Cohesion-Tension in Xylem
The tallest trees on Earth lift water more than 100 metres into the air, higher than any suction pump could ever pull it. There is no muscle and no motor doing this. Instead, water evaporating from leaves drags an unbroken chain of water molecules up the xylem, held together by their own stickiness. This lesson explains the cohesion-tension theory: how the sun, in effect, pulls a thread of water up a tree.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upThink about sucking a thick milkshake up a straw. You create low pressure at the top, and the whole column of liquid moves up together.
Before reading: predict where the "sucking" comes from in a plant (there is no mouth), and why the water doesn't simply break apart into separate drops as it is pulled up.
Learning Intentions
goalsKnow
- What transpiration, cohesion and tension mean
- The role of osmotic pressure in pulling water into the root
- The three parts of the cohesion-tension theory
Understand
- How evaporation from leaves creates tension (negative pressure)
- Why hydrogen bonding lets the water column resist breaking
- How osmotic pressure draws water into root hair cells
Can Do
- Use the cohesion-tension theory to explain water movement in xylem
- Link transpiration, cohesion and osmotic pressure into one continuous flow
- Explain why xylem must be unbroken and able to withstand tension
Scan these before reading
vocabMisconceptions To Fix
watch outOdd one out: Three of these statements about water movement in xylem are correct. Which one is wrong?
Tension, The Pull from Transpiration
How evaporation at the top creates a pull at the bottom
Core Content
Tension, The Pull from Transpiration
+5 XPHow evaporation at the top creates a pull at the bottom
When water evaporates from the leaves it leaves behind a pulling force in the xylem, because the escaping molecules tug on the ones still attached below them.
Transpiration is the evaporation of water from the leaf, mainly through the stomata. As water molecules leave, they create a tension (negative pressure, a pull) in the xylem at the top of the plant. This tension is the driving force that lifts water, the sun's energy, evaporating the water, ultimately does the lifting.
This is why the system works against gravity: the pull comes from the top, not a push from the bottom. The continuous flow it produces, from roots to leaves, is called the transpiration stream.
Pause, copy the highlighted link between transpiration and tension into your book.
The negative pressure (pull) created in the xylem as water evaporates from the leaves is called _____.
Cohesion, Why the Column Doesn't Break
Hydrogen bonding holds the water thread together
Cohesion, Why the Column Doesn't Break
+5 XPHydrogen bonding holds the water thread together
We just saw tension pulling from the top. That raises a question: why doesn't the water column simply snap apart under that pull? This card answers it → because water molecules stick to one another.
Water molecules attract one another strongly enough that a column of water in a narrow tube behaves like a single thread that can be pulled without breaking.
Cohesion is the attraction between water molecules due to hydrogen bonding. It holds the water in the xylem together as a continuous, unbroken column, so when tension pulls at the top, the entire column moves up as one, like pulling a rope. Water molecules also adhere to the xylem walls, helping the column resist breaking.
Text description
- Top: the leaf loses water vapour by transpiration, which creates tension (a pull) on the water in the xylem.
- Middle: the xylem holds a continuous column of water molecules; cohesion (hydrogen bonds between them) keeps it unbroken, so it is pulled up as one.
- Bottom: water enters the root by osmosis to replace it.
Pause, copy the highlighted definition of cohesion and how it keeps the column unbroken.
Cohesion between water molecules is due to hydrogen bonding and keeps the xylem water column unbroken.
Water is pushed up the xylem by a pump in the roots.
Transpiration from the leaves creates the tension that pulls the transpiration stream upward.
Trace a Water Molecule Root to Leaf
Do the activity in your book, then check what you have understood.
Activity 1
Trace a Water Molecule Root to Leaf
Pattern C, Worked journey
In your book, trace a single water molecule from soil to leaf, naming the force responsible at each stage: (1) entering the root, (2) being held in the column, (3) being lifted up the xylem, (4) leaving the leaf. Use the three terms osmotic pressure, cohesion and transpiration/tension in the correct places. End with one sentence explaining why the xylem column must be unbroken for this to work.
Match each part of the cohesion-tension theory to the job it does:
- Transpiration
- Tension
- Cohesion
- Osmotic pressure
- Draws water from the soil into the root hair cell
- Evaporation from the leaf that creates the pull
- Hydrogen bonding that stops the column breaking
- The negative pressure transmitted down the column
Osmotic Pressure and the Whole Theory
Bringing tension, cohesion and osmotic pressure together
Osmotic Pressure and the Whole Theory
+5 XPBringing tension, cohesion and osmotic pressure together
We just saw tension and cohesion in the stem. That raises a question: how does water get into the plant in the first place, and how do the three forces combine? This card answers it → osmotic pressure feeds the root while tension and cohesion lift the stream.
Water enters the root by osmosis, is held together by cohesion, and is pulled up by the tension of transpiration, three forces acting as one continuous system.
Cohesion-tension theory: (1) water is drawn from the soil into root hair cells by osmotic pressure (the root cell sap has a lower water potential than the soil water); (2) this water joins a continuous column in the xylem held together by cohesion (hydrogen bonding); (3) transpiration from the leaves creates tension that pulls the whole transpiration stream upward, carrying water and dissolved minerals from root to leaf.
The dissolved minerals are simply carried along in this stream. This is why a plant that cannot transpire (e.g. blocked stomata) also struggles to take up minerals: stop the pull, and the whole stream stalls.
Add the highlighted three-part cohesion-tension theory to your notes before the check below.
In cohesion-tension theory, what role does osmotic pressure play?
Why air bubbles can kill a drought-stressed tree
Where this biology is doing real work outside the classroom.
Why air bubbles can kill a drought-stressed tree
Because xylem water is under tension, literally being stretched, it is vulnerable. In a severe drought, the pull becomes so strong that the water column can snap and an air bubble forms, an event called cavitation or embolism. Once a vessel fills with air it can no longer carry water, like a broken straw. Foresters and climate scientists now track cavitation to predict which trees will die in heatwaves, and it explains the mass die-backs seen in Australian forests during extreme droughts. The same cohesion that lets a tree drink from 100 metres up is also its weak point when water runs short.
Use the cohesion-tension theory to explain how osmotic pressure, the transpiration stream and the cohesion of water molecules control the movement of water and dissolved minerals through the xylem.
Explain with a named theory, and the question names the three mechanisms it wants: osmotic pressure, the transpiration stream, and cohesion. Each carries two marks, so each needs a full causal account rather than a mention.
6 marks, two per mechanism. Take them in the order water travels — into the root, held together in the column, pulled out at the leaf — and finish by saying where the minerals fit and where the energy comes from.
Water enters the plant at the root hair cells by osmosis. The cell sap inside them has a lower water potential than the water in the surrounding soil, so water moves down the water-potential gradient into the root, and on into the root xylem. This osmotic pressure is what supplies the column of water from below.
Water molecules attract one another through hydrogen bonding, an attraction called cohesion. In the narrow xylem vessels this holds the water as a continuous, unbroken column that behaves like a single thread, so that a pull applied at the top is transmitted all the way down and the whole column moves as one, much like pulling a rope. Water molecules also adhere to the lignified vessel walls, which helps support the column, and the lignin stops the vessels collapsing inward under the pull.
At the leaf, transpiration is the evaporation of water, mainly through the stomata. As water molecules leave, they draw on the molecules still attached below them, creating a tension — a negative pressure, a pull — at the top of the xylem. Because cohesion holds the column unbroken, that tension is transmitted right down to the roots, and the whole column is drawn upward. The continuous flow this produces from root to leaf is the transpiration stream.
Dissolved mineral ions taken up at the root are simply carried along in this moving stream of water, so the same mechanism that lifts the water distributes the minerals to the leaves. This is why a plant whose stomata are blocked, and which therefore cannot transpire, also takes up minerals poorly: stopping the pull stalls the whole stream.
No part of the plant pumps the water. The energy that does the lifting comes from the evaporation of water at the leaf surface, and therefore ultimately from the sun. The three mechanisms combine as one system: osmotic pressure feeds water in at the bottom, cohesion holds the thread together, and transpiration pulls it up from the top.
Water enters the plant at the root hair cells by osmosis: their cell sap has a lower water potential than the surrounding soil water, so water moves down the water-potential gradient into the root and on into the root xylem, and this osmotic pressure supplies the column from below. Within the xylem, hydrogen bonding between water molecules — cohesion — holds the water as a continuous, unbroken column that behaves like a single thread, so a pull applied at the top is transmitted all the way down and the whole column moves as one, like a rope being drawn up; water also adheres to the lignified vessel walls, which helps support the column, while the lignin prevents the vessels collapsing inward under the pull. At the leaf, transpiration — the evaporation of water mainly through the stomata — removes water molecules, and as they leave they draw on those still attached below, creating a tension, or negative pressure, at the top of the xylem. Because cohesion keeps the column unbroken, that tension is transmitted down to the roots and the whole column is drawn upward, producing the continuous flow from root to leaf known as the transpiration stream. Dissolved mineral ions absorbed at the root are carried along in this moving stream, which is why a plant that cannot transpire also takes up minerals poorly: stopping the pull stalls the stream. No part of the plant pumps the water; the energy that does the lifting comes from evaporation at the leaf surface and therefore ultimately from the sun, so the water is pulled from above rather than pushed from below, with osmotic pressure feeding it in, cohesion holding the thread together and transpiration providing the pull.
A student coats the underside of every leaf of a potted plant with petroleum jelly, sealing the stomata. Predict the effect on the movement of water and dissolved minerals through the xylem, and justify your prediction using the cohesion-tension theory.
Predict asks for the outcome together with the reasoning that produces it, and the question names the theory the reasoning must come from. Every stage of the prediction should be traceable to one of the theory's three mechanisms.
5 marks. Work out what the intervention actually blocks, follow the consequence through tension, then through the column, then to the minerals, and finish with what would happen at the root.
Most stomata are on the underside of the leaf, and transpiration is the evaporation of water through them. Coating that surface with petroleum jelly seals the stomata, so water can no longer escape as vapour and transpiration from those leaves very nearly stops.
Transpiration is what creates the tension at the top of the xylem: water molecules leaving the leaf draw on those still attached below them, producing the negative pressure that lifts the column. With evaporation stopped, that tension is not generated, and there is no longer a pull at the top of the plant.
I predict that the movement of water up the xylem will slow dramatically and very nearly stop. Cohesion still holds the column together and the vessels remain intact, but cohesion only transmits a pull — it does not create one. Osmotic pressure at the root continues to draw water in, yet without tension at the top there is no pressure difference to drive the transpiration stream upward.
The uptake and distribution of dissolved minerals will fall for the same reason, because minerals are carried to the leaves dissolved in the moving stream rather than transported by any separate mechanism; when the stream stalls, their delivery stalls with it. Over time the leaves will also lose their transpirational cooling and the plant may overheat, while the root cells, still respiring, continue to take up water osmotically with nowhere for it to go. The experiment shows that the transpiration stream is driven from the leaf, not the root.
Most stomata are on the underside of the leaf, and transpiration is the evaporation of water through them, so coating that surface with petroleum jelly seals the stomata and very nearly stops transpiration from those leaves. Transpiration is what creates the tension at the top of the xylem, since water molecules leaving the leaf draw on those still attached below them and produce the negative pressure that lifts the column; with evaporation stopped, that tension is no longer generated. I therefore predict that the movement of water up the xylem will slow dramatically and very nearly cease. Cohesion still holds the water column together by hydrogen bonding and the vessels remain intact, but cohesion only transmits a pull rather than creating one, and although osmotic pressure continues to draw water into the root hair cells, without tension at the top there is no pressure difference to drive the transpiration stream upward. The uptake and distribution of dissolved minerals will fall for the same reason, because minerals are carried to the leaves dissolved in the moving stream rather than by any separate mechanism, so when the stream stalls their delivery stalls with it. The leaves will also lose transpirational cooling and the plant may overheat, while root cells continue to absorb water osmotically with nowhere for it to go. The result demonstrates that the transpiration stream is driven from the leaf rather than pushed from the root.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 14 marks
+5 XPAnalyseBand 6(6 marks) 1. Use the cohesion-tension theory to explain how osmotic pressure, the transpiration stream and the cohesion of water molecules control the movement of water and dissolved minerals through the xylem.
2 marks: osmotic pressure draws water into root hair cells (water potential gradient) · 2 marks: transpiration creates tension that pulls the transpiration stream up · 2 marks: cohesion (hydrogen bonding) keeps the column unbroken so it moves as one, carrying dissolved minerals
UnderstandBand 4(3 marks) 2. Explain why an air bubble (an embolism) in a xylem vessel stops water moving through that vessel, using the cohesion-tension theory.
1 mark: states that the theory requires a continuous, unbroken water column · 1 mark: explains that a gas gap breaks cohesion, so tension can no longer be transmitted down the column · 1 mark: concludes that water below the bubble is no longer pulled up, so flow in that vessel stops
AnalyseBand 5–6(5 marks) 3. A mountain ash can lift water more than 90 metres, with no pump anywhere in the plant. Analyse how osmotic pressure, the transpiration stream and the cohesion of water molecules act together to achieve this, and identify which of the three provides the energy that does the lifting.
1 mark: osmotic pressure draws water from the soil into the root hair cells and into the root xylem · 1 mark: transpiration from the leaves creates tension at the top of the column · 1 mark: cohesion by hydrogen bonding transmits that tension down an unbroken column so the whole thread moves · 1 mark: adhesion to the vessel walls assists, and lignified walls resist collapse under the tension · 1 mark: identifies evaporation, ultimately solar energy, as the source of the energy, so the water is pulled rather than pushed
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (6 marks): Water enters the plant when osmotic pressure draws it from the soil into the root hair cells: the cell sap has a lower water potential than the surrounding soil water, so water moves in by osmosis. This water joins a continuous column in the xylem held together by cohesion, the hydrogen bonding between water molecules, which keeps the column unbroken so it behaves like a single thread. At the top, transpiration (evaporation of water from the leaves through the stomata) removes water molecules, creating a tension (negative pressure) that pulls on the cohesive column. Because the column is unbroken, this pull lifts the entire transpiration stream upward from root to leaf, and the dissolved minerals are carried along in the moving water. Thus osmotic pressure supplies water at the root, cohesion holds the column together, and the tension from transpiration pulls it all up, together controlling the movement of water and minerals through the xylem.
Q2 (3 marks): The cohesion-tension theory depends on the water in the xylem forming one continuous, unbroken column from root to leaf. Tension generated by evaporation at the leaf is transmitted downward only because each water molecule is hydrogen-bonded to the ones below it, so pulling the top pulls the whole thread, like pulling a rope. An air bubble puts a gap of gas in that thread. Gas cannot transmit tension the way a cohering liquid column can, so the pull from the leaf stops at the bubble and the water below it is no longer being pulled upward. Flow through that particular vessel therefore stops. The plant is not necessarily killed, because xylem consists of many parallel vessels and water can be routed around a blocked one, but the affected vessel is lost until the embolism is cleared.
Q3 (5 marks): Water enters at the bottom by osmotic pressure: mineral ions are actively loaded into the root, lowering the water potential there, so water moves from the soil into the root hair cells and on into the root xylem by osmosis. At the top, transpiration evaporates water from the mesophyll surfaces and out through the stomata, and each departing molecule tugs on those still attached below, generating a tension, a negative pressure, in the xylem. Cohesion is what makes that tension useful: hydrogen bonding between water molecules holds the column together as a single unbroken thread, so a pull applied at the leaf is transmitted all the way to the root and the whole column moves up as one, the transpiration stream. Adhesion between water and the vessel walls helps the column resist gravity, and the lignified walls stop the narrow vessels collapsing inward under the tension. The energy that does the lifting comes from evaporation, and therefore ultimately from the sun, not from the plant's own metabolism, which is why the water is pulled from above rather than pushed from below and why no pump is needed at any height.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now use the cohesion-tension theory to explain how water is pulled up the xylem. Next lesson examines what speeds up or slows down transpiration, the engine of this whole stream.
↩ Back to focus-area hubHow did your thinking change?
At the start you compared the xylem to sucking a milkshake. The "sucking" is transpiration, evaporation from the leaves creates tension at the top. The water doesn't break into drops because of cohesion: hydrogen bonds hold the column together so it is pulled up as one unbroken thread, while osmotic pressure keeps feeding water in at the root.
If you thought water was pushed up from the roots, revisit the misconception box: it is mainly pulled from the top, against gravity.