Leaves

1. Leaves Mr. Chapman Biology 20

2. Let’s do an experiment first... Extracting chlorophyll from leaves and making it glow.

3. Chlorophyll Fluorescence • If you remove chlorophyll molecules from their cells and expose them to bright light, the chlorophyll will absorb the light energy. • Recall that chlorophyll is the pigment found in leaves that absorbs light energy from the sun. • The absorbed energy will be lost as heat or released as dull-coloured light, known as fluorescence. This is what we are hoping to see.

4. Procedure • Use the mortar and pestle to crush a handful of spinach leaves. • Add enough methanol to make approximately 10 mL of extract. Use a graduated cylinder to collect and measure the extract. • Place the filter paper in the funnel, and hold the funnel over a beaker. A second person should pour the extract through the funnel to filter the extract.

5. Procedure Continued... • Carefully transfer the extract to a test tube, and hold it in front of a light in the classroom for about one minute. • Quickly take the test tube and go to the storage room, closing the door so it is pitch black. Can you see any fluorescence from the test tube? • Clean up all of the materials you used for the lab. Wash all of the materials with soap and dry themwith paper towel.

6. Questions to Answer After Mini-Lab • What color does the fluorescence appear? • Why did the chlorophyll have to be extracted before the fluorescence could be observed? • Why do plants have pigments that can absorb light? What are the three things a plant requires in order to perform photosynthesis? • Name one way in which you could have improved the experiment.

7. Leaves Structure of Leaves, and Various Types

8. Leaves Don’t All Look the Same

9. Structure • Leaves grow out of a plant’s stem, and are composed of a few basic parts: • Blade: usually broad and flat, but not always. Collects sunlight for the plant. • Petiole: the blade connects to the stem by a thin stalk called the petiole. • Auxiliary bud: grows between the petiole and the stem of the plant, marking where leaf ends.

11. Leaf Tissue • Like all plant parts, leaves have an outer layer of dermal tissue. • Inside the dermal tissue is an internal system of vascular tissue, surrounded by ground tissue. • Remember that the dermal tissue is often covered by cuticle, a waxy substance that prevents water from escaping the leaves.

12. Leaf Tissue Continued... • Between both layers of the dermal tissue (the top and bottom of the leaf) is mesophyll, which is composed of parenchyma tissue. • Xylem and phloem is the vascular tissue that runs through the mesophyll.

14. Stomata and Guard Cells • In most plants, the tops and bottoms of the leaves have different functions. • The top of the leaf is where the mesophyll has the most chloroplasts, allowing it to absorb the most light and perform photosynthesis. • The bottom of the leaf contains the stomata and is where transpiration and gas exchange occur.

16. Stomata and Guard Cells • During the day, the stomata of most plants are open, which allows the plant to acquire the carbon dioxide it requires for photosynthesis. • The stomata must be open in order for the plant to get carbon dioxide. • Guard cells plump up when the plant is full of water, opening up the stoma.

17. Stomata and Guard Cells • When the stomata are open and absorbing carbon dioxide, the plant is losing water. • If the plant loses water faster than it absorbs it in its roots, the guard cells deflate and then close. • The guard cells are always closed at night, and other factors have an affect on whether or not the stomata will open.

18. Leaf Characteristics • Some leaves are simple, with just one blade connected to the petiole. • Other leaves are compound, with many blades connected to the petiole. Multiple blades are called leaflets. • All of the leaflets and their petiole together are known as one leaf, because auxiliary bud is at the base of the petiole attached to the stem.

20. Leaf Adaptations • Not all leaves are leafy. Leaves are adapted depending on their particular environment. • Cactus leaves are sharp spines, protecting them from predators and preventing excess water loss. • Some plants store water in their leaves, and have very thick cuticles in order to prevent water loss.

21. Leaf Adaptations • Some plants that rest on water, like water lilies, have stomata on the upper part of the leaf instead of underneath, to allow gas exchange. • Many tropical plants have huge, broad leaves. This is due to the challenge the plants have to absorb light with all of the other plants blocking the way. • Some plants are predators, with leaves that attract and trap small insects, digesting them to get to their sweet, sweet nitrogen.