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Difficulty: MediumTransport Mechanisms in Plants

Arrange the following physiological processes in the correct chronological sequence to describe the complete pathway of water transport from the soil through a vascular plant into the atmosphere.

  1. 1Water enters root hair cells from the soil by osmosis down a water potential gradient.
  2. 2Water moves radially across the root cortex and crosses the endodermis into the root xylem vessels.
  3. 3Water ascends through stem xylem vessels as a continuous column sustained by cohesion and adhesion.
  4. 4Water evaporates from spongy mesophyll cell walls into intercellular spaces and diffuses out through stomata.

Answer

The correct sequence begins with osmotic water uptake at the root hair cells, followed by radial passage across the root cortex and endodermis into root xylem, then upward movement through stem xylem vessels via cohesion-tension, and terminates with evaporation and diffusion from leaf mesophyll out through stomata.
The transpiration stream operates as a continuous unidirectional pathway. Water first enters root hairs by osmosis, moves across the root cortex into the root xylem, ascends through the stem xylem under cohesion-tension, and finally evaporates from mesophyll cells and diffuses through stomata into the surrounding air.

Step-by-Step Solution

1
Identify the initial uptake point of water from the environment.
Soil water enters root hair cells via osmosis across a selectively permeable membrane.
Root hair cells present a large surface area with a lower water potential than soil water.
2
Trace the movement of water inward through root tissues.
Water travels across cortex parenchyma cells and past the endodermal Casparian strip into root xylem vessels.
Endodermal regulation ensures selective solute movement into the vascular elements.
3
Determine how water is transported long-distance through the stem.
Water is pulled upward through stem xylem vessels as an unbroken column.
Cohesive forces between water molecules and adhesive forces against xylem walls prevent column breakage under tension.
4
Identify the exit stage of water from the plant to the atmosphere.
Water evaporates from spongy mesophyll surfaces into sub-stomatal air cavities and diffuses out into the atmosphere.
This process (transpiration) maintains the transpiration pull driving continuous upward water transport.

Key Concept

Pathway and Mechanism of Transpiration Stream
Estimated Time:1m 30s
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Topics

Transport Mechanisms in Plants

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