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BIO NMDCAT Biological Importance of Water
nmdcat.online June 27, 2026

The movement of water up a tall tree against the force of gravity is best explained by the

A. Root pressure theory only
B. Capillary action due to the narrow diameter of xylem vessels alone
C. Cohesion-tension (transpiration pull) mechanism
✓
D. Active pumping of water molecules by living xylem cells

📝 Explanation

The widely accepted mechanism is the cohesion-tension theory. Transpiration from leaves generates negative pressure (tension), pulling the water column up. Cohesion between water molecules transmits this pull down the entire continuous column from roots to leaves.

📖 Additional Information

  • Root pressure theory only
  • Capillary action due to the narrow diameter of xylem vessels alone
  • Cohesion-tension (transpiration pull) mechanism
  • Active pumping of water molecules by living xylem cells

The widely accepted mechanism is the cohesion-tension theory. Transpiration from leaves generates negative pressure (tension), pulling the water column up. Cohesion between water molecules transmits this pull down the entire continuous column from roots to leaves.

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