Leaves Section 23.4

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

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

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

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

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

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?