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Phloem Loading

Hardbiology

Active phloem loading of sucrose into sieve elements raises osmotic pressure causing:

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About This Question

Subject
biology
Chapter
plant physiology
Topic
phloem loading
Difficulty
Hard
Year
2025
Tags
PhloemTransport

Solution

Correct Answer:

Turgor increase driving mass flow

  1. Munch's Mass Flow Hypothesis: Explains how sugars are transported in phloem from 'source' to 'sink'.
  2. Phloem Loading: Sugars are actively loaded into the sieve tube elements at the source (leaves).
  3. Osmosis: This high concentration of sugar lowers the water potential in the sieve tube, causing water to diffuse in from the adjacent xylem by osmosis.
  4. Pressure Build-up: The entry of water increases the hydrostatic pressure (turgor pressure) in the phloem.
  5. Bulk Flow: This high-pressure area 'pushes' the sap toward the low-pressure 'sink' (roots/fruits), where sugars are unloaded.

This hard difficulty biology question is from the chapter plant physiology, covering the topic of phloem loading. It appeared in the 2025 exam.

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