K325 / 2027

Lesson 8 of 12 / Nutrition and transport in plants

The transpiration stream

How can evaporation at a leaf move water from the roots?

I can explain transpiration, water-potential changes and transpiration pull through xylem.

  • I can define transpiration and relate it to gas exchange.Syllabus

    Syllabus K325, Topic 10(j). Define transpiration and relate it to gas exchange.

  • I can explain water movement using water potential.Syllabus

    Syllabus K325, Topic 10(k). Explain water movement using water potential.

  • I can trace water from roots through xylem using transpiration pull.Syllabus

    Syllabus K325, Topic 10(l). Trace water from roots through xylem using 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?

  1. The roots pump it up using energy.
  2. Evaporation of water from the leaves.
  3. Osmosis through every xylem cell.
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.

Can you follow water from soil to air?
1234Soil -> root -> xylem -> air

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. 1

    Begin with leaf water loss

    Name evaporation and then diffusion through stomata.

  2. 2

    Connect to the pull

    Water loss generates tension; cohesion helps the water column remain continuous.

  3. 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.

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?

  1. Water evaporates from mesophyll surfaces, then vapour diffuses through stomata.
  2. Guard cells pump liquid water out through the open stomata, using energy from ATP.
  3. Water vapour enters phloem and turns into sucrose.
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

Card 1 of 3. Answer in your head, then check.

How can evaporation at a leaf move water from the roots?

You can return to this lesson any time.