Course: Science 10 Name: Date: Topic: Structure and Function of Stomata/Density Unit: Cycling of Matter in Living Grade: 10 of Stomata Systems A. Purpose To generate and analyze SEM images of leaf stomata from various species of plants to determine correlations between stomata density and the environment. 1. Background All plant leaves need to “breathe”. The exchange of atmospheric gases is essential to photosynthesis, the process by which plants use sunlight to convert carbon dioxide and water into oxygen and fuel (carbohydrates). Leaves have special pores called stomata that make gas exchange possible while helping to control the loss of water. The stomata operate through the use of two tiny jellybean shaped cells called guard cells located in the outer layer of tissue called the epidermal layer. Most stomata are on the lower epidermis of the leaves on plants (bottom of the leaf). Unlike other plant epidermal cells, the guard cells contain chlorophyll to do photosynthesis. This allows the cells to expand/ contract to open or close the stomata. Guard cells swell, through the process of osmosis, to allow opening of the stomata (for CO2 to enter and excess O2 and H2O to leave), and they shrink in order to force the stomata shut (either partially or completely) to prevent dehydration. In order Plants must also permit the movement of water from the roots to the leaves through the process of evaporation/transpiration in order to make water available to the cells for photosynthesis. However, a constant concern for terrestrial plants is controlling the rate of transpiration to prevent dehydration (desiccation). To conserve water during dry times, the stomata remain closed to reduce the loss of water vapour. Due to the requirement for carbon dioxide, it is possible for the lack of moisture that forces the stomata to stay closed to prevent the process of photosynthesis from occurring. The number of stomata on the epidermal surface can tell you a lot about a plant. Usually, a high concentration of stomata indicates fast growth and a wet climate. Lower concentrations of stomata indicate lower rates of photosynthesis and growth or adaptations for dry weather. Stomata are useful to drought-threatened plants because they can close to prevent dehydration. When stomata close: 1.Guard cells actively pump out K+ ions 2. This raises the water potential inside the cells 3. Water exits the guard cells by osmosis (moves into adjacent cells) 4. Vacuoles shrink, guard cells become flaccid and stoma close In general, having these openings located on the underside of leaves helps to prevent further loss of moisture. However, this is not always an issue for plants growing in high moisture environments, and can result in some unusual stomata configurations. For example, water lilies grow in ponds where their leaves generally float directly on the surface of the water or slightly above it. To thrive in this environment, they have developed stomata on the water lily's large leaf surface (rather than underside) making gas exchange more efficient. Another adaptation that helps to keep lily leaves above the water where stomata can function, are the large number of spongy internal cells that promote flotation. Other adaptations plants use to reduce evaporation and infestation of insects is to use surface hairs. It is likely that in many cases, hairs interfere with the feeding of at least some small herbivores and, depending upon stiffness and irritability to the "palate", large herbivores as well. Hairs on plants growing in areas subject to frost keep the frost away from the living surface cells. In windy locations, hairs break-up the flow of air across the plant surface, reducing evaporation. Dense coatings of hairs reflect solar radiation, protecting the more delicate tissues underneath in hot, dry, open habitats. And in locations where much of the available moisture comes from cloud drip, hairs appear to enhance this process. Introduction a. Review how plants use solar energy, carbon dioxide and water to make their own food through photosynthesis, releasing oxygen as a by product. b. Identify the adaptation which allows plants to take in CO2 and expel excess O2 through the leaves (i.e. stomata). c. How do plants uptake of CO2 through leaf stomata? d. What are stomata and briefly explain how they work, and describe their roles in gas exchange with their environment (CO2 and O2 exchange, and H20 loss). e. Identify where the water that exits via the stomata originates from (i.e., uptake at the roots) and the role that stomata play in the solution to the problem of getting water from the roots up to the leaves for use in photosynthesis. f. Explain how stomata open and close. g. Make a model of stomata using 2 long balloons & adhesive tape. . 2. Clarifying a. List and explain factors that affect the photosynthetic rate of a leaf. b. Students should now be thinking about leaf design and are ready to think about making hypotheses that help to explain variations in leaf stomata density relative to environmental conditions plants are adapted to. c. Test this hypothesis with the following activity. 3. Labwork View leaf samples using the SEM simulator by going to the following website: King’s Univeristy URL to compare stomata concentration. 1. Begin viewing the leaf samples at low power (40X), manually manipulate the stage to find the first leaf sample. Adjust contrast and focus at low power before increasing magnification. 2. Increase magnification to 200X and then 400X. Save the image as a .jpg file and make a labeled drawing from this image (your drawing can be completed after both samples are imaged and the next group is using the SEM). 3. Examine the leaf sample searching for areas where there are numerous stomata, and where there is no dirt, damaged areas, or leaf veins. 4. Count all the stomata in one microscopic field. Record the number on your data table. Ivy leaf 200X magnification Spider plant leaf 150X magnification 5. Repeat counts for at least three other microscopic fields. Record all the counts. Determine an average number per microscopic field. Spider plant leaf 400X magnification Spider plant leaf 1,500X magnification 6. From the average number/400x microscopic field, calculate the stomata per mm2 by multiplying by 8. 7. Find a good example of stomata, increase magnification to 1000x (or higher). Save the image as a .jpg file and make a labeled drawing from this image (your drawing can be completed after both samples are imaged and the next group is using the SEM). 8. Using the scale bar at the bottom right side of the image, estimate the size of one stoma in micrometers (um). 9. Follow procedures 1-9 with the other leaf sample. Data Collection: Leaf 1 Leaf 2 Name of Leaf Stomata in field 1 Stomata in field 2 Stomata in field 3 Average Stomata in field Stomata/mm2 Estimated size of stoma (um) Analysis: a. Which leaf had the most stomata? Why do you think this was so? b. Was the size of the stomata the same for both leaves? c. What kind of factors might affect the size of the stomata? d. Why would plants vary stomata number more than stomata size? e. Describe the structure and function of any other distinguishing features that are found on the leaf. 4. Summary a. Was a SEM image needed to view the stomata? b. Could a magnifying glass or a laboratory microscope have been just as effective? 5. Homework a. Explain, in detail, how guard cells open and close stomata? b. At what time of day would stomata be closed and why? c. Why does the lower epidermis have more stomata than the upper epidermis of a leaf? d. Define transpiration. e. What two gases move in and out of the leaf stomata? f. What does a larger number of leaf stomata indicate about the growing climate of that plant? 6. Review a. How might stomata density serve as a bioindicator of climate change and of paleoclimates? b. In addition to preventing water loss during dry conditions, plants also respond to changes in the environment by closing stomata to protect against pollutants. With this in mind, discuss how stomata density might be affected along the urban – rural gradient. c. What are some of the specific adaptations possessed by plants that prevent small herbivores and insects from eating the leaf tissue? d. What are some of the specific adaptations that would allow some insects and parasites to gain access to leaf tissue? Research the photosynthetic and stomatal adaptations that cacti have to help them survive in desert environments.
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