Explain transpiration pull and cohesion in xylem
Connect evaporation at the leaf with tension transmitted through a continuous water column.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall the transpiration route and vessel structure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does cohesion refer to here?
What you will learn
- Explain how leaf loss creates transpiration pull
- Explain cohesion’s role in the water column
Loss at the leaf creates a pull
Evaporation from mesophyll surfaces removes water, which is replaced by water from leaf xylem. This generates a pull, or tension, on the xylem water column. The pull can draw more water along the stem from the roots. It is not a leaf squeezing water up a tube.
Xylem’s thick lignified walls help resist collapse under tension. The original vessel structure remains important: an extended open lumen conducts water, while strong walls support the passage.
Water molecules attract each other
Cohesion is attraction between water molecules. It helps maintain a continuous column so that tension at the leaf can be transmitted through the water in xylem. In this syllabus explanation, transpiration pull and cohesion work together.
An air break can disrupt continuity; roots and xylem are not a heart-driven blood circuit. Mature vessels lack cell contents, so describe physical pull on a water column rather than active pumping by vessel cytoplasm. Some real conditions complicate this model, so continuity is an important assumption.
Pause and explain
Which sequence explains the stream?
Worked example
Explain how leaf evaporation can draw water through a stem without vessel cells actively pumping it.
Show the worked solution
- Leaf water loss creates tension as water is replaced from xylem.
- Cohesion maintains a continuous column transmitting that pull.
Answer Transpiration pull acts through a continuous cohesive xylem water column; mature vessels do not actively pump with cytoplasm.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give a two-stage explanation linking transpiration with upward water movement.
Give me a hint
State the pull and how it is transmitted.
Compare my reasoning
- Water loss at mesophyll creates tension in leaf xylem.
- Cohesion keeps the water column continuous so the pull is transmitted.
Transpiration creates a pull; cohesion maintains a continuous xylem column carrying the tension along the pathway.
Look for these in your work
- Describe the pull
- Describe cohesion and continuity
Why do thick xylem walls help while the water column is under tension?
Give me a hint
Relate tension to deformation of the passage.
Compare my reasoning
- Tension can tend to collapse a weak tube.
- Thick lignified walls resist collapse and support it.
Strengthened lignified walls resist collapse, helping maintain the conducting passage under tension.
Look for these in your work
- Mention lignified walls
- Connect strength with maintaining the passage
An air gap interrupts the water column. Explain why the simple continuous-column account may fail there.
Give me a hint
Which assumption has changed?
Compare my reasoning
- The continuous water column is broken.
- Tension is not transmitted across that gap in the same way as through connected water.
The continuity assumption is lost; an uninterrupted cohesive-column explanation cannot simply be applied across the air gap.
Look for these in your work
- Identify the break in continuity
- Explain the model limit
Common mistakes
- Saying water boils throughout xylem to push the stream.
- Confusing cohesion between water molecules with an active cellular pump.
What can you explain now?
Which sequence explains the stream?
Compare with the explanation
Leaf evaporation → pull → cohesive xylem column
Loss and water replacement create tension transmitted through the column.
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Make it stick
Explain the stream using “evaporation”, “pull”, “cohesion” and “continuous column”.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.