Edexcel iGCSE Biology-3.6B & 3.7B Transpiration- Study Notes- New Syllabus

Transpiration in Plants

📝 Introduction

Transpiration = the loss of water vapour from the surface of a plant.
It mainly happens through stomata on the leaves when water evaporates from mesophyll cells and diffuses out.
It is not just “water loss” → it also pulls water upwards through the xylem, helping in nutrient transport and cooling.

🌬️ What is Transpiration?

Definition: The evaporation of water from the surface of a plant, mostly through the stomata of leaves.

• Water absorbed by root hair cells → xylem.
• Water moves up the plant → reaches leaves.
• Water evaporates from mesophyll cells into air spaces.
• Water vapour diffuses out through open stomata.

🔑 Factors Affecting Transpiration

• Humidity
  High humidity = air already moist → smaller concentration gradient → slower transpiration.
  Low humidity = dry air outside → steeper gradient → faster transpiration.
• Wind Speed
  Still air = water vapour builds up near leaf → reduces gradient → slower transpiration.
  Windy conditions = water vapour removed → steep gradient maintained → faster transpiration.
• Temperature
  Higher temperature → water molecules evaporate faster → increases transpiration.
  Also increases diffusion rate.
• Light Intensity
  Light → stomata open for photosynthesis → allows more water vapour to escape.
  Dark → stomata close → transpiration decreases.

📊 Summary Table

⚡ Quick Recap
Transpiration = evaporation of water from leaves (via stomata).
Rate depends on:
Wind → more wind = faster.
Temperature → higher = faster.
Light → open stomata = faster.
Humidity → higher = slower.