IB Biology HLTransport in Animals & PlantsPaper 1 & 2~11 min read
The Transpiration Stream
A tall tree lifts water a hundred metres into the air with no pump, no muscle and no energy of its own. The whole thing is powered by water evaporating off leaf cells at the top – and by the fact that water molecules refuse to let go of each other.
📚 What you need to know
Water evaporates from the surfaces of cells inside the leaf, and more water is drawn from the nearest xylem vessels to replace it.
Water molecules adhere to the cellulose in plant cell walls, which lets water travel through the walls. Movement due to adhesion in a narrow tube is capillary action.
Losing water from the xylem generates tension – a negative pressure – inside the vessel.
That tension is a pulling force, transmitted all the way down the stem to the roots by cohesion between water molecules.
This is transpiration pull, and it moves water upwards against gravity.
The explanation is called the cohesion-tension theory, and the continuous upward flow is the transpiration stream.
Transpiration also cools the plant, helps with the uptake of mineral ions, and keeps cells turgid so leaves and non-woody stems stay supported.
Start at the top, not the bottom
It is tempting to imagine roots pushing water upwards. Mostly they do not. The stream is pulled from above, and it begins with evaporation.
Inside a leaf, the cell walls lining the air spaces are damp. Water evaporates from these surfaces into the air spaces and diffuses out through the stomata.
Those cells now need water, so they take it from neighbouring cells, and ultimately from the nearest xylem vessel.
Water leaving the xylem puts the water still inside it under tension, a negative pressure.
Because water molecules cohere – hydrogen bonds hold them to each other – the whole column behaves like a rope. Pull the top and the entire thing moves.
That pull reaches all the way down to the ends of the xylem in the roots, drawing more water in from the soil.
The blue arrows inside the stem are the transpiration stream – a single unbroken column of water from root tip to leaf.
Cohesion, adhesion and tension
Three water properties do all the work here, and exam answers usually need at least two of them by name.
Property
What it means
What it does in the plant
Cohesion
Water molecules stick to each other by hydrogen bonds
Holds the column together so a pull at the top moves water at the bottom
Adhesion
Water molecules stick to other surfaces, especially cellulose
Lets water travel through cell walls and cling to the sides of the xylem, helping it rise
Tension
A negative pressure – the column is being pulled, not pushed
Generated when water leaves the xylem at the top; transmitted right down to the roots
Adhesion in a narrow tube also produces capillary action – water creeping up a fine tube on its own. The xylem is narrow enough for this to help, although on its own it could never lift water to the top of a tree.
The green arrow is the pull. Break the column with an air bubble and the pull is lost, which is why xylem vessels must stay full.
Here is the detail that turns a good answer into a very good one: the water in a xylem vessel is under negative pressure. It is being stretched, like a rope pulled from both ends. That is why the vessel walls have to be reinforced – without lignin the tube would collapse inwards.
Why transpiration is worth the water
Losing water is expensive, but the stream it creates does three useful jobs.
Cooling. Evaporation from the leaf takes heat with it, so the plant does not overheat in direct sun.
Mineral uptake. Mineral ions dissolved in the water are carried up to the leaves in the transpiration stream.
Support. Water keeps cells turgid. Turgid leaves stay flat and hold a large surface area to the light, and turgor holds up the stems of non-woody plants.
Worked examples
WE 1
Water to the top of a tree
Explain how water is transported from the roots to the leaves of a tall tree. (4 marks)
Step 1: the loss
Water evaporates from the surfaces of leaf cells and diffuses out through the stomata.
Step 2: the tension
Water is drawn out of the xylem to replace it, creating tension, or negative pressure, in the vessel.
Step 3: the column
Cohesion between water molecules holds the column together, so the pulling force is transmitted all the way down the stem to the roots.
Step 4: the name
This transpiration pull moves water upwards against gravity, and is described by the cohesion-tension theory.
Evaporation creates tension; cohesion passes the pull down the columnuse the words cohesion, tension and transpiration pull – they are the mark scheme terms
WE 2
The role of adhesion
Explain how adhesion contributes to the movement of water through a plant. (2 marks)
Point 1: what adhesion is
Water molecules are attracted to other surfaces, in particular the cellulose in plant cell walls.
Point 2: the effect
This allows water to move through the cell walls of the leaf, and to cling to the sides of narrow xylem vessels, producing capillary action that helps water rise.
Water sticks to the walls as well as to itselfcohesion is water to water, adhesion is water to something else – do not swap them
WE 3
A broken column
Suggest why an air bubble in a xylem vessel stops water moving up that vessel. (3 marks)
Point 1: how the pull works
Water moves up because tension at the top is transmitted down a continuous column of water held together by cohesion.
Point 2: what the bubble does
A bubble breaks the column, so there are no cohesive forces across the gap and the pull is not passed on.
Point 3: the consequence
Water below the bubble is no longer pulled upwards, so that vessel stops transporting, although water can move sideways into neighbouring vessels.
No continuous column means no transmitted pullthis is also why a potometer must be set up under water
💡 Exam tips
Always start the explanation at the leaf, not the root.
Say water is pulled, not pushed, and use the word tension.
Distinguish clearly: cohesion water to water, adhesion water to cellulose.
Name the theory: cohesion-tension theory.
Remember the three benefits: cooling, mineral uptake, turgor support.
Water travels in xylem, sugars travel in phloem. Never mix them up.
⚠ Common mistakes
Saying the roots pump water up. Root pressure exists but transpiration pull does most of the lifting.
Describing positive pressure in the xylem during transpiration. It is under tension, so pressure is negative.
Confusing cohesion and adhesion.
Saying water moves up by osmosis. Osmosis moves water into cells; the movement up the xylem is mass flow driven by tension.
Forgetting that the column must be unbroken.
Writing that transpiration is the plant excreting waste water. It is a consequence of open stomata.
Up next: Xylem Vessel Adaptations – the tube itself, and why a dead, hollow, lignified cell is exactly the right thing for the job.
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