. Both PS I and PS II use CHLOROPHYLL A as their reaction centre!

🛡️ How to avoid the -1 mark: Chlorophyll a is the King: It is the reaction centre for BOTH PS I (700 nm) and PS II (680 nm).

📖

Core NEET Syllabus Standard

NEET Biology Core Syllabus · Photosynthesis in Higher Plants Standard Benchmark

Core Scientific Principle Tested:

"Chlorophyll a forms the reaction centre: P700 in PS I and P680 in PS II."

Syllabus Focus: Light reactions - The electron transport chain and Photosystems I and II

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Chlorophyll a is always King: P680 and P700 under its wing!

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NEET 2026 Botany Question 102 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The Chief Photosynthetic Pigment Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Structure of the Light Harvesting Complex (LHC)

Contrast PS I (P700) and PS II (P680)

Conclusion

360° Option Elimination Engine Key Feature

NTA Trap Alert: Thinking Chlorophyll b is the Reaction Centre for One of the Photosystems

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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