Question
How does water move from roots to leaves against gravity? Explain the cohesion-tension theory (transpiration pull mechanism) with reference to the forces involved.
Solution — Step by Step
Root hair cells have a lower water potential than the soil solution, so water enters by osmosis. This creates root pressure — a mild push from below. Root pressure alone can raise water only a few metres, so it is not enough for tall trees.
Leaves lose water vapour through stomata during transpiration. As water evaporates from mesophyll cells, their water potential drops below that of neighbouring cells. This sets up a water potential gradient from leaf → stem → root — a continuous pull directed upward.
The evaporation at the leaf surface creates negative pressure (tension) inside the xylem. Think of it like sipping water through a straw — you create suction at the top, and the liquid column rises. This tension is transmitted all the way down the xylem vessel to the roots.
Water molecules are held together by hydrogen bonds — this is cohesion. Without cohesion, the tension would simply snap the water column and pull in air (a cavitation event). Because water is cohesive, the entire column behaves like a continuous thread that can be pulled upward without breaking.
Water also adheres to the walls of xylem vessels (tracheids and vessels) — this is adhesion. Adhesion counteracts the downward pull of gravity and prevents the water column from draining back when transpiration slows temporarily.
Water always moves from higher water potential to lower. The atmosphere has the most negative , so it is ultimately the final “sink” that drives the whole system.
Why This Works
The elegance of this mechanism — called the cohesion-tension theory, proposed by Dixon and Joly — is that the plant uses atmospheric demand (transpiration) as the engine rather than spending its own metabolic energy. Xylem transport is entirely passive.
The key physical property that makes it possible is water’s unusually high tensile strength due to hydrogen bonding. Water under tension in the xylem is in a metastable state — technically below its vapour pressure, but held liquid by cohesive forces. Measurements in tall trees show xylem pressure can reach −2 to −4 MPa (strongly negative).
Adhesion to hydrophilic xylem walls adds capillary action as a minor contributor, but the dominant force is the transpiration pull transmitted through the cohesive water column. Remove transpiration (block stomata, high humidity) and water movement slows dramatically — which is direct experimental evidence for this theory.
Alternative Method — Pressure Chamber Experiment (Scholander Bomb)
This is how scientists actually measure xylem tension, and NEET sometimes asks about it conceptually.
A freshly cut leaf is placed in a pressure chamber with the cut end protruding. Pressure is applied until xylem sap is just pushed back to the cut surface. The pressure required equals the tension that existed in the xylem before cutting. This elegantly confirms that xylem water is indeed under negative pressure (tension), not positive pressure.
Remember the sequence for NEET one-liners: Transpiration → Tension → Cohesion → Adhesion → Ascent of sap. Four words: T-T-C-A. Many students mix up the order in MCQs.
Common Mistake
Students often say root pressure is the main force for ascent of sap. It is NOT. Root pressure is a minor, supplementary force — it can only raise water 20–30 cm. In tall trees (30 m+), transpiration pull is overwhelmingly responsible. NEET 2023 had a direct question on this: the correct answer was transpiration pull, not root pressure. Also — xylem transport is passive (no ATP involved). Students sometimes confuse it with active transport.
Final answer: Water rises from roots to leaves via the cohesion-tension (transpiration pull) mechanism — transpiration lowers water potential in leaves, creating tension in the xylem; cohesion of water molecules (H-bonds) keeps the column intact; adhesion to xylem walls prevents backflow. The process is entirely passive — no ATP required.