CIE iGCSE Co-ordinated Sciences-B8.3 Transpiration- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-B8.3 Transpiration – Study Notes

CIE iGCSE Co-ordinated Sciences-B8.3 Transpiration – Study Notes -CIE iGCSE Co-ordinated Sciences – 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 the effects on the rate of transpiration of varying the following: temperature, wind speed and humidity
Explain how and why wilting occurs

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Transpiration in Plants

📖 Definition

Transpiration = Loss of water vapour from plant leaves → by evaporation from mesophyll cell surfaces + diffusion through stomata.

🔑 Explanation

Inside the leaf: Mesophyll cells are moist. Water evaporates from their surfaces into the air spaces.
From leaf to atmosphere: Water vapour diffuses out through open stomata.

This continuous loss creates a transpiration pull → sucking water up through the xylem.
Together with root pressure and cohesion/adhesion of water, transpiration helps move water up tall trees (e.g. redwoods >100 m tall).

🌊 Transpiration Stream

Water absorbed by root hairs (from soil by osmosis).
Moves through root cortex → enters xylem vessels.
Travels upwards in a continuous column through the stem.
Reaches leaves → passes into mesophyll cells.
Evaporates into air spaces → diffuses out through stomata.

This whole upward flow is called the Transpiration Stream.

📊 Summary Table

Step	Process	Structure Involved
1	Absorption of water	Root hairs
2	Osmosis across cortex	Root cortex
3	Long-distance upward flow	Xylem vessels
4	Evaporation into air spaces	Mesophyll cells
5	Diffusion to atmosphere	Stomata

📝 Quick Recap
Transpiration = loss of water vapour from leaves (mesophyll → stomata → air).
Creates transpiration pull → draws water up the xylem.
Important for:
Cooling the plant
Continuous water + mineral supply
Maintaining turgidity

Transpiration & Water Loss from Leaves

📌 Definition

Transpiration = Loss of water vapour from plant leaves.
Happens by evaporation from mesophyll cell surfaces → followed by diffusion through stomata.

🔬 Step-by-Step Process

Inside the leaf: Mesophyll cells contain water. Water evaporates from their cell walls into the air spaces in the spongy mesophyll.
Into air spaces: The air spaces in the leaf fill with water vapour.
Through stomata: Water vapour diffuses out through open stomata → into the atmosphere.
(diffusion occurs because concentration of water vapour inside leaf > outside air)

🌱 Why it matters

Creates a pull (suction) → draws water up the xylem.
Maintains continuous transpiration stream (root → stem → leaf).
Helps with cooling the leaf + transport of minerals & water.

📊 Factors Affecting Transpiration

Light → stomata open → ↑ transpiration.
Temperature → faster evaporation → ↑ transpiration.
Wind → removes water vapour → ↑ transpiration.
Humidity → high humidity slows diffusion → ↓ transpiration.

🧪 Experiments

Weight Potometer → measures mass lost (water vapour lost by transpiration).
Capillary Potometer → measures water uptake as an indirect measure of transpiration rate.
Vaseline Leaf Test → shows more water is lost from lower surface (more stomata there).

📌 Summary Table

Step	What happens
1. Mesophyll	Water evaporates from cell walls
2. Air spaces	Water vapour fills spaces
3. Stomata	Vapour diffuses out
4. Atmosphere	Water lost to environment

⚡ Quick Recap
Transpiration = loss of water vapour from leaves.
Pathway: Evaporation → mesophyll → air spaces → stomata → outside.
Maintains transpiration stream & cools leaves.
Controlled by: Light, Temp, Wind, Humidity.
More water lost from lower surface due to stomata.

Effect of Temperature & Wind Speed on Transpiration

📌 Introduction

Transpiration = loss of water vapour from leaf surface (via mesophyll → stomata → atmosphere).
Rate of transpiration is strongly affected by environmental factors like temperature and wind speed.

🔥 Effect of Temperature

↑ Temperature → ↑ kinetic energy of water molecules.
Water evaporates faster from mesophyll cell walls.
Increases water vapour concentration inside leaf → steeper concentration gradient with outside air.
Result = faster diffusion through stomata → ↑ transpiration rate.
Too high temperature → may cause stomata to close (to prevent water loss) → ↓ transpiration.

🌬️ Effect of Wind Speed

Still air → water vapour builds up around stomata → reduces concentration gradient → slower transpiration.
Wind blows away this moist layer → maintains steep concentration gradient between leaf and air.
Result = more rapid diffusion of water vapour → ↑ transpiration rate.
Very strong wind → stomata may close to avoid excess water loss → transpiration drops.

🧪 Practical Investigations

Using a Potometer

Measure water uptake under different temperatures and wind speeds.
Compare rate of bubble movement (capillary potometer) or mass loss (weight potometer).

Observations

Higher temp = faster transpiration.
Increased airflow (fan) = faster transpiration.

📊 Summary Table

Factor	Effect on Transpiration	Explanation
Temperature ↑	Transpiration ↑	Faster evaporation, steeper gradient
Wind Speed ↑	Transpiration ↑	Removes humid air, maintains gradient
Extreme heat/wind	Transpiration ↓	Stomata close to conserve water

⚡ Quick Recap
Temp ↑ → faster evaporation → ↑ transpiration.
Wind ↑ → removes moist air → ↑ transpiration.
Extreme heat/wind → stomata close → ↓ transpiration.
Tested with potometers (weight/capillary).

Factors Affecting Rate of Transpiration

📌 Introduction

Transpiration = loss of water vapour from mesophyll cells → through stomata → into atmosphere.
The rate of transpiration changes with environmental factors.

🔥 1. Temperature

Higher temp → ↑ kinetic energy of water molecules.
Water evaporates faster from mesophyll cell walls.
Steeper concentration gradient between inside of leaf & outside air.
Result → Transpiration rate increases.
Very high temp → stomata may close to prevent excess water loss → transpiration drops.

🌬️ 2. Wind Speed

Still air → water vapour builds up near stomata → reduces gradient → slows transpiration.
Wind → blows away moist air → maintains steep concentration gradient.
Result → Transpiration rate increases with wind.
Very strong winds → stomata may close → transpiration decreases.

💧 3. Humidity

High humidity (moist air) → reduces difference in water vapour concentration between leaf & air → transpiration slows.
Low humidity (dry air) → steep gradient → transpiration faster.

📊 Summary Table

Factor	Effect on Transpiration	Reason
Temperature ↑	Rate ↑	Faster evaporation + steeper gradient
Wind speed ↑	Rate ↑	Removes humid air around stomata
Humidity ↑	Rate ↓	Gradient reduced (air already moist)
Low humidity	Rate ↑	Steep gradient (air is dry)

⚡ Quick Recap
Temp ↑ → faster evaporation → ↑ transpiration.
Wind ↑ → removes moist air → ↑ transpiration.
Humidity ↑ → gradient reduced → ↓ transpiration

Wilting in Plants

📌 What is Wilting?

Wilting = when plant leaves and stems droop because they lose turgor pressure due to excessive water loss.

🔎 How Wilting Occurs (Step by Step)

High transpiration rate (hot, dry, windy conditions) or low water absorption by roots (drought, dry soil).
More water is lost from leaves than absorbed by roots.
Leaf and stem cells lose water → vacuoles shrink.
Loss of water = loss of turgor pressure in cells.
Cells become flaccid → tissues can no longer stay firm.
Leaves and stems bend or droop → wilting.

🧠 Why Wilting Occurs (Significance)

Wilting is a protective response.
Leaves droop → reduce surface area exposed to sunlight.
This lowers transpiration and conserves water.
But if water loss continues and is not replaced → plant may die (permanent wilting).

📊 Types of Wilting

Type	Cause	Effect
Temporary wilting	Hot day, high transpiration, but soil still moist	Plant recovers at night when transpiration ↓
Permanent wilting	Soil water unavailable (drought, dry soil)	Plant cannot recover → may die

⚡ Quick Recap
Wilting = loss of turgor pressure → cells flaccid → plant droops.
Caused by more water lost than absorbed.
Temporary wilting = plant recovers later.
Permanent wilting = plant cannot recover, may die.
Protective role = drooping leaves reduce transpiration.

CIE iGCSE Co-ordinated Sciences-B8.3 Transpiration- Study Notes- New Syllabus