Question
Explain the transpiration pull (cohesion-tension) theory of water transport in plants. How does water rise to the top of tall trees against gravity?
(NCERT Class 11, commonly asked in NEET)
Solution — Step by Step
Tall trees like redwoods can be over 100 metres tall. Water must travel from the roots to the topmost leaves, overcoming gravity and friction in the narrow xylem vessels. Root pressure alone cannot push water this high (it can only account for a few metres). So what pulls the water up?
The answer: transpiration pull — a pulling force generated at the leaves that draws water upward through the xylem.
When water evaporates from the leaf surface through stomata (transpiration), it creates a water deficit in the mesophyll cells. This pulls water from the nearby xylem vessels in the leaf veins.
As water leaves the xylem in the leaf, it creates a negative pressure (tension) that is transmitted downward through the continuous water column in the xylem — all the way to the roots. This tension pulls water upward like a chain being pulled from the top.
For this continuous water column to work without breaking, two forces are essential:
- Cohesion: The attraction between water molecules (due to hydrogen bonding). This keeps the water column intact — water molecules stick to each other and resist being pulled apart.
- Adhesion: The attraction between water molecules and the walls of the xylem vessels. This prevents the water column from pulling away from the vessel walls.
Together, cohesion and adhesion maintain an unbroken water column from root to leaf — this is why the model is also called the cohesion-tension theory (proposed by Dixon and Joly in 1894).
Root hair (absorption by osmosis) → Root cortex → Endodermis (Casparian strip forces apoplastic water into symplast) → Xylem of root → Xylem of stem → Xylem of leaf veins → Mesophyll cells → Evaporation through stomata
The driving force at each step is the water potential gradient — water moves from higher water potential (soil) to lower water potential (leaf mesophyll to atmosphere). The atmosphere has the lowest water potential, so it provides the ultimate driving force.
Why This Works
The transpiration pull theory is the most widely accepted explanation for long-distance water transport in plants. The key physical property that makes it possible is the high tensile strength of water — water in narrow xylem tubes can sustain tensions of up to MPa without the column breaking. This is more than enough to lift water 100+ metres against gravity (which requires only about MPa per 10 metres).
Transpiration is both beneficial (drives water transport, cools the leaf) and costly (90-95% of water absorbed by roots is lost through transpiration). Plants regulate this trade-off by opening and closing stomata.
For NEET, know the three forces: transpiration pull (main driving force for long-distance transport), root pressure (pushes water short distances, causes guttation), and capillarity (contributes minimally). Transpiration pull can generate tensions of up to MPa — far exceeding what is needed.
Common Mistake
The most common error: writing that root pressure is the main force for water transport in tall trees. Root pressure can push water only a few metres and is absent in most tall trees. The main force is transpiration pull. Root pressure is significant only in short herbaceous plants and is responsible for phenomena like guttation (water droplets on leaf edges in the morning).
Another mistake: calling the water column in xylem “pumped” upward. There is no biological pump involved. The movement is purely physical — driven by evaporation at the top and cohesive forces holding the water column together. The xylem vessels are dead at maturity and play no active role.