Lesson 6 of 9 / Nutrition and transport in flowering plants
The transpiration stream
How can evaporation at a leaf move water from the roots?
In this lesson: Define transpiration and briefly explain water movement through stems by transpiration pull.
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 stomata. Water first evaporates from moist mesophyll cell surfaces into leaf air spaces, then vapour diffuses out through open stomata into the surrounding air.
These two stages are different. Evaporation changes liquid water into vapour; diffusion describes its net movement down a water-vapour concentration gradient. Stomata also allow carbon dioxide to enter for photosynthesis, so gas exchange and water loss share a route.
Water lost from leaves is replaced by water from the xylem. The loss creates a pulling force, called transpiration pull, on the continuous water columns in xylem. This draws replacement water up the stem from the roots.
Root uptake supplies the water entering the transport system. Water moves through the stem in the hollow xylem vessels; the vessels do not actively pump water upwards. Reducing leaf water loss generally reduces this pull under otherwise comparable conditions.
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.
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.