Transpiration
BiologyΒ· 7 min read
1. Transpiration and the Cohesion-Tension Theoryβ β ββββ± 15 min
Transpiration
The passive loss of water vapour from the aerial parts of a plant (mostly leaves) via diffusion through open stomata. It is an inevitable consequence of opening stomata for photosynthetic COβ uptake.
Example:
A large oak tree can lose >100 litres of water per day via transpiration on a hot dry day.
The currently accepted model for water movement up xylem from roots to leaves is the cohesion-tension theory, which relies on two key properties of water.
Outline the sequence of events that pulls water from root xylem to leaf mesophyll, according to cohesion-tension theory.
- 1
- Water evaporates from mesophyll cell surfaces into leaf air spaces, then diffuses out through open stomata. This removes water from the leaf mesophyll.
- 2
- Evaporation creates tension (a pulling force) on water in the mesophyll cell walls, pulling more water from the leaf xylem into mesophyll cells.
- 3
- Water molecules are cohesive (stick together via hydrogen bonding), so the entire column of water in xylem is pulled upwards as a continuous, unbroken chain.
- 4
- This pull lowers pressure in the root xylem, drawing more water into the root from the soil via osmosis, maintaining the continuous column.
Exam tip:
You will almost always lose marks if you mention cohesion but not tension, or vice versa β always link both terms in your answer.
2. Factors Affecting Transpiration Rateβ β β βββ± 20 min
Transpiration rate depends on the steepness of the water potential gradient between the leaf air spaces and the external air, and the rate of diffusion. Any factor that changes either of these alters transpiration rate:
Factor | Effect on Transpiration Rate | Reason |
|---|---|---|
Increased light intensity | Increases (to a plateau) | Stomata open in light for photosynthesis, increasing diffusion pathway |
Increased temperature | Increases | Higher kinetic energy increases evaporation rate, lowers external relative humidity |
Increased air humidity | Decreases | Reduces the water potential gradient between leaf and air |
Increased wind speed | Increases | Removes stagnant humid air around stomata, maintains steep gradient |
A student measures transpiration rate at 15Β°C and 25Β°C, with all other factors held constant. Predict and explain the difference in rate.
- 1
Prediction: Transpiration rate will be higher at 25Β°C than 15Β°C.
- 2
- Higher temperature increases the kinetic energy of water molecules in the leaf mesophyll.
- 3
- This increases the rate of evaporation from mesophyll cell surfaces into leaf air spaces.
- 4
- Higher temperature also reduces the relative humidity of external air, increasing the steepness of the water potential gradient between the leaf and the environment. Both factors increase water loss.
3. Measuring Transpiration with a Potometerβ β β βββ± 20 min
Potometer
A piece of apparatus that estimates transpiration rate by measuring the rate of water uptake by a cut plant shoot. Water uptake is approximately equal to water loss via transpiration, so it gives a reliable estimate.
In a standard potometer, an air bubble is introduced to a capillary tube. As the plant takes up water to replace water lost via transpiration, the bubble moves along the capillary. The distance moved per unit time is used to calculate transpiration rate.
A student sets up a potometer, but the air bubble does not move after 10 minutes in bright light. Suggest three possible reasons and their fixes.
- 1
- Reason: Air entered the xylem when cutting the shoot, breaking the water column, so no water can be pulled up. Fix: Cut the shoot under water to prevent air entry, then reassemble the potometer.
- 2
- Reason: Joints between the shoot and potometer are not sealed, so water leaks out instead of being taken up. Fix: Seal the joint with petroleum jelly to make it watertight and airtight.
- 3
- Reason: Stomata are all closed, so no transpiration occurs. Fix: Leave the set up in bright light for 10 more minutes to allow stomata to open before taking measurements.
Exam tip:
Always state that potometers estimate transpiration rate, never directly measure it, to get full marks.
4. Xerophyte Adaptations to Reduce Transpirationβ β ββββ± 15 min
Xerophytes are plants adapted to survive in dry habitats with limited water, so they have evolved a range of structural and physiological adaptations to reduce transpiration rate and avoid dehydration.
Thick waxy cuticle: reduces non-stomatal water loss through the leaf epidermis
Sunken stomata: trap humid air in pits around stomata, reducing the water potential gradient
Reduced leaf surface area (e.g. cactus spines): lowers total surface area for water loss
Leaf hairs: trap stagnant humid air around stomata, reducing diffusion out
Stomatal closure during the day: reduces water loss when temperatures are highest
Explain how sunken stomata reduce transpiration rate.
- 1
- Sunken stomata are located in deep pits below the surface of the leaf.
- 2
- Water vapour that diffuses out of the stomata gets trapped inside the pit, rather than being blown away by wind.
- 3
- This increases the humidity of the air immediately outside the stomatal pore, reducing the water potential gradient between the leaf interior and external air.
- 4
- A smaller gradient reduces the rate of diffusion of water vapour out of the leaf, lowering overall transpiration rate.
5. Common Pitfalls
Wrong move:
Stating that tension pulls water up xylem but omitting mention of cohesion between water molecules
Why:
Cohesion is required to maintain a continuous water column; without it the column would break and no movement can occur
Correct move:
Always mention both cohesion (water molecules stick together) and tension (pull from evaporation) when explaining water movement up xylem
Wrong move:
Claiming that potometers directly measure transpiration rate
Why:
Potometers measure water uptake, and some water taken up is used for processes other than transpiration
Correct move:
Always state that potometers estimate transpiration rate, as water uptake is approximately equal to water loss
Wrong move:
Saying increased humidity increases transpiration rate because there is more water in air
Why:
Transpiration relies on diffusion down a water potential gradient; more water in external air reduces the gradient
Correct move:
Increased external humidity reduces the water potential gradient between the leaf and air, so transpiration rate decreases
Wrong move:
Cutting the plant shoot in air when setting up a potometer
Why:
Cutting in air allows air bubbles to enter the xylem, breaking the water column and stopping water movement
Correct move:
Always cut the shoot under water to prevent air from entering the xylem during potometer setup
Wrong move:
Stating that transpiration is only harmful to plants
Why:
Transpiration has key functional roles for plant survival
Correct move:
Recognise that transpiration supplies water for photosynthesis, transports mineral ions up the plant, and cools leaves via evaporation
6. Quick Reference Cheatsheet
Concept | Key Exam Point |
|---|---|
Transpiration | Loss of water vapour via stomata, inevitable for gas exchange |
Cohesion-tension | Cohesion = H bonds between water; Tension = pull from evaporation |
Potometer | Estimates rate = measures water uptake, not direct measurement |
Increased light β | Increased rate (more open stomata) |
Increased temperature β | Increased rate (faster evaporation) |
Increased humidity β | Decreased rate (lower water potential gradient) |
Increased wind speed β | Increased rate (removes humid air) |
Xerophyte adaptations | All act to reduce transpiration rate in dry habitats |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Factors affecting transpiration rate
- 2023 Β· 4
Potometer experiment analysis
- 2021 Β· 1
Cohesion-tension theory MCQ
Going deeper
- practical guidePotometer setup for CIE practicalsCovers common sources of error and fixes
What's Next
Transpiration is a core process that links to almost all areas of plant physiology in CIE A-Level Biology. Understanding water movement helps explain how plants obtain and distribute mineral ions required for growth and photosynthesis, and how plants adapt to different environments. Potometer experiments are a common topic in both written and practical CIE papers, so mastering its setup, sources of error and limitations is critical for exam success. Transpiration also connects to abiotic stress responses, such as stomatal closure controlled by plant hormones. Next, you can explore the transport of organic solutes in phloem, and deepen your understanding of A-Level practical biology.