Transpiration is caused by the evaporation of water from moist leaf tissues followed by the diffusion of water vapour out through open stomata. It is an unavoidable consequence of opening stomata for gas exchange and is required at both SL and HL in IB Biology subtopic B3.1.
The mechanism
- Water is transported to the leaf through xylem vessels.
- It moves from the xylem into mesophyll cells and their moist cell walls.
- Water evaporates from these cell walls into the leaf's internal air spaces.
- The air inside the leaf usually contains more water vapour than the external atmosphere. Water vapour therefore diffuses through open stomata down its concentration gradient.
- This water loss creates tension in the xylem. Because water molecules cohere through hydrogen bonding, a continuous column of water is drawn upwards from the roots in the transpiration stream.
Plants open stomata so that carbon dioxide can diffuse into leaves for photosynthesis. However, this also allows water vapour to escape. Guard cells can reduce water loss by closing stomata, but closure also restricts carbon dioxide uptake.
| Factor | Effect on transpiration rate |
|---|---|
| Higher temperature | Increases evaporation and usually increases diffusion |
| Lower humidity | Creates a steeper water-vapour concentration gradient |
| Increased wind speed | Removes humid air near the leaf, maintaining the gradient |
| Greater light intensity | Usually causes stomata to open for photosynthesis |
| Low soil water availability | Promotes stomatal closure, reducing transpiration |
A common misconception is that transpiration is simply the plant “sucking up” water. Transpiration specifically means water loss from the plant; the resulting tension helps pull replacement water through the xylem.
Exam technique
For an explain question, give a linked sequence: evaporation from mesophyll cell walls → diffusion through stomata → movement down a water-vapour concentration gradient. If asked about a factor, state how it changes either evaporation, the concentration gradient, or stomatal opening rather than merely saying that transpiration increases or decreases.