Lesson 8 of 12 / Nutrition and transport in flowering plants
The transpiration stream
How can evaporation at a leaf move water from the roots?
I can define transpiration and briefly explain water movement through stems by transpiration pull.
I can define transpiration as stomatal water-vapour loss.Syllabus
Syllabus K224 / K225, Topic 8(i). Define transpiration as stomatal water-vapour loss.
I can briefly explain stem water movement by transpiration pull.Syllabus
Syllabus K224 / K225, Topic 8(j). Briefly explain stem water movement by transpiration pull.
About 6 min
Make a guess. You are not marked.
Water rises from the roots to leaves 30 metres up a tree. What mainly pulls it up?
Show the answer
Evaporation of water from the leaves.
Water vapour lost from leaves is replaced from the xylem, which pulls a continuous column of water upward.
The key idea
Water evaporates from mesophyll cell surfaces and diffuses out as vapour. This loss creates a pull that draws water up continuous columns in xylem.
If soil water has higher water potential, water enters root hairs by osmosis across their partially permeable membranes.
Blue line: xylem water column. Green arrows: net route of replacement water and vapour loss. Numbers match the stages. Osmosis across membranes and bulk flow in xylem are different processes.
Explanation
Transpiration is the loss of water vapour from the leaves through the stomata. First, water evaporates from the wet surfaces of the mesophyll cells into the air spaces. Then the water vapour diffuses out through the open stomata.
These are two different steps: evaporation turns liquid water into vapour, and diffusion moves the vapour out. The same open stomata let carbon dioxide in for photosynthesis, so a leaf cannot take in carbon dioxide without losing some water.
The water lost from the leaves is replaced by water from the xylem. This loss pulls on the columns of water in the xylem. The pull is called transpiration pull, and it draws water up the stem from the roots.
The roots take in the water that enters the xylem. Water moves up the stem inside hollow xylem vessels; the vessels do not pump it. If the leaves lose less water, the pull is weaker.
Step by step
- 1
Evaporate
Water evaporates from moist mesophyll surfaces.
- 2
Diffuse out
Water vapour leaves through open stomata.
- 3
Replace the loss
Transpiration pull draws water upwards through xylem from the roots.
Worked example
Why blocking stomata matters
In a simplified comparison, most stomata are blocked while other conditions remain similar. Predict the effect on transpiration and xylem water flow.
One way to explain it
Less water vapour escapes, so transpiration decreases. The resulting transpiration pull is reduced, so the rate of water flow up the xylem generally decreases.
Why this answer works
- Blocking pores restricts vapour diffusion.
- Reduced leaf water loss reduces the pull.
- Do not describe xylem as an active water pump.
Watch out for this
A student says: "Water travels up every xylem vessel by osmosis through its cell membranes." What is wrong with this?
Show the answer
Mature xylem vessels are hollow and dead, without functioning cell membranes. Water moves along them by bulk flow; osmosis occurs where water crosses partially permeable membranes elsewhere.
Check your understanding
Which sequence correctly describes the final stages of transpiration?
Show the answer
Water evaporates from mesophyll surfaces, then vapour diffuses through stomata.
Keep the change of state and the subsequent movement through the pore as separate steps.
Quick recall
How can evaporation at a leaf move water from the roots?