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Difficulty: MediumTransport Mechanisms in Plants

In tall dicotyledonous trees, water and dissolved mineral salts are continuously transported from the roots up to the leaves. Which structural feature and transport mechanism account for this upward, unidirectional flow?

  1. A
    Living sieve tubes transport minerals bidirectionally via active translocation
  2. Lignified, hollow xylem vessels transport water unidirectionally driven by transpiration pullAnswer
  3. C
    Companion cells actively pump water and sucrose upward through phloem under positive root pressure
  4. D
    Non-lignified xylem parenchyma cells translocate dissolved mineral salts bidirectionally by mass flow

Answer

Lignified, hollow xylem vessels transport water unidirectionally driven by transpiration pull
Xylem vessels are made of dead, empty, lignified cells joined end-to-end to form continuous hollow tubes. Evaporation of water vapor through stomata creates transpiration pull (tension), pulling water and mineral salts unidirectionally upward from roots to leaves.

Step-by-Step Solution

1
Identify the vascular tissue responsible for water and mineral transport
Xylem tissue conducts water and dissolved inorganic minerals.
Phloem conducts synthesized organic food (sucrose and amino acids), whereas xylem conducts water and mineral ions.
2
Determine the direction of movement and primary driving force
Movement in xylem is strictly unidirectional (roots to leaves) powered by transpiration pull.
Evaporation of water from leaf mesophyll cells creates negative pressure (tension) that pulls water upwards through the continuous column of dead, lignified xylem vessels.

Key Concept

Xylem Structure and Transpiration Pull Mechanism
Estimated Time:1m 0s
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Topics

Transport Mechanisms in Plants

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