Plants move water upward through xylem vessels mainly by transpiration pull, supported by cohesion, adhesion and capillary action. No mechanical pump is needed because evaporation from leaves generates tension that pulls a continuous column of water from the roots.
The Mechanism
- Water evaporates from moist mesophyll cell walls inside a leaf and diffuses out through the stomata. This loss of water is transpiration.
- Water is drawn from the xylem into leaf cells to replace the water lost. This generates tension, or negative pressure, in the xylem.
- Cohesion occurs because hydrogen bonds attract water molecules to one another. When water molecules leave the top of the xylem, they pull the molecules below them upward, maintaining a continuous water column.
- Adhesion is the attraction of water to polar or charged materials in xylem walls. It helps water resist the downward pull of gravity.
- Together, cohesion and adhesion produce capillary action, which assists water movement through the narrow xylem vessels.
| Process | Contribution to water transport |
|---|---|
| Transpiration | Creates the tension that pulls water upward |
| Cohesion | Keeps water molecules connected in a continuous column |
| Adhesion | Attracts water to xylem walls and opposes gravity |
| Capillary action | Supports upward movement in narrow vessels |
A common misconception is that capillary action alone transports water to the tops of tall trees. Capillary action contributes, but the principal driving force is transpiration pull; cohesion transmits this pull down the xylem.
Exam Technique
This is assessed at both SL and HL, particularly through A1.1 Water and its link to plant transport. For an explain question, give a linked sequence: evaporation through stomata → tension in xylem → cohesion through hydrogen bonding → upward movement of the water column. Do not write that water is “pumped” or that transpiration directly pushes water upward.
