CIE iGCSE Biology-8.3 Transpiration- Study Notes- New Syllabus
CIE iGCSE Biology-8.3 Transpiration- Study Notes – New syllabus
CIE iGCSE Biology-8.3 Transpiration- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe transpiration as the loss of water vapour from leaves
- 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 vapour
- Investigate and describe the effects of variation of temperature and wind speed on transpiration rate
Supplement
- 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 stomata - 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 molecules
- Explain the effects on the rate of transpiration of varying the following factors: temperature, wind speed and humidity
- Explain how and why wilting occurs
Transpiration – Definition and Explanation
📘 What is Transpiration?
Transpiration is the process by which water vapour is lost from the aerial parts of a plant, especially through small pores in the leaves called stomata. It is a natural and essential part of a plant’s water regulation system.
🌿 Where and How Does Transpiration Occur?
Main Site: Leaves (specifically through stomata, mostly on the underside)
- Water is absorbed from the soil by the root hairs.
- It travels upwards through the xylem vessels in the stem.
- Water reaches the mesophyll cells in the leaf where it evaporates.
- The water vapour diffuses out through open stomata into the atmosphere.
🌟 Why is Transpiration Important?
- Cooling the Plant: As water evaporates from leaf surfaces, it cools the plant—just like sweat cools our bodies.
- Water Movement (Transpiration Pull): It creates a suction force that helps pull water and dissolved minerals up from the roots.
- Transport of Minerals: Dissolved nutrients like nitrates are carried with the water through the xylem.
- Maintains Turgidity: It keeps cells firm and helps in the support of leaves and young stems.
📊 Key Factors That Affect Transpiration Rate
| Factor | Effect on Transpiration |
|---|---|
| Light intensity | Increases transpiration (opens stomata wider) |
| Temperature | Warmer air increases evaporation and diffusion |
| Humidity | Lower humidity increases transpiration rate |
| Wind | Wind moves saturated air, speeding up evaporation |
| Stomatal opening | Wider stomata lead to more water loss |
📝 Summary
How Water Vapour Leaves the Leaf – The Process of Evaporation and Diffusion
🌿 Where Does Water Go in a Leaf?
Inside each leaf, mesophyll cells are loosely packed and surrounded by air spaces. These cells are always moist due to water transported from the roots via xylem.
🔁 Step-by-Step Process
- Evaporation from Mesophyll Surfaces: Water evaporates from the moist walls of mesophyll cells into internal air spaces.
- Build-up of Water Vapour: As evaporation continues, water vapour concentration increases inside the leaf.
- Diffusion Through the Stomata: When stomata are open, water vapour diffuses out into the atmosphere.
- Loss of Water Vapour: This escape of water vapour through stomata is known as transpiration.
💧 Why Is This Process Important?
- Maintains a continuous stream of water from roots to leaves.
- Enables transport of dissolved minerals (e.g. nitrates, potassium).
- Helps cool the leaf surface by evaporation.
🔍 Key Statement
Factors Affecting the Rate of Transpiration
To study transpiration, scientists use a potometer a device that measures the rate of water uptake by a plant, which reflects how quickly water is lost through transpiration.
By altering environmental conditions such as temperature or wind speed, we can investigate their impact on transpiration.
🔥 Effect of Temperature on Transpiration
- Higher temperatures increase the rate of transpiration.
- Water molecules gain more kinetic energy, leading to faster evaporation from mesophyll surfaces.
- Water vapour diffuses out more rapidly due to a steeper concentration gradient.
- Observation: The potometer shows faster movement of the air bubble under warm conditions.
🌬️ Effect of Wind Speed on Transpiration
- Faster wind increases the rate of transpiration.
- Wind removes the humid air layer surrounding the leaf, keeping the outside air drier.
- This enhances the diffusion of water vapour out of the leaf.
- Observation: In windy conditions (e.g. near a fan), the potometer’s air bubble moves faster.
🧠 Summary Table: Factors Affecting Transpiration Rate
| Factor | Effect on Transpiration | Reason |
|---|---|---|
| Temperature ↑ | Transpiration ↑ | Faster evaporation and diffusion of water vapour |
| Wind Speed ↑ | Transpiration ↑ | Removes humid air layer, increases diffusion gradient |
🧾 Conclusion
How Internal Leaf Structure Affects Water Vapour Loss
Transpiration mainly occurs through stomata, but the internal structure of the leaf plays a crucial role in how much water vapour is lost. Inside the leaf, mesophyll cells are surrounded by large air spaces, which promote evaporation and diffusion.
🧱 1. Large Internal Surface Area of Mesophyll
- Mesophyll cells, especially spongy mesophyll, are loosely packed with large air spaces between them.
- These air spaces are in direct contact with moist mesophyll surfaces.
Water evaporates into the air spaces, building a high concentration of water vapour inside the leaf.- A larger internal surface area means more evaporation, increasing transpiration rate.
🌬️ 2. Size and Number of Stomata
- Stomata are the exit points for water vapour.
- More stomata = more openings = greater water loss potential.
- Larger stomatal pores = faster diffusion of vapour out of the leaf.
🔄 Linking It Together
- A large internal surface area increases water evaporation from mesophyll cells.
- More or larger stomata increase diffusion of water vapour out of the leaf.
- Together, they contribute to a higher transpiration rate.
🧠 Why Does This Matter?
Understanding this structural relationship helps us explain:
- Why some plants lose water more quickly than others.
- How leaves are adapted to dry vs. humid environments.
- How plants regulate stomatal openings to avoid excessive water loss.
How Water Moves Up Through the Xylem
Water in plants travels from the roots to the top leaves against gravity. This upward movement is driven by a natural mechanism involving transpiration and the physical properties of water molecules.
🔍 What’s the Big Question?
How does water climb up a tall plant without any pump? The answer is the transpiration pull, made possible by cohesion between water molecules.
💨 1. The Driving Force: Transpiration Pull
- Water evaporates from the mesophyll surfaces inside the leaf.
- This water vapour exits through the stomata (transpiration).
- Evaporation creates a suction effect, pulling water up from the xylem—just like sucking through a straw.
🧲 2. Cohesion Between Water Molecules
- Water molecules are polar they attract each other via hydrogen bonds.
- This attraction is called cohesion.
- Cohesion ensures water moves as a continuous column through the xylem vessels.
🧵 3. A Continuous Column of Water
- As water evaporates at the top of the plant, it pulls the next water molecules up.
- This pull continues all the way down to the roots.
- No energy is used this is a passive process powered by water loss from the leaves.
📌 Summary of the Mechanism
| Step | What Happens |
|---|---|
| 1 | Water evaporates from the leaf surface (transpiration) |
| 2 | Creates a pull on the water column in the xylem |
| 3 | Cohesion holds water molecules together |
| 4 | Water is pulled up from roots through the xylem vessels |
🧠 Why Is This Important?
- Ensures delivery of water and minerals to all parts of the plant.
- Helps cool the plant through evaporative cooling.
- Supports photosynthesis by supplying water to the leaf cells.
Factors Affecting the Rate of Transpiration
Transpiration is the loss of water vapour from leaves, mostly through stomata. Several environmental factors can increase or decrease how quickly this happens.
🌡️ 1. Temperature
- Higher temperature → faster evaporation from mesophyll cells
- Warm air gives water molecules more energy to escape as vapour
- Effect: Increases transpiration rate
- Lower temperature → slower evaporation and diffusion
🌬️ 2. Wind Speed
- High wind blows away moist air from the leaf surface
- This maintains a steep concentration gradient for water vapour
- Effect: Windy conditions = faster transpiration
- Still air allows water vapour to build up, slowing diffusion
💧 3. Humidity
- High humidity means the air is already moist
- This reduces the concentration gradient for water vapour
- Effect: More humidity = less transpiration
- Dry air increases the gradient → more water loss
📝 Summary Table
| Factor | Effect on Transpiration Rate | Reason |
|---|---|---|
| Temperature | Increases when hot | Faster evaporation of water from mesophyll cells |
| Wind Speed | Increases with strong wind | Removes humid air around leaf; maintains diffusion gradient |
| Humidity | Decreases in humid conditions | Reduces the water vapour gradient for diffusion |
How and Why Wilting Occurs
📌 What is Wilting?
Wilting is when a plant’s leaves and stems droop or collapse due to a lack of water. It’s a clear sign that the plant is losing water faster than it can absorb from the soil.
💦 Why Does Wilting Happen?
- Transpiration causes water loss from the leaves.
- If water uptake by roots doesn’t match this loss, plant cells lose turgor pressure.
- Cells become flaccid, and leaves/stems droop as they lose firmness.
🧬 Scientific Explanation
Plant cells absorb water by osmosis, filling their central vacuoles and building up turgor pressure against the cell wall. This keeps tissues firm. When water is lost (e.g., hot weather or dry soil), the vacuoles shrink, cells lose pressure, and the plant wilts.
❗ If not corrected, this can lead to permanent cell damage or death.
🌤️ Factors That Can Lead to Wilting
| Condition | How It Affects Water Loss |
|---|---|
| Hot weather | Increases evaporation and transpiration |
| Dry or windy conditions | Accelerates water vapour loss |
| Low soil moisture | Roots can’t absorb enough water |
| Damaged roots | Reduces water uptake |
| Blocked xylem vessels | Interrupts water flow to the leaves |
🔁 Is Wilting Reversible?
- Yes, if treated early: Watering restores turgor pressure and upright posture.
- No, if prolonged: Extended water loss can permanently damage cells.