IB Biology SLTopic 2 — Transport in Animals & PlantsPaper 1 & 2Core idea~12 min read
The Transpiration Stream
A tall tree lifts water a hundred metres into the air with no pump, no muscle and no energy spent on the lifting. The whole thing is done by water evaporating off a leaf. Once you see how, it is one of the neatest bits of biology in the course.
📚 What you need to know
Water evaporates from the surfaces of cells inside the leaf and diffuses out through the stomata.
That water is replaced by water pulled from the nearest xylem vessels.
Losing water from the xylem creates tension — a negative pressure — inside it.
Cohesion between water molecules passes that pull all the way down to the roots. This is transpiration pull.
Adhesion to the vessel walls helps water climb narrow tubes. That is capillary action.
Together these make the cohesion–tension theory, and the continuous upward flow is the transpiration stream.
Start at the top, not the bottom
Most students try to explain this from the roots upwards, as if the plant pushes water up. It does not. Nothing at the bottom is pushing. The pull comes from the top, and it starts with evaporation.
Inside a leaf, the mesophyll cells are wet on the outside and surrounded by air spaces. Water evaporates off those wet surfaces into the air spaces, then diffuses out through the open stomata into the atmosphere. That water has to be replaced, and the nearest supply is the xylem.
The energy for all of this comes from the sun, which drives the evaporation. The plant spends none of its own ATP lifting water.
Tension, cohesion, adhesion
These three words do all the work in this topic, and students lose marks by muddling them. Here is what each one means in plain terms.
Tension
When water leaves the top of the xylem, nothing pushes in behind it — so the water that is left is under a pull. That is what tension means here: a negative pressure, the opposite of the positive pressure in your arteries. The xylem is being sucked, not squeezed.
Cohesion
Water molecules are attracted to each other by hydrogen bonds, so they stick together. That means water in a narrow tube behaves like a rope rather than a pile of loose beads. Pull the top of a rope and the whole rope comes. Pull the top of the water column and the whole column comes, all the way down to the roots.
Adhesion
Water molecules are also attracted to the cellulose in the vessel walls. This sticking-to-the-sides is adhesion, and in a narrow tube it drags water upwards. You have seen it happen when a drop of water climbs into a thin straw on its own. That effect is capillary action.
Break the column — let an air bubble in — and the rope snaps. That vessel can no longer carry water.
If you only remember one line for the exam, make it this: evaporation creates tension, cohesion transmits it, adhesion helps at the walls. Nearly every mark on this topic sits inside that sentence.
Why the plant bothers
Transpiration looks like pure loss — the plant leaks water all day. But the stream it creates does three useful jobs.
Cooling. Evaporation takes heat energy with it, so the leaf stays cooler on a hot day. Same reason sweating works.
Mineral transport. Mineral ions absorbed by the roots are carried up in the water, so the stream doubles as a delivery service to every leaf.
Support. Water inside the cells keeps them turgid. That turgor pressure holds leaves out flat to catch light and keeps the stems of non-woody plants upright. It is why a plant that runs short of water wilts.
Wilting explained properly: less water means cells lose turgor, so they no longer push out against their walls. Without that internal pressure the leaves and soft stems go floppy. Nothing has broken — the plant is just under-inflated.
What changes the rate
Transpiration is evaporation followed by diffusion, so anything that speeds up either one speeds up the whole stream.
Factor
Effect on rate
Why
More light
Increases
Stomata open for photosynthesis, so more water vapour can escape
Higher temperature
Increases
Molecules have more kinetic energy, so evaporation and diffusion are faster
Higher humidity
Decreases
The air outside is already moist, so the concentration gradient is smaller
More wind
Increases
Moist air is blown away, keeping a steep gradient at the stoma
🧠 A quick way to reason it out
Ask yourself: does this factor make the air just outside the stoma wetter or drier? Drier outside air means a steeper gradient, which means faster diffusion out. Wind dries it, humidity wets it. That one question answers the whole table.
Worked examples
WORKED EXAMPLE
Explain how water moves from the roots to the leaves of a tall tree. [4 marks]
Start at the top — that is where the cause is
Water evaporates from the surfaces of mesophyll cells and diffuses out of the stomata.
Link 2
This water is replaced from the xylem, and removing water from the xylem creates tension, a negative pressure.
Link 3
Cohesion between water molecules means the column of water does not break, so the tension is transmitted all the way down to the roots.
Link 4
The whole column is therefore pulled upwards against gravity, and adhesion to the vessel walls assists the climb.
Cohesion–tension theory: 4 linked pointsNever write “the roots push the water up”. There is no pushing anywhere in this answer.
WORKED EXAMPLE
Explain why transpiration rate increases on a windy day. [3 marks]
Step 1: what sits outside the stoma on a still day?
A layer of moist air builds up, because water vapour that has diffused out stays there.
Step 2: what does wind do to it?
Wind blows that moist air away and replaces it with drier air.
Step 3: the consequencedrier air outside → steeper concentration gradient → faster diffusion out of the stomataSteeper water vapour gradient, so faster transpiration
WORKED EXAMPLE
In a potometer, an air bubble moved 60 mm along a capillary tube of radius 0.50 mm in 4.0 minutes. Calculate the rate of water uptake in mm3 per minute. [3 marks]
Step 1: the water taken up fills a cylinderV = π r2 × distanceStep 2: put the numbers inV = π × (0.50)2 × 60 = π × 0.25 × 60V = 47.1 mm3Step 3: divide by time47.1 ÷ 4.0 = 11.8Rate = 11.8 mm3 per minute (3 s.f.)A potometer measures water UPTAKE, which is slightly more than water lost by transpiration.
💡 Exam tip
Answer top down: evaporation first, roots last. Answers written bottom-up usually miss the mechanism entirely.
Use the word tension, and say it means negative pressure. Both halves can be credited.
Keep cohesion (water to water) and adhesion (water to wall) apart in your head. Examiners test this deliberately.
The theory has a name: cohesion–tension theory. Naming it is often a free mark.
For rate questions, always finish with gradient and diffusion. That is the language the mark scheme uses.
If a question mentions a potometer, note in one line that it measures uptake, not transpiration itself.
⚠ Common mix-up
“The roots pump water up.” Roots do not pump. The pull comes from evaporation at the leaves.
Cohesion and adhesion swapped. Cohesion is water sticking to water. Adhesion is water sticking to the wall.
Saying the xylem is under high pressure. It is under tension — negative pressure. The opposite.
Confusing transpiration with translocation. Transpiration moves water in xylem; translocation moves sugars in phloem.
Thinking transpiration is a waste. It cools the plant, carries minerals and supports the leaves.
Writing “water is sucked up”. Use tension, pull or transpiration pull — “sucked” rarely gets credit.
Up next: Xylem Vessel Adaptations — how a dead, hollow tube manages to survive being permanently sucked from the inside without collapsing.
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