Edexcel iGCSE Biology-2.42 B-2.43 B structure of the leaf and gas exchange- Study Notes- New Syllabus

Leaf Adaptations for Gas Exchange & Role of Stomata

📝 Introduction

Leaves are the main site of gas exchange in plants.
They need O₂ for respiration and CO₂ for photosynthesis.
Adaptations in leaf structure make diffusion of gases quick and efficient.

🌟 Leaf Adaptations for Gas Exchange

• Large surface area 
  Broad, flat leaf captures more sunlight and provides a bigger area for diffusion.
• Thin structure 
  Short distance for gases to diffuse in and out.
• Air spaces in spongy mesophyll 
  Gases (O₂, CO₂) diffuse easily between cells and stomata.
• Stomata present 
  Tiny pores mostly on the underside of the leaf.
  Open to allow CO₂ in and O₂ out.
• Moist surface 
  Helps dissolve gases, making diffusion easier.
• Veins (xylem & phloem)
  Xylem brings water → keeps cells turgid → stomata function properly.
  Phloem removes sugars → maintains concentration gradient.

🔘 Role of Stomata in Gas Exchange

Definition: Stomata are tiny pores in the epidermis of the leaf, mostly on the lower surface.

Each stoma is surrounded by two guard cells.

Functions:

• Gas Exchange
  CO₂ enters for photosynthesis.
  O₂ (from photosynthesis) exits.
  O₂ (for respiration) enters at night.
  CO₂ (from respiration) exits.
• Regulation of Water Loss (Transpiration)
  Stomata also allow water vapour to escape.
  This can cause wilting if uncontrolled.
• Opening & Closing Mechanism
  Guard cells become turgid → stoma opens.
  Guard cells become flaccid → stoma closes.
  Controlled by water availability and light.

📊 Summary Table

⚡ Quick Recap
Leaves are broad, thin, moist, with air spaces → ideal for gas exchange.
Stomata: tiny pores surrounded by guard cells.
Allow CO₂ in for photosynthesis & O₂ out as by-product.
At night → only respiration → O₂ in, CO₂ out.
Guard cells open stomata in light and close them to prevent water loss.