ACTIVITY 4. THE PLANT LEAF: AN ORGAN DESIGNED FOR PHOTOSYNTHESIS

Now that our study of light, pigments, and photosynthesis is complete, it is time to take a brief look at the place where the majority of photosynthesis occurs in a plant. While stems and some flower parts are green, the plant organ designed for optimal photosynthesis is the leaf. In a typical flowering plant, the leaf is broad to provide surface area for light absorption, but very thin since light cannot penetrate deeply into living tissue.

A diagram of a typical leaf is shown below. It is cut in both longitudinal and cross section to show the cellular structure within. Note the different cell types which carry out different functions in the leaf.

Most of the chloroplasts within the leaf are located within the mesophyll cells. Most of the mesophyll region contains spaces where water and gases can diffuse through the leaf. The epidermis forms a protective layer on both sides of the leaf. It has openings called stomata that allow gas exchange; usually carbon dioxide enters, whereas oxygen leaves the leaf. The vein contains tubes that bring water into the leaf from the roots and collect sugars for distribution throughout the plant.

Now it is time to study the cross section of a real leaf. View this stained leaf section which is displayed as if using a "virtual light microscope". Magnify the image and move to a region that contains the clearest view of internal structure, including a stoma. Perform a screen capture of the magnified leaf section, then label a stoma, mesophyll, epidermis, and a vein. Submit your labeled image to WebAssign for question 15.

The openings within the epidermis are called stomata (singular is stoma). On either side of a stoma lies a guard cell, a unique cell type that can change shape to open or close the stoma. Water vapor escapes from the leaf through open stomata on a hot day. If there is insufficient water in the soil under these conditions, the stomata will close to prevent water loss. The image below shows both an open and a closed stoma on the surface of a real leaf.

These scanning electron micrographs show an open stoma (top) and a closed stoma (bottom) within the same leaf. Note that the guard cells around the open stoma are thicker than those surrounding the closed stoma. Under conditions in which water loss is not a problem, potassium ions enter the guard cells creating a hypotonic environment around the cells. Water then enters the cells by osmosis causing them to swell and the stoma opens. The cell walls prevent the guard cells from bursting under pressure.

When you have studied the stomata images and caption, answer question 16.