Question:
In sieve tubes source to sink pressure gradient is maintained by:
In sieve tubes source to sink pressure gradient is maintained by:
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Companion cells and sieve tubes form a functional unit in phloem transport.
Updated On: Jun 20, 2026
- Phloem parenchyma
- Sieve cells
- Sieve tubes only
- Companion cells
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The Correct Option is D
Solution and Explanation
Step 1: Understanding translocation in phloem.
Food transport in plants occurs through phloem by the pressure flow hypothesis, where a source-to-sink pressure gradient drives movement of sugars.
Step 2: Role of sieve tubes.
Sieve tubes are the main conducting elements but they lack nuclei and metabolic machinery, so they cannot maintain gradients independently.
Step 3: Role of companion cells.
Companion cells are metabolically active and provide ATP and regulatory support to sieve tube elements. They actively load and unload sugars, maintaining osmotic pressure differences.
Step 4: Why other options are incorrect.
Phloem parenchyma plays a storage role, sieve cells are primitive conducting cells (gymnosperms), and sieve tubes alone cannot maintain gradients.
Step 5: Mechanism explanation.
Companion cells actively load sucrose into sieve tubes at the source and help unloading at the sink, maintaining continuous pressure gradient.
Step 6: Final conclusion.
Thus, companion cells are responsible for maintaining the source-sink pressure gradient.
\[ \boxed{(4)} \]
Food transport in plants occurs through phloem by the pressure flow hypothesis, where a source-to-sink pressure gradient drives movement of sugars.
Step 2: Role of sieve tubes.
Sieve tubes are the main conducting elements but they lack nuclei and metabolic machinery, so they cannot maintain gradients independently.
Step 3: Role of companion cells.
Companion cells are metabolically active and provide ATP and regulatory support to sieve tube elements. They actively load and unload sugars, maintaining osmotic pressure differences.
Step 4: Why other options are incorrect.
Phloem parenchyma plays a storage role, sieve cells are primitive conducting cells (gymnosperms), and sieve tubes alone cannot maintain gradients.
Step 5: Mechanism explanation.
Companion cells actively load sucrose into sieve tubes at the source and help unloading at the sink, maintaining continuous pressure gradient.
Step 6: Final conclusion.
Thus, companion cells are responsible for maintaining the source-sink pressure gradient.
\[ \boxed{(4)} \]
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