Question:
Assertion (A): In Euglena, forward movement is produced mainly by the effective stroke of the flagellum.
Reason (R): During the recovery stroke, the flagellum becomes stiff and pushes the body forward.
Assertion (A): In Euglena, forward movement is produced mainly by the effective stroke of the flagellum.
Reason (R): During the recovery stroke, the flagellum becomes stiff and pushes the body forward.
Reason (R): During the recovery stroke, the flagellum becomes stiff and pushes the body forward.
Show Hint
Effective stroke = Rigid, powerful backward sweep.
Recovery stroke = Flexible, low-resistance forward return.
This dual-phase movement is similar to the swimming stroke used by a human swimmer.
Recovery stroke = Flexible, low-resistance forward return.
This dual-phase movement is similar to the swimming stroke used by a human swimmer.
Updated On: Jul 22, 2026
- Both (A) and (R) are true. (R) is correct explanation for (A)
- Both (A) and (R) are true. (R) is not correct explanation for (A)
- (A) is true. But (R) is false
- (A) is false. But (R) is true
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Question:
The question presents an Assertion about flagellar locomotion in Euglena and a Reason explaining the structural state of the flagellum during the recovery stroke.
Step 2: Key Concepts and Approach:
We analyze the biophysical mechanism of flagellar movement, which occurs in two phases:
1. The effective (or power) stroke.
2. The recovery stroke.
Step 3: Detailed Explanation:
• Assertion (A): During flagellar locomotion in Euglena, forward movement is driven by the effective stroke.
In this phase, the flagellum is held relatively rigid and sweeps backwards like an oar, pushing the water back and propelling the cell forward. Thus, Assertion (A) is true.
• Reason (R): During the recovery stroke, the flagellum bends and becomes highly flexible.
It is pulled forward with minimal resistance to return to its starting position.
It does not become stiff, nor does it push the body forward in this phase. Thus, Reason (R) is false.
Step 4: Final Answer:
Assertion (A) is correct, but Reason (R) is incorrect, corresponding to option (C).
The question presents an Assertion about flagellar locomotion in Euglena and a Reason explaining the structural state of the flagellum during the recovery stroke.
Step 2: Key Concepts and Approach:
We analyze the biophysical mechanism of flagellar movement, which occurs in two phases:
1. The effective (or power) stroke.
2. The recovery stroke.
Step 3: Detailed Explanation:
• Assertion (A): During flagellar locomotion in Euglena, forward movement is driven by the effective stroke.
In this phase, the flagellum is held relatively rigid and sweeps backwards like an oar, pushing the water back and propelling the cell forward. Thus, Assertion (A) is true.
• Reason (R): During the recovery stroke, the flagellum bends and becomes highly flexible.
It is pulled forward with minimal resistance to return to its starting position.
It does not become stiff, nor does it push the body forward in this phase. Thus, Reason (R) is false.
Step 4: Final Answer:
Assertion (A) is correct, but Reason (R) is incorrect, corresponding to option (C).
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