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Zorluk: ZorTransport Mechanisms in Plants

During rapid transpiration in a tall dicotyledonous tree, water moves continuously from the soil through the plant body to the atmosphere along a water potential (Ψ\Psi) gradient. Which of the following statements correctly describes the physical state of water and the primary driving mechanism within the xylem vessels during this process?

  1. Water is maintained under negative hydrostatic pressure (tension) and pulled upward due to transpirational evaporation at the leaf surface.Cevap
  2. B
    Water is pushed upward under positive hydrostatic pressure generated by the active ATP-dependent secretion of water into xylem vessels by root endodermal cells.
  3. C
    Water moves down an increasing water potential gradient from the leaf mesophyll cells to the root xylem elements via active translocation.
  4. D
    Water flows bidirectionally through xylem vessel elements propelled by osmotic pressure gradients created by translocated sucrose.

Cevap

Water is maintained under negative hydrostatic pressure (tension) and pulled upward due to transpirational evaporation at the leaf surface.
According to the cohesion-tension theory, transpiration causes evaporation of water from leaf mesophyll cells into sub-stomatal air spaces. This lowers the water potential of the leaves and creates a continuous suction force that puts the water inside dead xylem vessels under tension (negative pressure). The cohesive properties of water molecules maintain an unbroken column pulled upward from the roots.

Adım Adım Çözüm

1
Analyze the water potential gradient along the transpiration stream.
Water moves spontaneously from regions of higher (less negative) water potential in the soil and roots toward lower (more negative) water potential in the leaf air spaces and atmosphere.
Evaporation of water vapor through stomata creates a strong suction force at the top of the plant.
2
Evaluate the mechanical state of water in xylem vessels according to the cohesion-tension theory.
The continuous evaporation generates tension (negative hydrostatic pressure) in the xylem sap, pulling the water column upward intact due to hydrogen bonding (cohesion between water molecules and adhesion to vessel walls).
Passive transpirational pull under tension is the primary driving mechanism for long-distance xylem transport in tall plants during high transpiration rates.

Anahtar Kavram

Cohesion-Tension Theory and Water Potential Gradient in Xylem Transport
Tahmini Süre:2m 0s
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Konular

Transport Mechanisms in Plants

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