8.3 Transpiration
- Syllabus
- 0610–2026–2027
- Topic
- 8.3
- Level
- —
Learning objectives
8.3.1Transpiration as the loss of water• Describe transpiration as the loss of water vapour from leaves8.3.2Water evaporates from the surfaces• State that water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour8.3.3Effects of variation of temperature• Investigate and describe the effects of variation of temperature and wind speed on transpiration rate8.3.4Water vapour loss is related• Explain how water vapour loss is related to: the large internal surface area provided by the interconnecting air spaces between mesophyll cells and the size and number of stomata8.3.5Mechanism by which water moves upwards• Explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules8.3.6Effects on the rate of transpiration• Explain the effects on the rate of transpiration of varying the following factors: temperature, wind speed and humidity8.3.7Why wilting occurs• Explain how and why wilting occurs
Define transpiration
Transpiration is the loss of water vapour from the leaves of a plant.
Water reaches a leaf as liquid in the xylem but leaves it as water vapour. The loss occurs mainly through stomata in the leaf surface.
Transpiration names the overall loss of water vapour from leaves. It is not the upward movement of liquid water in xylem, and it is not translocation of sucrose in phloem.
Trace water vapour out of a leaf
Water evaporates from the moist surfaces of mesophyll cells into the interconnecting air spaces inside the leaf. The water vapour then diffuses out through the stomata.
| Stage | State and location of water | Process |
|---|---|---|
| 1 | liquid water on mesophyll-cell surfaces | evaporation |
| 2 | water vapour in leaf air spaces | diffusion toward the outside air |
| 3 | water vapour passing through stomata | diffusion out of the leaf |
Diffusion occurs because the concentration of water vapour is usually higher inside the leaf air spaces than in the surrounding air.
Evaporation is the liquid-to-vapour change at mesophyll surfaces; diffusion is the later net movement of vapour through the air spaces and stomata.
Investigate temperature and wind effects on transpiration
Change temperature or wind speed one factor at a time and measure transpiration rate while keeping the other conditions constant.
| Investigation choice | What to do |
|---|---|
| response measure | record plant mass loss per unit time, or use bubble movement/water uptake in a potometer as a proxy |
| temperature | compare several controlled temperatures while wind speed stays constant |
| wind speed | compare controlled air speeds or fan distances while temperature stays constant |
| controls | same species, leaf area, duration, light intensity and humidity |
| reliability | repeat each condition and calculate a mean rate |
Increasing temperature increases transpiration rate. Increasing wind speed also increases transpiration rate. Plot the measured rate against the changed factor and describe the pattern using the data.
A potometer measures water uptake, not water loss directly. Water uptake is used as an estimate of transpiration only when the apparatus is airtight and other water uses are recognised as a limitation.
Link leaf structure to water-vapour loss
A leaf presents a large moist internal surface to air and provides stomatal openings through which that air connects to the atmosphere.
| Leaf feature | Effect on water-vapour loss |
|---|---|
| many interconnecting spaces between mesophyll cells | expose a large internal surface and provide a route for vapour movement |
| many stomata | provide a larger total area of exit pores |
| larger stomatal openings | provide a larger total diffusion area to the outside air |
Together, the large mesophyll surface and stomatal exit area allow more water to evaporate and diffuse out at the same time, increasing water-vapour loss.
The air spaces do not make water themselves. They expose moist mesophyll surfaces and connect them to stomata, where vapour leaves the leaf.
Explain transpiration pull in xylem
Water loss from the leaf creates a pulling force at the top of the xylem called transpiration pull.
The pull draws a column of water molecules upwards through the xylem. Forces of attraction between water molecules hold the molecules together, so the pull is transmitted down the continuous column.
water vapour lost from leaves → transpiration pull develops → continuous water column is pulled upward → attraction between water molecules keeps the column together
Water is not pushed up the xylem by active transport. The syllabus mechanism is a pull from transpiration acting on a cohesive column of water.
Explain how temperature, wind and humidity affect transpiration
Environmental factors change evaporation from mesophyll surfaces or the water-vapour concentration gradient between the leaf and the surrounding air.
| Change | Effect on rate | Why |
|---|---|---|
| higher temperature | increases | water molecules have more kinetic energy, so evaporation and diffusion are faster |
| higher wind speed | increases | moving air removes the humid layer near the leaf, maintaining a steep concentration gradient |
| higher humidity | decreases | outside air already contains more water vapour, so the concentration gradient is smaller |
The highest rate is expected in warm, windy, low-humidity air; the lowest in cool, still, high-humidity air, provided stomata and water supply do not become limiting.
High humidity does not speed evaporation. It reduces the water-vapour gradient from leaf to air, so net diffusion and transpiration slow down.
Explain how and why plants wilt
Wilting occurs when water loss by transpiration is faster than water uptake by the roots.
Plant cells lose water, their vacuoles shrink and turgor pressure falls. The cells become flaccid and no longer press firmly against one another, so leaves and non-woody stems droop.
water loss exceeds uptake → cells lose water → turgor pressure falls → cells become flaccid → leaves and stems wilt
A wilted plant has lost turgor; its cells are flaccid, not turgid. Wilting is caused by an imbalance between water loss and uptake, not simply by high temperature alone.