Overview
This lecture covers gas exchange in dicotyledonous plant leaves, focusing on leaf structure, gas movement, and adaptations for reducing water loss, especially in xerophytic plants.
Leaf Structure and Gas Exchange
- The leaf of dicotyledonous plants has three main layers: upper epidermis, palisade mesophyll, and spongy mesophyll.
- Most stomata (pores) are found on the lower surface of the leaf; each stoma is surrounded by two guard cells.
- Stomata allow gas exchange: carbon dioxide (CO₂) enters and oxygen (O₂) exits.
- The spongy mesophyll has air spaces that facilitate gas diffusion and maintain concentration gradients.
- The palisade mesophyll, packed with chloroplasts, is where most photosynthesis occurs.
- The vascular bundle contains xylem (water/mineral transport) and phloem (sucrose transport).
Gas Movement and Concentration Gradients
- CO₂ diffuses into the spongy mesophyll for photosynthesis, maintaining a gradient from the atmosphere to the leaf.
- O₂, produced during photosynthesis, diffuses out of the stomata since its internal concentration becomes higher than outside.
- The guard cells control the opening and closing of stomata; stomata usually close at night to limit water loss.
Plant Adaptations to Reduce Water Loss
- Xerophytic plants have features to minimize water loss while enabling gas exchange in dry environments.
- Adaptations include curled leaves (trap humid air), hair-like projections (trap moisture), and sunken stomata (reduce evaporation).
- Thick cuticles and extensive root systems further limit water loss and improve water uptake.
Key Terms & Definitions
- Stoma/Stomata — pore(s) on the leaf surface for gas exchange, formed by two guard cells.
- Guard Cells — cells that open and close stomatal pores.
- Mesophyll — inner tissue of the leaf; includes palisade and spongy types.
- Palisade Mesophyll — tightly packed cells, rich in chloroplasts, main site of photosynthesis.
- Spongy Mesophyll — loosely packed cells with air spaces for gas diffusion.
- Vascular Bundle — tissue containing xylem (water/minerals) and phloem (sucrose).
- Xerophyte — plant adapted to survive in dry environments through specialized features.
Action Items / Next Steps
- Review the structure of dicotyledonous leaf layers and their functions.
- Study the adaptations of xerophytic plants for reducing water loss.