The big idea: A plant has no heart, yet it must move water from the roots all the way up to the leaves.
Water enters the roots, travels up dead tubes called xylem vessels, and finally evaporates out of the leaves as water vapour. This loss of water vapour from the leaves is called transpiration.
Transpiration is not just waste — it is the very thing that pulls the water up the whole plant.
The transpiration stream: water enters at the root hairs, travels up the xylem and evaporates from the leaf as water vapour. Evaporation at the top pulls the whole water column up — the water molecules stick together (cohesion).
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- Transpiration
- The loss of water vapour from the leaves (and other above-ground parts) of a plant, mostly through tiny pores called stomata.
- Xylem
- The plant tissue made of long, hollow, dead tubes (xylem vessels) that carry water and dissolved minerals upwards from the roots to the leaves.
- Root hair cell
- A root cell with a long, thin extension that gives a large surface area for absorbing water and minerals from the soil.
- Stoma (plural: stomata)
- A tiny pore on the underside of a leaf through which water vapour leaves and gases are exchanged.
- Transpiration stream
- The continuous, one-way flow of water from the roots, up the xylem, to the leaves — driven by transpiration.
One direction only: The xylem carries water up in one direction: roots → stem → leaves.
It never carries water back down. (The other transport tissue, phloem, is what moves dissolved sugars — that is a different topic.)
If there is no pump, what makes the water climb metres up a tree? The answer is evaporation at the top, working together with the fact that water molecules stick to each other.
Follow the chain of cause and effect from the leaf downwards.
The cohesion-tension mechanism, step by step
- Water evaporates from the wet surfaces of the leaf's mesophyll cells and diffuses out through the stomata — this is transpiration.
- Losing this water leaves the leaf cells short of water, so they pull water out of the nearest xylem vessel.
- Pulling on the top of the water column puts the whole column under tension (a pulling force, like sucking a straw).
- Because water molecules stick to one another (cohesion), the column does not break — the pull is passed all the way down.
- So water is pulled up the xylem from the roots, and fresh water is drawn into the root hairs from the soil to replace it.
Cohesion and tension — the two key words: Cohesion = water molecules stick to each other (because of hydrogen bonding), so they form one continuous, unbroken column.
Tension = the pull created when water evaporates at the leaf; this pull is transmitted down the unbroken column.
Together they are called the cohesion-tension mechanism: evaporation pulls, cohesion holds the column together, and the water rises.
This only works because the xylem is built for the job. The pulling force (tension) is strong enough to collapse an ordinary tube, so xylem vessels are specially adapted.
How xylem vessels are adapted: Xylem vessels are dead, hollow cells stacked end to end with the end walls removed, so they form one long continuous pipe with nothing blocking the flow.
Their walls are thickened with lignin (they are lignified), which keeps the vessel strong so it does not collapse under the tension of the water column.
Their narrow diameter helps keep the water column continuous.
| Xylem adaptation | What it is | Why it helps water transport |
|---|---|---|
| Hollow, dead cells | The cells lose their contents and die, leaving an empty tube | Water flows through with nothing blocking the way |
| No end walls | The end walls between stacked cells break down | The cells join into one long, continuous pipe from root to leaf |
| Lignified walls | The walls are thickened and strengthened with lignin | The vessel does not collapse under the tension (pull) of the water column |
| Narrow diameter | Each vessel is a thin tube | Helps the water column stay continuous and supports cohesion |
Finally, the rate of transpiration is not fixed — it speeds up or slows down with the weather. Anything that makes water evaporate faster from the leaf raises the rate.
What changes the transpiration rate: Transpiration is fastest in hot, dry, windy and bright conditions — the same weather that dries washing on a line.
Light opens the stomata; heat gives water molecules more energy to evaporate; wind blows away the moist air at the leaf surface; low humidity means the air outside is dry, so water diffuses out faster.
High humidity does the opposite and slows transpiration down.
| Factor | Increasing it does what to the rate? | Why |
|---|---|---|
| Light | Increases the rate | Light opens the stomata, so more water vapour can escape |
| Temperature | Increases the rate | Warmer air gives water molecules more energy, so they evaporate faster |
| Humidity | Decreases the rate | Moist air outside means a smaller difference, so less water diffuses out |
| Wind / air movement | Increases the rate | Wind blows away humid air at the leaf, keeping the difference large |
A memory hook: Think of drying the washing: it dries fastest when it is hot, dry and windy — exactly the conditions that speed up transpiration.
And remember the pull: transpiration at the top → tension → cohesion holds the column → water rises.
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
How this is tested: On Paper 2 an Explain question regularly asks how transpiration drives water movement through a plant — you must link evaporation at the leaf → tension → cohesion → water pulled up.
An Outline question may ask for two adaptations of xylem vessels (hollow tube, no end walls, lignified walls) or for the environmental factors that change the transpiration rate.
On Paper 1A you might have to locate lignin in a transverse section of a root (it is in the xylem) or select the conditions that raise the rate.
IB-style question — explain how transpiration moves water up
Explain how transpiration drives the movement of water up through a plant. [3]
How to score all three marks
- Start at the leaf. Water evaporates from the leaf's mesophyll cells and is lost as vapour through the stomata (transpiration), so the leaf cells pull water from the xylem.
- Name the force. This pulling places the water column under tension (it is pulled, not pushed), and the pull is transmitted down the xylem.
- Use cohesion. Because water molecules stick together by cohesion, the column stays unbroken, so water is pulled up from the roots to replace what was lost. (Mark 1: evaporation/transpiration at the leaf. Mark 2: creates tension / a pull. Mark 3: cohesion keeps the column continuous so water rises.)
Final answer
Water evaporates from the leaf (transpiration), creating tension that pulls on the water column; because water molecules cohere, the column stays unbroken and water is pulled up the xylem from the roots.
✓ Why this scores full marks: It follows the cause-and-effect chain in order — evaporation, then tension (the pull), then cohesion holding the column.
Naming both key words, tension and cohesion, is what earns the explanation marks; just saying 'water goes up the xylem' is not enough.
The transpiration stream: water enters at the root hairs, travels up the xylem and evaporates from the leaf as water vapour. Evaporation at the top pulls the whole water column up — the water molecules stick together (cohesion).
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