Transpiration Pull Works Because Water Columns Resist Breaking
Water rises through the xylem of tall trees because evaporation from leaves pulls on a continuous column held together by cohesion. Questions ask how large the holding force is and how stomatal behaviour changes the water loss.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Water moves across a membrane from dilute to concentrated
Water crosses the membrane from dilute to concentrated
For osmosis, track water—not solute—across a semi-permeable membrane. The concentration difference determines the net direction.
Cohesion-tension theory
Transpiration from the leaves creates a tension (negative pressure) in the xylem. Water molecules cohere to one another through hydrogen bonds and adhere to the walls of the xylem vessels, so the column is pulled up as one. The forces opposing the ascent of sap in the tallest trees are about 50 atm, but the cohesive (tensile) strength of the water column is far greater, of the order of 350 atm, so the column does not break.
Reading a potometer
A potometer measures water uptake by a cut shoot, which is close to the rate of transpiration. Bubble movement tracks stomatal opening, light, humidity and temperature. A succulent such as Crassula uses CAM and keeps its stomata closed during the day, so in sunlight its twig takes up almost no water and the bubble does not move.
OlympIQ exam lens
- ✓Transpiration pull plus cohesion and adhesion lifts water in the xylem
- ✓Opposing forces about 50 atm; cohesive force of the water column about 350 atm
- ✓Potometer reading follows water uptake, which follows stomatal opening
- ✓CAM succulents keep stomata closed by day, so a potometer bubble hardly moves
Do not expect a plant in strong sunlight to transpire rapidly in every case. A CAM plant closes its stomata in the day, so light alone does not set the transpiration rate; stomatal opening does.
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