Cohesion-Tension and Water Movement in the Xylem
Evaporation from leaf cell walls creates tension that pulls water through the xylem. Hydrogen bonding gives the water column cohesion. Stomatal closure reduces transpiration-driven flow, while gas bubbles can interrupt individual columns.
What a learner can do afterwards
- Explains where the pulling force comes from and where in the plant it acts.
- Links cohesion in the column back to hydrogen bonding between water molecules.
- Uses the theory to predict what happens to the flow when the stomata close.
1 · Read
Water evaporates from wet mesophyll cell walls into leaf air spaces. Water vapour then diffuses mainly through stomata. As water is lost, tension develops in the remaining water films. Tension means pressure below atmospheric pressure. It pulls replacement water from the leaf xylem and is transmitted down connected water columns. The main pull for transpiration-driven flow begins in the leaves.
Hydrogen bonds between neighbouring water molecules produce cohesion, helping transmit tension through a continuous column. Adhesion is attraction between water and xylem walls; it helps support water against gravity. Neither guarantees continuity. Gas bubbles can interrupt a column. This blockage, an embolism, reduces flow through the affected vessel, while other water-filled vessels may still conduct.
Closing stomata reduces water vapour loss. Transpiration-driven tension generally weakens and xylem flow slows. Closure does not instantly remove all tension or stop every route of water loss. Some water can escape through the cuticle or incompletely closed stomata, and stored water can continue moving within the plant.
With open stomata and ample soil water, warmer, drier, windier air usually raises transpiration. At steady water balance, root uptake rises to replace that loss. The rates need not match immediately because stored water can supply the leaves. In drought, stomatal closure or limited soil water may prevent higher uptake despite hot weather.
Mature xylem vessels are dead, hollow conducting tubes with no living cytoplasm. Pressure differences drive water through their open passage, or lumen; the vessels do not need to make ATP for this flow. Root pressure can push water upward in some conditions, but cannot account for ascent to the tops of tall trees by itself. Transpiration provides the main pull.
Leaf evaporation creates tension. Cohesion transmits the pull through connected water columns. Stomatal closure reduces flow; bubbles can interrupt individual vessels.
Lesson, questions and flashcards written for this dataset. OpenStax sections on leaves and transport of water and solutes in plants were consulted for biological facts.
Readings and flashcards written for this dataset. OpenStax Biology and Biology for AP Courses sections on leaves and plant water transport were consulted for biological facts.
More to read
A connected column carries the pull
Water lost from a leaf must be replaced. Evaporation from wet mesophyll walls creates tension in the remaining water films. Replacement water is drawn from the leaf xylem, and connected liquid columns transmit the pull down the plant.
Hydrogen bonding produces cohesion between water molecules. Adhesion is their attraction to vessel walls. These interactions help support the column, but they do not make it unbreakable.
A gas bubble can interrupt a column and reduce flow through that vessel. This blockage is an embolism. Other water-filled vessels can still conduct water, so a local interruption need not stop transport throughout the plant.
Compare water loss with replacement
Root uptake and transpiration need not match at every moment. Water stored in the plant can supply leaves temporarily. Later uptake can replace that stored water.
With open stomata and ample soil water, warm, dry, moving air usually increases transpiration. Once the plant reaches steady water balance, greater loss requires greater uptake. Hot weather alone does not guarantee this response. Drought may limit the water available or cause stomata to close.
Root pressure can contribute to upward transport in some conditions. It cannot account for water reaching the tops of tall trees by itself. Transpiration-driven tension supplies the main pull.
From leaf water films to conducting tubes
Inside a leaf, water evaporates from moist mesophyll cell walls into air spaces. Water vapour leaves mainly through stomata, whose openings are controlled by guard cells.
Closing stomata restricts water loss. The transpiration pull generally weakens and xylem flow slows. Some loss can continue through the cuticle or incompletely closed pores, so closure does not instantly remove every source of water loss or all existing tension.
The mature xylem vessels carrying replacement water are dead and lack living cytoplasm. Their hollow lumen provides a passage for pressure-driven flow. ATP production inside a vessel is not required for this movement, although living plant tissues carry out many other energy-requiring processes.
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