Biology • Year 11 • Module 2 • Lesson 17
Transpiration: Factors and Measurement
Lock in the core vocabulary, the four-factor framework, and the potometer's role as an indirect measure of transpiration rate.
1. Complete the factors table
The table below lists four environmental factors that affect transpiration rate. For each factor, state whether increasing that factor increases or decreases transpiration rate, then write the mechanism in your own words. 12 marks (1 mark each for direction, 2 marks each for mechanism)
| Factor | Direction of effect on transpiration rate when the factor increases | Mechanism, what is actually happening in the leaf or at the stomatal pore? (2 marks each) |
|---|---|---|
| Temperature | ||
| Humidity | ||
| Wind speed | ||
| Light intensity |
2. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: transpiration, potometer, stomatal aperture, water potential gradient, humidity, wind, temperature, light intensity, boundary layer, xerophyte. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | The evaporation of water from mesophyll cells and its diffusion through stomata to the external atmosphere. | |
| 2.2 | An apparatus used to measure the rate of water uptake by a cut shoot as an indirect indicator of transpiration rate. | |
| 2.3 | The width of the stomatal pore opening, controlled by guard cell turgor, which regulates water vapour loss. | |
| 2.4 | The difference in water potential between the humid leaf air spaces and the drier external air that drives transpiration by diffusion. | |
| 2.5 | The concentration of water vapour in the surrounding air; high levels reduce the water potential gradient and slow transpiration. | |
| 2.6 | Air movement that removes water vapour from around the leaf, steepening the diffusion gradient and increasing transpiration rate. | |
| 2.7 | Affects transpiration by increasing the kinetic energy of water molecules and the water potential gradient between leaf and air. | |
| 2.8 | Drives stomatal opening via guard cell K¹+ uptake, indirectly increasing transpiration rate when present. | |
| 2.9 | A thin, still, humid layer of air at the leaf surface that reduces the effective water potential gradient and slows transpiration. | |
| 2.10 | A plant adapted to arid conditions, with structural features such as sunken stomata, thick cuticle, or succulent tissue that minimise water loss. |
3. True or false, with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 8 marks (1 for T/F, 1 for correction where needed)
3.1 A potometer directly measures how much water a plant transpires. T / F
3.2 High humidity increases transpiration rate because water molecules move faster in humid air. T / F
3.3 Wind increases transpiration rate by removing the boundary layer of humid air around the leaf. T / F
3.4 Light directly evaporates water from leaf cells, which is why it increases transpiration rate. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What does the air bubble in a potometer capillary tube represent, and which direction does it move?
4.2 What is the function of the waxy cuticle in a xerophyte?
4.3 How do sunken stomata reduce transpiration? Name the mechanism.
4.4 What is the function of guard cells in the context of transpiration control?
4.5 What does CAM photosynthesis allow a xerophyte to do that dramatically reduces daytime transpiration?
5. Build a concept map
Draw labelled arrows between the five terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “increases”, “reduces”, “acts through”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: transpiration rate · water potential gradient · stomatal aperture · boundary layer · humidity.
Q1, Factors table
Temperature: Increases transpiration rate. Mechanism: higher temperature increases the kinetic energy of water molecules in the leaf mesophyll, speeding evaporation into leaf air spaces; warm air also has greater capacity to hold water vapour, so the atmosphere is further from saturation, the water potential gradient between the humid leaf interior and the drier outside air steepens, driving faster diffusion of water vapour through stomata. [Direction 1 mark; mechanism 2 marks]
Humidity: Decreases transpiration rate. Mechanism: transpiration is driven by the difference in water vapour concentration between the near-saturated leaf air spaces (~99% relative humidity) and the outside air. When outside humidity is high, the gradient is small; at 100% humidity there is no gradient and no net transpiration occurs. [Direction 1 mark; mechanism 2 marks]
Wind speed: Increases transpiration rate (up to moderate wind; very high wind can cause stomatal closure). Mechanism: in still air a humid boundary layer accumulates immediately outside the stomata, partially saturating the air and reducing the effective water potential gradient. Wind removes this boundary layer, replacing it with drier bulk air and restoring a steep gradient. [Direction 1 mark; mechanism 2 marks]
Light intensity: Increases transpiration rate. Mechanism: light drives stomatal opening indirectly, guard cells photosynthesise, accumulate K¹+ ions by active transport, water enters by osmosis, guard cells become turgid, and the stomatal pore widens. A wider aperture provides more pathway for water vapour to diffuse out. [Direction 1 mark; mechanism 2 marks]
Q2, Term–definition matches
2.1 transpiration • 2.2 potometer • 2.3 stomatal aperture • 2.4 water potential gradient • 2.5 humidity • 2.6 wind • 2.7 temperature • 2.8 light intensity • 2.9 boundary layer • 2.10 xerophyte.
Q3, True / false with correction
3.1 False. A potometer measures water uptake by the cut shoot, not transpiration directly. Over 95% of water taken up is transpired, so it is a valid indicator of transpiration rate, but a small proportion is used in photosynthesis and cell growth.
3.2 False. High humidity decreases transpiration rate because it reduces the water potential gradient between the humid leaf interior and the outside air, when the atmosphere already contains a high concentration of water vapour, less net diffusion occurs through the stomata.
3.3 True.
3.4 False. Light increases transpiration indirectly by triggering guard cell K¹+ accumulation, causing osmotic water entry and stomatal opening. Light does not directly evaporate water from mesophyll cells, that evaporation is driven by temperature and the water potential gradient.
Q4.1, Air bubble in potometer
The air bubble marks the water column in the capillary tube. As the shoot transpires, water is drawn up from the capillary and the bubble moves toward the shoot. The distance the bubble moves per unit time (mm/min) is the measure of water uptake rate.
Q4.2, Function of waxy cuticle in xerophyte
The thick waxy cuticle deposits a layer of hydrophobic cutin on the epidermis that is essentially waterproof. It blocks cuticular transpiration, water diffusing through the cuticle rather than through stomata, significantly reducing total water loss from the leaf surface.
Q4.3, Sunken stomata mechanism
Stomata recessed into crypts or pits below the leaf surface allow transpired water vapour to accumulate in the crypt, creating a humid still-air boundary layer immediately outside the pore. This reduces the effective water potential gradient at the stomatal pore, slowing diffusion of water vapour from the leaf interior.
Q4.4, Function of guard cells
Guard cells control the width of the stomatal pore (stomatal aperture). By actively accumulating or releasing K¹+ ions they alter their own water potential, causing osmotic water entry or exit that inflates or deflates the guard cells, opening or closing the pore and therefore regulating the rate of water vapour loss.
Q4.5, CAM photosynthesis
CAM plants open their stomata at night to fix CO² into organic acids, then close stomata during the hot, dry day. CO² stored overnight is released inside the leaf during the day for photosynthesis. By keeping stomata closed during the hottest, driest period, CAM plants eliminate daytime transpiration almost entirely.
Q5, Sample concept map
A correct map should include arrows such as:
- water potential gradientdrives → transpiration rate
- humidityreduces → water potential gradient
- boundary layerreduces effective → water potential gradient
- stomatal aperturecontrols pathway for → transpiration rate
- humidityaccumulates to form → boundary layer (or: wind removes boundary layer, increasing gradient)
Any biologically valid linking phrases are accepted. Award full marks for at least 5 correctly labelled arrows that respect causal direction.