How leaves pull water skywards

Science · Organisms & Life Processes · ages 17-18

Name ______________________ Date ____________

Which interaction gives water cohesion within a continuous xylem column?
- Hydrogen bonds between water molecules
- Air bubbles connecting neighbouring water molecules
- ATP-powered pumps joining neighbouring water molecules
Where does the main pulling force for transpiration-driven xylem flow begin?
- In the roots, where pressure pushes the column upward
- In the stem, where living vessel cells squeeze the column
- In the leaves, where evaporation creates tension
Root pressure alone explains how water reaches the tops of tall trees.
Circle one: True False
Compare identical plants with open stomata and ample soil water after each reaches a steady water balance. Which air conditions would usually give the highest root water uptake?
- Cool, humid, still air
- Warm, dry, moving air
- Warm, humid, still air
A plant closes its stomata while other conditions remain similar. What usually happens to transpiration-driven xylem flow?
- Flow always accelerates because closed pores increase evaporation
- Flow usually slows as transpiration-driven tension weakens
- Flow must reverse because the roots become the strongest sink
What does adhesion contribute to water movement in the xylem?
- Water molecules bind to one another, providing the upward pump
- Water adheres to the xylem walls, helping support the column
- Xylem wall cells use ATP to carry each water molecule upward
A gas bubble interrupts the water column in one xylem vessel. Other vessels remain water-filled. What does cohesion-tension theory predict?
- Flow through that vessel is reduced because its water column is broken
- Flow through that vessel rises because the bubble adds extra pushing pressure
- Flow throughout the plant must stop because all other vessels become blocked
A mature xylem vessel has no living cytoplasm but still carries water. Which conclusion follows?
- Its wall must make ATP in place of the missing cytoplasm
- Every water molecule must supply ATP before entering the vessel
- ATP production inside that vessel is not required for its water flow

Answer key

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How leaves pull water skywards W1-mt_9_TORolYIl-s1

Hydrogen bonds between water molecules · Hydrogen bonds attract neighbouring water molecules. Cohesion helps transmit tension, although bubbles can interrupt a column.
In the leaves, where evaporation creates tension · Evaporation from mesophyll cell walls creates tension in leaf water films. This pulls replacement water from the xylem.
False · Root pressure can contribute to upward movement, but cannot account for tall-tree ascent by itself. Transpiration supplies the main pull.
Warm, dry, moving air · Warm, dry, moving air usually raises transpiration when stomata remain open. At steady water balance, root uptake replaces that greater loss.
Flow usually slows as transpiration-driven tension weakens · Closing stomata reduces water vapour loss. The transpiration pull generally weakens and flow slows, but some water loss and flow may continue.
Water adheres to the xylem walls, helping support the column · Adhesion is attraction between water and xylem walls. It helps support water against gravity but is not a pump or a guarantee against embolism.
Flow through that vessel is reduced because its water column is broken · A bubble interrupts the continuous water column and reduces flow in the affected vessel. Other water-filled vessels can still conduct.
ATP production inside that vessel is not required for its water flow · The vessel conducts despite lacking living cytoplasm. Pressure differences move water through its lumen without ATP production inside the vessel.