This topic asks you to trace moisture out of foliage, predict how conditions change the rate, and explain how the cells flanking each pore balance moisture conservation with gas exchange. Think in stages rather than memorising isolated arrows.
Use the route and gradient method
Moisture first evaporates from damp mesophyll cell surfaces into internal air spaces. It then diffuses as vapour through open stomata. The rate depends on how quickly evaporation occurs, how steep the vapour difference is between the internal spaces and surrounding air, and how open the stomata are. Apply each environmental change to one of those three controls.
| Change | Reasoning |
|---|---|
| Warmer conditions | Evaporation is faster, so transpiration increases. |
| Faster air movement | Vapour is removed from around the foliage, keeping a steep diffusion difference. |
| More humid air | The vapour difference becomes smaller, so diffusion slows. |
| Brighter light | The cells flanking each pore usually open the stomata, giving vapour a wider exit route. |
| Moisture shortage | The cells flanking each pore lose turgor and close the stomata, reducing further loss. |
The pair of cells beside a stoma shows the central trade-off. When the pair takes in liquid and becomes turgid, the cells bend apart and open the pore. When they become flaccid, the pore closes. Closure conserves moisture, but it also restricts carbon dioxide entry and can slow photosynthesis. Transpiration is not simply harmful: evaporation cools foliage, and vapour loss helps draw a liquid column with dissolved mineral ions upwards through xylem. A potometer tracks liquid uptake by a cut shoot, which estimates rather than directly measures transpiration. A plant wilts when loss from its shoot becomes faster than uptake at its roots, because its cells lose the firmness that supports it.
Avoid these vapour-loss mistakes
| Tempting idea | How to reject it |
|---|---|
| Treating transpiration as sugar transport or root pumping | Follow the physical route: moisture changes to vapour inside foliage and diffuses to the air. |
| Assuming ordinary warmth destroys the pores or stops vapour loss | For normal growing conditions, connect higher temperature to faster evaporation, not to enzyme damage. |
| Thinking humid air gives the plant more moisture to lose | Humidity describes the air outside. When that air is already moist, the diffusion difference from inside to outside is smaller. |
| Imagining liquid pours through stomata or crosses the waxy cuticle | The normal exit is vapour through pores. The cuticle limits loss across the rest of the surface. |
| Choosing the sunlit upper surface automatically | For typical foliage, stomatal abundance matters more than which side faces the light; most pores are on the lower surface. |
Try it
A leafy shoot that had been standing in still air is placed in front of a running fan. What happens to its rate of transpiration, and why?
- It rises, because moving air sweeps humid air away from the pores
- It falls, because the moving air cools the leaf down
- It rises, because the wind forces water back out through the pores
- It is unchanged, because air outside cannot affect what happens inside a leaf
A plant left unwatered on a hot windowsill droops and its leaves go limp. Which statement explains the drooping?
- The plant has used up the food that was stored in its soil
- Its leaves have stopped photosynthesising, so they collapse
- The stem has lost the lignin that held it up
- It is losing water faster than its roots can replace it