A-Level Biology: Gas Exchange in Plants

Introduction

• Focus on understanding gas exchange in plants, specifically in dicotyledonous leaves.
• Content is relevant for both A-Level and GCSE studies.

Structure of a Dicotyledonous Leaf

• Key Structures for Gas Exchange:
  • Lower Side of Leaf: Contains most stomata.
  • Stoma/Stomata: Singular is stoma, plural is stomata. It is the gap formed by two guard cells, creating a pore for gas exchange.
  • Guard Cells: Surround the stoma and regulate its opening.

Process of Gas Exchange

• Carbon Dioxide (CO2):
  • Diffuses in through stomata.
  • Moves into the spongy mesophyll, which has ample space for diffusion and helps maintain a concentration gradient.
  • Further diffuses up to the palisade mesophyll where photosynthesis occurs.
• Oxygen (O2):
  • Diffuses out of the leaf.
  • Produced in high quantities due to photosynthesis, leading to higher concentrations in the spongy mesophyll compared to the atmosphere.

Leaf Anatomy Related to Gas Exchange

• Vascular Bundle:
  • Contains xylem and phloem.
  • Xylem transports water and dissolved minerals.
  • Phloem transports sucrose.
• Spongy Mesophyll:
  • Characterized by large gaps between cells, facilitating gas diffusion.
• Palisade Mesophyll:
  • Long, oblong cells packed with chloroplasts for maximum light absorption during photosynthesis.

Stomata Functionality

• Role in Photosynthesis:
  • Stomata open to allow CO2 in for photosynthesis.
  • Close at night to minimize water loss when photosynthesis is not occurring.
• Water Conservation:
  • Balance between CO2 intake and water loss.
  • Stomata closure is aided by guard cells becoming less turgid.

Adaptations in Xerophytic Plants

• Adaptations for Dry Conditions:
  • Curled Leaves: Traps moisture, creating humidity to reduce water potential gradient and evaporation.
  • Trichomes (Hairs): Trap evaporated water, increasing humidity around the leaf surface.
  • Sunken Stomata: Located within folds to trap moisture and reduce evaporation.
  • Thicker Cuticle: Prevents water loss through evaporation.
  • Extensive Root Systems: Reach deeper water sources.

Conclusion

• Understanding the intricate balance of gas exchange and water conservation in plants is crucial, especially in extreme environments.
• Adaptations like those in xerophytic plants illustrate the plant's ability to thrive under harsh conditions.