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Difficulty: MediumTransport Mechanisms in Plants

During water absorption by root cells, water moves through the root cortex via both apoplastic and symplastic pathways. Which anatomical feature in the endodermis blocks the movement of water along the apoplast pathway, forcing it to pass through the living cytoplasm via the symplast pathway?

  1. The Casparian strip impregnation of suberin in endodermal cell wallsAnswer
  2. B
    Lignified rings and spirals within the xylem vessel elements
  3. C
    Living sieve plates located at the ends of sieve tube elements
  4. D
    Dense cytoplasm and prominent nuclei of pericycle cells

Answer

The Casparian strip impregnation of suberin in endodermal cell walls
The Casparian strip is a band of suberin deposited in the cell walls of the root endodermis. Because suberin is waxy and impermeable to water, it blocks passive apoplastic flow through cell wall spaces, compelling water and dissolved mineral ions to enter the plasma membrane and travel symplastically, thereby enabling selective mineral absorption.

Step-by-Step Solution

1
Identify the two pathways of water movement in root cortex tissues.
Water moves through cell walls/intercellular spaces (apoplast pathway) or through interconnected cell cytoplasm via plasmodesmata (symplast pathway).
Understanding the routes available for water transport across the root cortex towards the vascular cylinder.
2
Analyze the structural modification present at the endodermal layer.
Endodermal cells possess radial and transverse cell walls impregnated with a waxy, hydrophobic substance called suberin, known as the Casparian strip.
Suberin prevents water and dissolved minerals from moving freely through non-living cell walls.
3
Determine the functional consequence of the Casparian strip.
Apoplast movement is completely blocked, forcing water and solute molecules to cross the selectively permeable plasma membrane into the symplast pathway.
This allows the plant root to exert metabolic control over which mineral ions enter the vascular stream.

Key Concept

Apoplast and Symplast Transport across the Root Endodermis
Estimated Time:1m 0s
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Topics

Transport Mechanisms in Plants

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