Lesson 7 of 10 / Nutrition and transport in plants
The transpiration stream
How can evaporation at a leaf move water from the roots?
In this lesson: Explain transpiration, water-potential changes and transpiration pull through xylem.
About 6 min
The key ideaWater 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 aerial parts of a plant, mainly through stomata in the leaves. Water evaporates from moist mesophyll cell walls into leaf air spaces, then water vapour diffuses out through open stomata down its concentration gradient.
Opening stomata allows carbon dioxide to enter for photosynthesis but also provides a route for water loss. Transpiration is therefore a consequence of gas exchange. Evaporation and diffusion are different steps: liquid water becomes vapour, then vapour moves through the pore.
Water lost from the leaf is replaced from nearby cells and xylem. Where water crosses cell membranes, it moves by osmosis from higher to lower water potential. Evaporation lowers water potential in the leaf and produces tension in the xylem water columns. Cohesion between water molecules helps maintain a continuous column, transmitting the transpiration pull down the plant.
Water enters root hairs from soil when the soil has higher water potential, moves across the root and into the xylem, then flows upwards towards leaves. Flow within the dead, hollow xylem vessels is bulk flow under tension, not repeated osmosis through living xylem cells or active pumping of water.
Step by step
- 1
Begin with leaf water loss
Name evaporation and then diffusion through stomata.
- 2
Connect to the pull
Water loss generates tension; cohesion helps the water column remain continuous.
- 3
Trace replacement water
Soil -> root hair -> root cortex -> xylem -> leaf, using osmosis where membranes are crossed.
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.
Is this true? "Water travels up every xylem vessel by osmosis through its cell membranes."
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.