Q.44 Given below are two statements : One is labelled as Assertion (A) and the other is
labelled as Reason (R).

Assertion (A) :The thylakoid reactions of photosynthesis take place in the
specialized internal membranes of chloroplast called thylacoids.

Reasons (R) :The end products of these thylakoid reactions are used for the
synthesis of sugars in the carbon fixation reactions.

In the light of the above statements, choose the most appropriate answer from the
options given below :
(1)Both (A) and (R) are correct and (R) is the correct explanation of (A)
(2)Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
(3)(A) is correct but (R) is not correct
(4)(A) is not correct but (R) is correct

This is a classic Assertion (A)-Reason (R) question from exams like GATE Life Sciences. Let’s break it down step by step, evaluate each statement against photosynthesis biology, then analyze the options.

Quick Facts on Photosynthesis

Photosynthesis occurs in chloroplasts and has two main stages:

• Light-dependent reactions (thylakoid reactions): Happen in thylakoid membranes. Light energy splits water, producing ATP and NADPH, plus oxygen.

• Light-independent reactions (carbon fixation, Calvin cycle): Happen in the stroma. ATP and NADPH reduce CO₂ to sugars (glucose).

Now, evaluate the statements:

• Assertion (A): “The thylakoid reactions of photosynthesis take place in the specialized internal membranes of chloroplast called thylakoids.”

  • True. Thylakoids are stacked into grana within chloroplasts. Photosystems I and II, embedded in thylakoid membranes, capture light for the reactions. Standard textbook fact (e.g., in Campbell Biology).

• Reason (R): “The end products of these thylakoid reactions are used for the synthesis of sugars in the carbon fixation reactions.”

  • True. End products are ATP and NADPH, which power the Calvin cycle (carbon fixation) in the stroma to make sugars like G3P → glucose.

Key point: Both A and R are factually correct. But does R explain A? No—R describes what happens after thylakoid reactions (output usage), not why they occur in thylakoids. The location (A) is due to thylakoids housing pigments and electron transport chains for light capture, independent of downstream sugar synthesis (R).

Option Analysis

1. Both (A) and (R) are correct and (R) is the correct explanation of (A)
   Incorrect. R is true but doesn’t explain why reactions are in thylakoids—it just links outputs to the next stage.

2. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
   Correct. Matches perfectly: facts right, no explanatory link.

3. (A) is correct but (R) is not correct
   Incorrect. R is accurate—ATP/NADPH are used in carbon fixation.

4. (A) is not correct but (R) is correct
   Incorrect. A is spot-on; thylakoids are the site.

Answer: (2)

Example: Imagine thylakoids as solar panels (site of energy capture, A). Panels produce electricity (ATP/NADPH), used later to run a factory making sugar (R). The factory need doesn’t explain why panels are on the roof—that’s for sunlight access.

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Thylakoid Reactions of Photosynthesis: Assertion Reason Explained for GATE Life Sciences

Thylakoid reactions of photosynthesis power the light-dependent stage in chloroplasts, converting sunlight into chemical energy. If you’re prepping for GATE Life Sciences, mastering this topic is key—especially Assertion-Reason questions testing location and outputs.

What Are Thylakoid Reactions of Photosynthesis?

Thylakoid reactions occur in the specialized internal membranes of chloroplast called thylakoids. These flattened sacs stack into grana, housing photosystems for light absorption.

• Light hits chlorophyll in Photosystem II.

• Water splits (2H2O→4H++4e−+O22H2O→4H++4e−+O2).

• Electrons flow via carriers, pumping protons for ATP synthesis.

• NADPH forms in Photosystem I.

End products: ATP and NADPH—fuel for sugar synthesis.

Role in Carbon Fixation Reactions

Thylakoid reactions of photosynthesis link directly to carbon fixation reactions (Calvin cycle) in the stroma.

• ATP and NADPH reduce CO₂ to G3P (sugar precursor).

• No light needed here—just chemical energy from thylakoids.

This split ensures efficiency: membranes optimize light capture; stroma handles fixation.

Solved: Assertion (A) and Reason (R) on Thylakoid Reactions

Assertion (A): The thylakoid reactions of photosynthesis take place in the specialized internal membranes of chloroplast called thylakoids.
Reason (R): The end products of these thylakoid reactions are used for the synthesis of sugars in the carbon fixation reactions.

Option Breakdown

1. Both correct, R explains A → No: R shows output use, not location reason.

2. Both correct, R does NOT explain A → Yes (Answer).

3. A correct, R incorrect → No: R is true.

4. A incorrect, R correct → No: A is factual.

Pro Tip: Location (thylakoids) suits electron transport; outputs (R) are downstream.

GATE Life Sciences Prep Tips

• Memorize: Thylakoids = light reactions; stroma = dark reactions.

• Practice PYQs: Search “thylakoid reactions photosynthesis assertion reason.”

• Visualize: Thylakoids like “power plants” feeding sugar factories.

Master thylakoid reactions of photosynthesis to ace Section A questions. Questions?

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