Investigating Stomata Learning Objectives: To be able to identify stomata and compare stomata on leaves of a plant To understand the function of stomata and the role they play in a plant To understand what causes stomata to open and close Learn how mathematical models can be used to simplify a biological process (Ball Berry model). Picture from www.biologyjunction.com Background: Stomata are microscopic pores in the epidermis of the leaves and usually are found on the bottom of leaves. Stomata are comprised of two guard cells that open and close to allow air to pass into and out of the leaf. Plants need carbon dioxide (CO2) to fix sugars to generate biomass through photosynthesis. CO2 enters the leaves through the stomata. When stomata are open, CO2 is able to flow into the cells. But why would stomata need to close? Plants need water and when stomata are open, water transpires out of the plant. On average, plants lose 10-30 molecules of water for every CO2 absorbed and can be high as 400 molecules of water lost for every CO2 fixed. The humidity inside a leaf is about 100% but outside the leaf, the humidity is less. Because of this gradient, or difference, water will tend to escape the leaf to the atmosphere where it is drier. This is called osmosis. Thus, by closing stomata, the plant can preserve water. Picture from https://evolution.berkeley.edu
In plant science, mathematical models can be used to simulate a biological process. Using mathematical models helps scientists simplify and understand the basics behind a biological mechanism. For example, there are many different factors that affect the opening and closing of stomata. The Ball Berry model is a simple model that explains what factors cause stomata to open and to close. It also measures the rate of stomata opening and closing, or stomatal conductance. The Ball Berry model is: (A RH) g s = g 0 + m [CO 2 ] where, g s is stomatal conductance g 0 is the minimum stomatal conductance m is the slope RH is the relative humidity A is the rate of photosynthesis [CO 2 ] is the concentration of CO 2 at the surface of the leaf This model gives insight of what environmental factors cause stomata to open and close. The pictures below show open and closed stomata on a tobacco leaf. The picture on the left shows closed stomata on a tobacco leaf and the picture on the right shows open stomata on a tobacco leaf. Outlined in blue are the stomata and outlined in red are the epidermal cells. The epidermal cells are equivalent to skin cells. One element that can change stomatal conductance is the CO2 concentration around the leaf. If CO2 concentration increases, stomatal conductance decreases, or stomata are closed more. On the contrary, if CO2 concentration decreases, then stomata are open more often or stomatal conductance increases.
Another component that changes stomatal conductance is humidity. If the humidity is high, then stomata stay open and stomatal conductance will increase. If the humidity is low, stomata will shut and stomatal conductance will decrease. Additionally, the rate photosynthesis can alter stomatal conductance. If photosynthesis decreases, then stomatal conductance decreases. However, if photosynthesis increases, then stomatal conductance increases. Finally, other environmental factors that alter stomatal conductance are light, wind, and the type of plant. The guard cells that surround each stoma regulate water loss and CO2 gain. The physical opening and closing of stomata is accomplished through osmosis. Notice in figure A below, that when the stoma is open, the guard cells are large and turgid or swollen. This occurs from an accumulation of potassium ions (K+) in the guard cells that causes water to flow into the guard cells. Water moves into the cells because of a difference of water potential. An increase of solute concentration will lower the water potential and causes water to move from a region with higher water potential to a region of lower water potential. In figure B, the potassium ions have moved into the epidermal cells, causing water to move out of the guard cells. The guard cells become flaccid and the stoma closes. leaf through the stomata A Potassium ions B Potassium ions Turgid guard cell, filled with water Stoma open Epidermal cell Flaccid guard cell Stoma closed Epidermal cell This process is extremely important for plants to monitor water loss as well as CO2 gain. The atmosphere contains about 3,100 mi 3 (3.41 x 10 15 gallons) of water vapor. Each year, two times the total amount of the water in the atmosphere passes through stomata. This provides an idea of how important plants and stomata are on a global scale. In Activity A, we are going to identify stomata and we are going to understand what makes stomata open and close. Activity A: Lettuce look at stomata! Materials: Lettuce leaves (not iceberg), forceps, tape, microscope slide, cover glasses, small pipette, water Procedure: 1. Soak a lettuce leaf in water 2. Cut a small part off the lettuce leaf make sure section is flat
3. Make a wet mount and use the forceps to place the bottom of the leaf on the slide 4. Putt the slide under the microscope 5. Locate the stomata and epidermal cells 6. Count how many stomata you can see 7. Try to identify open and closed stomata 8. Flip leaf over on the slide 9. Try to locate stomata and epidermal cells Discussion questions: 1) Did you see any stomata on the top of the leaf? Why or why not? 2) What do you think would happen if we dropped salt water on the leaf? What would the stomata look like and why? 3) What is the role of stomata for plants? 4) Why are stomata important? Glossary: Biomass total mass of a plant Epidermis single, outer layer of cells that cover leaves, flowers, roots, and stems of plants. It is the boundary between the plant and the environment Flaccid droopy through a lack of water Gradient a difference between the concentration of solutes between two regions Osmosis process which molecules, or solutes, tend to pass through a membrane from a less concentrated solution into a more concentration solution
Photosynthesis process which green plants use sunlight to fix sugars from carbon dioxide Transpiration (transpire) process where plats absorb water through roots and then water vapor is released through stomata in leaves Turgid swollen, inflated, filled with water Solute is a substance that dissolved into a solution Stoma (stomata) a small pore in the epidermis of leaves or stems of plants. It forms a slit that allows movements of gasses in and out of the leaf Stomatal conductance rate of carbon dioxide entering the leaf through the stomata or the rate of water vapor exiting the leaf through the stomata Water potential caused by osmosis, it is the tendency of water to move from one area to anther because of solute concentration