Water moves from the soil into the root by osmosis, rises in the xylem, and leaves the leaf as vapour. The force that keeps it moving is the pull created by transpiration.
This lesson is part of transport in plants. The next lesson, interpreting transpiration data, uses this route to read tables and graphs.
What is the route, stage by stage?
Learn it in five stages. Each one has a structure and a reason.
- Root hair cell. Water enters by osmosis because the cell sap has a lower water potential than the soil water.
- Root cortex. Water passes across cell to cell towards the centre.
- Xylem in the root and stem. Water enters the xylem vessels and travels upwards.
- Leaf mesophyll. Water moves to cells in the leaf and evaporates from their surfaces.
- Stomata. Water vapour diffuses out into the air.
What makes the water move?
Evaporation from leaf cells pulls water from the xylem below. Water molecules stick together, so the pull passes down as a continuous column.
This is called the transpiration pull. The column does not break because the molecules cling to each other and to the xylem walls.
Worked example: explain the effect of wind
Question: a plant loses water faster on a windy day. Explain why, and state what happens to water uptake.
- Wind blows away the water vapour near the stomata.
- A steeper diffusion gradient forms between the leaf and the air.
- Transpiration speeds up, so the pull on the xylem increases.
- More water is drawn up from the roots, so uptake increases.
Answer: “Wind removes water vapour near the stomata, which makes the diffusion gradient steeper. Transpiration is faster, so the pull is stronger and the plant takes up more water.”
The mistake that loses marks
The usual weak answer says “water goes up the xylem because it is the water transport tissue”. It names the path but never gives the force.
| Weak | Strong |
|---|---|
| Water goes up the xylem | Transpiration pulls water up through the xylem |
| Roots suck water up | Water enters root hairs by osmosis |
| Leaves lose water | Water vapour diffuses out of the stomata |
Check yourself
A student covers the lower surface of a leaf with petroleum jelly. Explain why the plant takes up less water over the next hour.
Answer
Most stomata are on the lower surface, and the jelly blocks them. Less water vapour leaves, so transpiration slows and the pull on the xylem falls. Less water is drawn up, so uptake drops.
What to study next
Read the numbers in interpreting transpiration data. Then see the sugar journey in explaining translocation.
If you want a teacher to question you on each stage, see online one-to-one Biology tuition.