Transpiration
Water enters the leaf via the xylem. Inside the leaf, water evaporates from the moist surfaces of the mesophyll cells (specifically the spongy mesophyll) into the air spaces between these cells.
Once in the air spaces, the water is in gaseous form (water vapour). It then diffuses out of the leaf through tiny pores called stomata (singular: stoma).
Note: Evaporation is a change of state from liquid to gas. Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
How does water move up against gravity? It is not pumped by the plant. Instead, it relies on physical properties of water:
- Cohesion: Water molecules are attracted to each other by hydrogen bonds. This holds them together in a continuous column within the xylem.
- Adhesion: Water molecules are attracted to the walls of the xylem vessels, helping to support the column.
As water evaporates from the top of the leaf (transpiration), it pulls the next molecule up. Because of cohesion, this pull is transmitted down the entire continuous column of water in the xylem, drawing more water up from the roots. This is known as the transpiration pull.
3. Factors Affecting Rate (Supplement LOs 3 & 6)
The rate of transpiration depends on how fast water vapour can diffuse out of the stomata. This is governed by the concentration gradient (the difference in water vapour concentration between the air spaces inside the leaf and the outside air).
- Temperature: Higher temperature increases the kinetic energy of water molecules, causing them to evaporate faster and diffuse more rapidly. It also allows the air to hold more water vapour.
- Wind Speed: Wind blows away the humid air layer surrounding the stomata (the boundary layer). This maintains a steep concentration gradient between the inside of the leaf and the outside air, increasing the rate of diffusion.
- Humidity: High humidity means the outside air already contains a lot of water vapour. This reduces the concentration gradient between the inside and outside of the leaf, slowing down diffusion.
4. Structural Adaptations (Supplement LO 4)
The rate of transpiration is directly related to the structure of the leaf:
- Internal Surface Area: The spongy mesophyll layer has many interconnecting air spaces. This provides a large internal surface area for water to evaporate from, maximizing the rate of transpiration.
- Stomata Size and Number:
- Number: More stomata provide more exit points (pores) for water vapour to diffuse out, increasing the total rate.
- Size: Larger stomatal pores allow water vapour to escape more easily. The size of the pore is controlled by guard cells swelling or shrinking.
5. Wilting (Supplement LO 7)
Wilting occurs when the rate of water loss through transpiration exceeds the rate at which roots can absorb water from the soil.
- Mechanism: Water normally exerts turgor pressure against the cell walls of plant cells, keeping them rigid and the plant upright. When water is lost faster than it is replaced, turgor pressure drops.
- Result: The cells become flaccid (limp), and the stems/leaves droop. This is a protective mechanism to reduce surface area and slow down further water loss.
Experiment: Effect of Wind Speed on Transpiration
A common investigation uses a potometer (which measures the rate of water uptake by a shoot, assuming uptake equals transpiration) or weighs a leafy shoot over time.
| Condition | Expected Result | Explanation |
|---|---|---|
| High Wind Speed | Increased rate of water uptake/transpiration | Wind removes humid air from around the stomata, maintaining a steep concentration gradient for diffusion. |
| Low Wind Speed (Still Air) | Decreased rate of water uptake/transpiration | A layer of humid air builds up around the stomata, reducing the concentration gradient. |
| High Temperature | Increased rate | Higher kinetic energy increases evaporation from mesophyll cells and increases the capacity of air to hold water vapour. |
| High Humidity | Decreased rate | The outside air is already saturated with water vapour, reducing the concentration gradient between the leaf interior and exterior. |
Structural Impact on Rate:
- Leaf with many stomata: Transpires faster than a leaf with few stomata because there are more pores for diffusion.
- Leaf with large air spaces (spongy mesophyll): Transpires faster because the large surface area allows more water to evaporate into the air spaces simultaneously.
Why Wilting Happens: A Step-by-Step Example
- Condition: It is a hot, dry, windy day.
- Process: Transpiration rate becomes very high due to the steep concentration gradient and high kinetic energy.
- Limitation: The roots cannot absorb water from the soil fast enough to match this loss.
- Cellular Effect: Water leaves the vacuoles of the parenchyma cells in the stem and leaves.
- Physical Result: Turgor pressure decreases. The cells become flaccid.
- Observation: The plant wilts (leaves droop).
Mistake 1: Confusing Evaporation and Diffusion
- Incorrect: "Water evaporates out of the stomata."
- Correct: Water evaporates from the mesophyll cell surfaces into the air spaces. It then diffuses out of the leaf through the stomata as water vapour. Evaporation is a phase change; diffusion is movement down a gradient.
Mistake 2: Attributing Wilting to Photosynthesis
- Incorrect: "Wilting occurs because photosynthesis stops."
- Correct: Wilting occurs due to loss of turgor pressure caused by water deficit. While stomatal closure (to prevent wilting) does stop photosynthesis, wilting itself is a mechanical failure of cell rigidity, not a metabolic failure.
Mistake 3: Misidentifying the Force Holding Water Together
- Incorrect: "Water molecules are held together by adhesion."
- Correct: Water molecules are held together by cohesion (attraction between water molecules). Adhesion is the attraction between water molecules and the xylem vessel walls.
Tip 1: Explaining the Effect of Humidity
When asked to explain why high humidity reduces transpiration, you must explicitly mention the concentration gradient.
- Examiner Accepts: "High humidity reduces the concentration gradient of water vapour between the air spaces in the leaf and the outside air."
- Why: Examiners look for the link between the environmental condition (humidity) and the physical principle driving diffusion (gradient). Simply saying "it is wet outside" is insufficient.
Tip 2: Describing the Transpiration Pull Mechanism
When explaining how water moves up the xylem, you must use the terms cohesion and transpiration pull.
- Examiner Accepts: "Water molecules are cohesive, forming a continuous column. Evaporation at the top creates a tension (pull) that draws the column upwards."
- Why: This directly addresses the physical properties of water required to explain movement against gravity without active transport.
Tip 3: Investigating Wind Speed
When planning an investigation on wind speed, ensure you identify the correct variables.
- Examiner Accepts: "Independent variable: wind speed (e.g., fan at different settings). Dependent variable: mass loss of leaf / distance moved by air bubble in potometer. Controlled variables: temperature, humidity, light intensity."
- Why: This shows you understand experimental design. Failing to list controlled variables like temperature often leads to lost marks.
- The rate of transpiration exceeds the rate of water absorption by roots. 2. Loss of turgor pressure in plant cells (cells become flaccid).