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Q.116 Botany ⭐ 360° Option Elimination Active
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NEET 2023 Botany Question 116 Solution

Topic: Photosynthesis in Higher Plants · Official NTA Booklet Code G5
NEET 2023 Botany Question 116: Photosystem II Reaction Centre (P680)

Question

The reaction centre in PS II has an absorption maxima at

  1. (1)780 nm
  2. (2)680 nm
  3. (3)700 nm
  4. (4)660 nm
✓
Verified Against NTA Final Answer Key
Option (2) : 680 nm
📚 Photosynthesis in Higher Plants Easy
📋 Given Parameters & Problem Setup
Photosystem Photosystem II (PS II)
Reaction Centre Chlorophyll a molecule
⚡

The 30-Second Exam Hall Shortcut: ⚡ PS II = P680 vs. PS I = P700 Rule

⏱️ Save ~20 seconds

The reaction centre chlorophyll a of PS II absorbs light maximally at 680 nm (called P680), whereas the reaction centre of PS I absorbs maximally at 700 nm (called P700)!

1 Photosystem I has reaction centre P700P_{700} (peak at 700 nm in red spectrum).
2 Photosystem II has reaction centre P680P_{680} (peak at 680 nm in red spectrum).
3 The prompt asks specifically for PS II.
4 Match Option (2) 680 nm directly.
🛡️
Why You Can 100% Trust This Shortcut: Canonical NCERT Photosynthesis photochemical reaction centre biophysics.
📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Organization of the Light Harvesting Complex (LHC)

Photosystems comprise hundreds of pigment molecules bound to proteins forming antennae that funnel excitation energy to a unique single chlorophyll aa reaction centre:

Photosystem=Antenna Pigments (LHC)+Core Reaction Centre (Chlorophyll a)\text{Photosystem} = \text{Antenna Pigments (LHC)} + \text{Core Reaction Centre (Chlorophyll } a)
Step 2

Spectroscopic Absorption Maxima

The two photosystems are designated based on the red absorption peak wavelength of their respective reaction centre chlorophyll aa molecules:

PS II:Reaction centre P680  ⟹  λmax⁡=680 nm (Red Light)PS I:Reaction centre P700  ⟹  λmax⁡=700 nm (Far-Red Light)\begin{aligned} \text{\textbf{PS II:}} & \quad \text{Reaction centre } \mathbf{P_{680}} \implies \lambda_{\max} = \mathbf{680\text{ nm (Red Light)}} \\[4pt] \text{PS I:} & \quad \text{Reaction centre } P_{700} \implies \lambda_{\max} = 700\text{ nm (Far-Red Light)} \end{aligned}
Step 3

Verbatim NCERT Confirmation

NCERT states verbatim: "In PS II the reaction centre chlorophyll a has an absorption peak at 680 nm, and is called P680."

⭐

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:

780 nm lies in the near-infrared region, outside the photosynthetically active radiation (PAR, 400-700 nm) spectrum of oxygenic phototrophs.

⚠️
Student Fallacy (Why candidates pick this trap): Conflating bacterial infrared bacteriochlorophyll peaks with plant PS II.
2 2
✓ 100% Correct
Verdict & Scientific Reason:

Exact NCERT peak absorption wavelength for the chlorophyll a reaction centre of PS II (P680P_{680}).

💡
Why this is airtight: None; textbook verbatim.
3 3
✗ Incorrect
Verdict & Scientific Reason:

700 nm is the absorption peak of Photosystem I (P700P_{700}), not Photosystem II.

⚠️
Student Fallacy (Why candidates pick this trap): Inverting the photosystem labels (confusing PS I with PS II).
4 4
✗ Incorrect
Verdict & Scientific Reason:

660 nm is the absorption peak for the red-absorbing form of phytochrome (PrP_r), not the PS II reaction centre.

⚠️
Student Fallacy (Why candidates pick this trap): Confusing phytochrome photobiology with photosynthetic photosystems.
⚠️

NTA Trap Alert: PS I vs. PS II Numbering Inversion Trap

PS II acts first in non-cyclic electron transport (Z-scheme), but was discovered second, causing students to associate PS II with 700 nm and PS I with 680 nm.

🛡️ How to avoid the -1 mark: Remember: PS II operates at the shorter wavelength: P680P_{680}. PS I operates at the longer wavelength: P700P_{700}.
📖

Core NEET Syllabus Standard

NCERT Biology · Photosynthesis in Higher Plants Standard Benchmark
Core Scientific Principle Tested:

"In PS II the reaction centre chlorophyll a has an absorption peak at 680 nm, and is called P680."

Syllabus Focus: The Electron Transport System (Light Reaction)
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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