Xylem, phloem, transpiration and translocation
The two transport tissues and the two processes they carry out.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 8.1.1Describe the functions of xylem and phloem and identify their positions in the plant.
- 8.1.2Describe transpiration and the factors affecting its rate.
6 minute read
Xylem, phloem, transpiration and translocation
Plants have two separate transport tissues, carrying different things in different directions.
Xylem versus phloem
| Xylem | Phloem | |
|---|---|---|
| Carries | water and mineral ions | sucrose and amino acids |
| Direction | upwards only, roots → leaves | both ways, source → sink |
| Cells | dead, hollow, lignified | living |
| Process | transpiration | translocation |
A helpful pairing: xylem carries water; phloem carries food.
Transpiration
Transpiration is the loss of water vapour from the leaves, mainly through the stomata, by evaporation and diffusion. This loss pulls water up the xylem in a continuous transpiration stream, dragging up the mineral ions with it. Transpiration also cools the leaf and keeps cells turgid.
Factors affecting the rate
- Higher temperature → faster evaporation → faster transpiration.
- Lower humidity → steeper water vapour gradient out of the leaf → faster.
- More wind → blows away accumulated vapour, maintaining the gradient → faster.
- Higher light intensity → stomata open for photosynthesis → faster.
Notice that three of these work by the same mechanism: they keep the concentration gradient of water vapour steep.
Wilting
If water loss exceeds uptake, cells lose water, become flaccid, lose turgor pressure, and the plant wilts. Wilting reduces the surface area exposed to the sun and the stomata close, both of which slow further loss.
Think of it like this
The transpiration stream is a drinking straw with the top continuously evaporating away. Water leaving the top pulls the whole column up behind it — nothing pushes from the bottom.
Worked examples
Method, step by step
Explain why transpiration is faster on a windy day than on a still day.
- 1Water vapour diffuses out of the stomata into the air just outside the leaf.
- 2On a still day this vapour accumulates, reducing the concentration gradient.
- 3Wind blows the humid air away, keeping the gradient steep, so diffusion out of the leaf continues rapidly.
Wind removes the water vapour that collects around the leaf, maintaining a steeper water vapour concentration gradient, so water diffuses out of the stomata faster.
Common misconceptions
- Thinking the roots push water up. Water is *pulled* up by evaporation from the leaves; root pressure is a minor contributor.
- Saying transpiration is useless water loss. It is largely an unavoidable consequence of having stomata open for CO₂, but it also cools the leaf and delivers mineral ions.
- Mixing up the tissues — remember xylem is dead and hollow, which is precisely what lets water flow through it freely.
In the exam
- When explaining why wind or low humidity increases transpiration, use the phrase "maintains a steeper concentration gradient of water vapour" — that is the marking point.
- For a potometer question, remember it strictly measures water *uptake*, which is only an estimate of transpiration since some water is used in photosynthesis.