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Differentiate between active water absorption and passive water absorption.

GeneralClass 12AllAnswered 27 Mar 2026
Answer

Parameter

Active Water Absorption

Passive Water Absorption

Energy Requirement

Requires metabolic energy (ATP)

No energy required (passive process)

Driving Force

Force originates in the root (root pressure)

Force originates in shoot (transpiration pull)

Mechanism

Active ion uptake into xylem → osmotic water entry → positive pressure in xylem

Water loss via transpiration → negative pressure (tension) in xylem → water pulled upward

Pressure in Xylem

Positive pressure (root pressure)

Negative pressure (tension/suction)

Occurrence

Common in herbaceous plants, grasses, and when transpiration is low (humid conditions, night)

Operates in all plants, dominant during high transpiration

Proportion

Only 1-4% of total water absorption

96-99% of total water absorption

Essential Factor

Root pressure is essential

Transpiration is essential

Observable Phenomenon

Guttation (water exudation from hydathodes)

Wilting when water supply insufficient

Conditions Favored

High soil moisture, high humidity, low temperature

High temperature, low humidity, wind

Key Distinction: Active absorption is a root-driven process, while passive absorption is shoot-driven.

Practical Significance: Understanding this helps explain why plants wilt during hot afternoons (excessive passive absorption exceeding root supply) and show guttation on humid mornings (active absorption continues when transpiration is minimal).

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