1. Overview
Transpiration is the inevitable consequence of gas exchange in plants, acting as the "engine" that pulls water and dissolved minerals from the roots to the leaves. This process is vital for maintaining cell turgidity, cooling the plant, and ensuring the leaves have enough water for photosynthesis.
Key Definitions
- Transpiration: The loss of water vapour from leaves.
- Stomata: Tiny pores mainly found on the underside of a leaf that allow gases to diffuse in and out.
- Mesophyll Cells: Specialized cells within the leaf (palisade and spongy) where evaporation occurs.
- Cohesion: The force of attraction between water molecules, holding them together in a continuous column.
- Adhesion: The force of attraction between water molecules and the walls of the xylem vessels.
- Potometer: A piece of apparatus used to measure the rate of water uptake (and therefore the rate of transpiration).
Core Content
The Process of Transpiration (Step-by-Step)
- Water is absorbed by root hair cells via osmosis.
- Water travels through the root cortex into the xylem vessels.
- Water moves up the xylem into the mesophyll cells of the leaf.
- Evaporation: Water evaporates from the moist surfaces of the mesophyll cells into the interconnecting air spaces.
- Diffusion: Water vapour builds up in the air spaces, creating a high concentration. It then diffuses out of the leaf through the stomata, moving down a concentration gradient into the surrounding air.
Investigating Environmental Factors
To measure transpiration, we use a potometer. By changing one condition while keeping others constant, we can observe the effect on the rate:
- Temperature: Use a heater or a lamp to increase the temperature.
- Observation: Increased temperature increases the rate of transpiration.
- Wind Speed: Use a fan at different speed settings or distances.
- Observation: Increased wind speed increases the rate of transpiration.
Extended Content (Extended Curriculum Only)
Leaf Structure and Water Loss
The rate of transpiration is facilitated by the leaf’s internal anatomy:
- Large Internal Surface Area: The spongy mesophyll layer has many interconnecting air spaces. This provides a huge surface area for water to evaporate from cell walls into the air.
- Stomata Number and Size: The more stomata a leaf has, or the wider they are open, the faster water vapour can diffuse out.
The Transpiration Pull (Mechanism)
Water moves up the xylem in a continuous, unbroken column. This is explained by the Cohesion-Tension Theory:
- As water vapour is lost from the leaves, it creates a "negative pressure" (suction) at the top of the xylem.
- This creates a transpiration pull that draws the water column upwards.
- Cohesion: Water molecules are polar and stick to each other. This ensures the column doesn't break.
- Adhesion: Water molecules stick to the cellulose in xylem walls, helping to support the column against gravity.
Factors Affecting Transpiration Rate (The "Why")
| Factor | Effect on Rate | Explanation |
|---|---|---|
| Temperature | Increase | Increases kinetic energy of water molecules, so they evaporate and diffuse faster. |
| Wind Speed | Increase | Moves water vapour away from the leaf surface, maintaining a steep diffusion gradient. |
| Humidity | Decrease | High humidity means there is a lot of water vapour in the air outside the leaf. This reduces the diffusion gradient between the leaf and the atmosphere. |
Wilting
Wilting occurs when the rate of transpiration is greater than the rate of water uptake at the roots.
- Mechanism: Cells lose water and their turgor pressure decreases. The cells become flaccid.
- Result: The plant stems and leaves lose their mechanical support and droop. This is actually a survival mechanism as it often causes stomata to close, reducing further water loss.
Key Equations
Rate of Transpiration (using a potometer): $$\text{Rate} = \frac{\text{Distance moved by air bubble (mm)}}{\text{Time taken (min)}}$$
- Distance: Measured in mm or cm.
- Time: Measured in minutes or seconds.
- Unit: e.g., mm/min.
Note: If the capillary tube's diameter is known, you may be asked to calculate the volume ($\pi r^2 \times \text{distance}$).
Common Mistakes to Avoid
- ❌ Wrong: Water "evaporates out of the stomata."
- ✓ Right: Water evaporates from the mesophyll surfaces and then diffuses as vapour through the stomata.
- ❌ Wrong: Thinking transpiration and evaporation are the same thing.
- ✓ Right: Transpiration is the total process of water loss from a plant; evaporation is just the phase change from liquid to gas inside the leaf.
- ❌ Wrong: High humidity increases transpiration because there is more water.
- ✓ Right: High humidity decreases transpiration because it makes the concentration of water outside the leaf similar to the inside, slowing down diffusion.
Exam Tips
- Command Word "Explain": When asked to explain the effect of wind or temperature, you must mention the diffusion gradient or kinetic energy. Simply saying "it goes faster" is a "describe" answer, not "explain."
- Practical Contexts: You will often see questions featuring a bubble potometer. Remember to state that the air-tight seal (usually made with Vaseline) is essential for accurate results.
- Numerical Values: Be prepared to work with values such as 5.1 mm/min. Always check if the question asks for the rate per minute or per hour.
- Stomata Location: Remember that most stomata are on the lower epidermis to prevent excessive water loss from direct sunlight. Questions often ask you to compare transpiration between the upper and lower surfaces.