1. Leaves
Section 23.4

2. How is the leaf adapted to make photosynthesis more efficient?
Most leaves have a thin flattened part called the blade – maximize area over which light can be absorbed
Blade attached to stem by a thin branch called a petiole
Leaves are covered top and bottom by dermal tissue
Thick outer wall that resists tearing
Covered by a waxy cuticle - waterproof barrier that protects tissues and limits the loss of water through evaporation
Vascular tissue flows through leaves in veins – connecting the leaves to the rest of the plants
Leaves contain a special ground tissue called mesophyll, where photosynthesis occurs

3. Leaf structure
Photosynthesis predominantly occurs in pallisade mesophyll layer
Transpiration – loss of water through leaves
Walls of mesophyll cells are kept moist to facilitate gas exchange
Also keeps leaves cool on a hot day
Water replaced through xylem

4. What role do stomata play in maintaining homeostasis?
When Photosynthesizing, leaves take in Carbon Dioxide and give off Oxygen
When respiring they take in oxygen and give off carbon dioxide
Stomata control gas exchange between spongey mesophyll and exterior
Why can’t the stomata be left open all the time?
Water loss would be too great
Guard cells maintain the balancing act between gas exchange and water loss
Guard cells open and close in response to changes in water pressure
General rule – stomata are open during day time and closed during night time

5. Transpiration and wilting
Osmotic pressure keeps a plant’s leaves and stems rigid and stiff
High transpiration rates can lead to wilting
Wilting leads to loss of water and pressure in a plants cells
When a leaf wilts it’s stomata close
Therefore, wilting helps plants to conserve water

6. Leaf adaptations
Leaves can adapt to live in a variety of different environments

7. Key point questions
How is the structure of a leaf adapted to make photosynthesis more efficient?
What is the role of the palisade mesophyll layer?
Why might desert plants have two or more palisade mesophyll layers?
How do stomata help maintain photosynthesis?
Will stomata be open or closed on a hot day?