Structures and Functions in Living Organisms · 5 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 19% of your exam marks.
Transpiration and the roles of xylem and phloem are tested on almost every paper in recent years.
The rate of transpiration depends on how quickly water vapour can leave the leaf and how steep the water-vapour gradient is between the inside of the leaf and the air outside. Four environmental factors matter most:
| Factor | Effect when increased | Why |
|---|---|---|
| Light intensity | Transpiration increases | Brighter light makes the guard cells open the stomata wider, so more water vapour can escape |
| Temperature | Transpiration increases | Warmer water molecules have more kinetic energy; they evaporate faster and diffuse out more quickly |
| Wind speed (air movement) | Transpiration increases | Moving air sweeps water vapour away from the leaf as soon as it leaves the stomata, keeping the concentration gradient steep |
A dry, hot, sunny, windy day is therefore the worst combination for water loss. A still, cool, humid, dark night is the best for keeping water in the plant.
Explain why light intensity or temperature affects the rate of transpiration
What comes up: two common 2-mark explanations — one about how changing light intensity affects water loss from a shoot, and one about why lower leaf temperature slows transpiration.
Write — light intensity (two marks): (1) As light intensity increases, the guard cells open the stomata wider (or more stomata open), so more water vapour can escape. (2) As a result, more water is lost through the stomata (rate of transpiration increases). If light intensity plateaus and rate stops rising, add that all stomata are already fully open.
Write — temperature (two marks): (1) Lower temperature means water molecules have less kinetic energy. (2) They therefore evaporate and diffuse more slowly, reducing the rate at which water vapour leaves through the stomata.
Watch out: for the light question, simply stating "stomata open in light" earns one mark — you need to connect this to the increased water loss to earn the second. For temperature, stating "lower temperature means slower diffusion" without explaining the kinetic-energy reason will usually score only one of the two marks.
Explain why increased air movement (wind) increases the rate of water loss
What comes up: a 3-mark question asking you to explain the mechanism by which wind or moving air increases the rate of water loss from a plant shoot.
Write (three marks): (1) Moving air sweeps water vapour away from the leaf surface as soon as it leaves the stomata. (2) This removes the humid air that would otherwise build up next to the leaf. (3) The concentration gradient (diffusion gradient) of water vapour between the inside of the leaf and the air outside is therefore kept steeper, so water vapour diffuses out more rapidly.
Watch out: saying only "wind blows the water away" is one mark. The full answer needs to explain what that removal does to the concentration gradient — that is where marks two and three are earned.
| Humidity | Transpiration decreases | High humidity means lots of water vapour is already in the air, so the gradient between leaf and air is shallow and diffusion slows down |