Downtown Campus Biology > Biology 182 > Lesson 7 Activities > Step 4
In Class Activity - Observing Stomata |
Time: This activity will take 20 - 30 min. to complete.
Kit # 168, K (microscopes, if there isn't one already at your desk)
This lab was adapted from an activity created by Mike Tveten, biology instructor at Pima Community College's Northwest Campus. Thanks, Mike.
Introduction
You first learned about stomata in lesson 6. In this lab activity, we'll review the structure and function of stomata, and look at actual stomata under the microscope.
The surface of a leaf is covered by a transparent epidermis ("epi"="outer", "dermis" = "skin"). The epidermis has a waxy cuticle, which minimizes water loss through the leaf surface. But we know that plants do lose water because they wilt. So, where does the water leave the plant and why?
Stomata are pores in the surface of the leaf. (Stomata is plural, and a singular pore is called a stoma or stomate.) Each stoma is surrounded by two guard cells. When these two guard cells take up water, they bend and open up the pore between them. When these guard cells begin to lose water, they shrink back, and the pore closes up.
From Bio 100 textbook (author?) |
The stomata have two major functions - to permit gas exchange and transpiration
Gas exchange
Plants need to take in carbon dioxide and release oxygen as part of photosynthesis. Plants also take in oxygen and release carbon dioxide during cellular respiration. However, the epidermis and cuticle prevent the movement of in and out of the leaf. The stomata serve as channels into the leaf, allowing the gases to diffuse directly into and out of the cells from within the leaf tissue.
Transpiration Stomata also allow controlled release of water molecules into the atmosphere. Although the plant cannot afford to lose too much water to the environment, the plant must have a way to carry water and minerals from the roots, up the stem, and out to the leaves. Transpiration is the loss of water through the stomata. By allowing some water molecules to escape the leaves through the stomata, there is a negative pressure gradient created that helps draw additional water up through the plant from the roots. In addition to bringing water and minerals to the leaves, transpiration also helps keep the plant cool on hot days - a method of evaporative cooling. |
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Fig 39.10 from textPlants have a balancing act to follow - they have to let in enough carbon dioxide for photosynthesis and enough oxygen for cellular respiration, without losing too much water. So, stomata open and close in response to environmental cues. Temperature, humidity, the concentration of carbon dioxide, and even the presence or absence of light affect the turgor of the guard cells, which controls the opening and closing of the stomata.
Desert plants have a real challenge to minimize water loss, since their environment is so hot and dry. We'll learn more about desert plant adaptations in Lesson 15, but I'll mention a few plant adaptations to desert environments now.
Small leaves to minimize water loss
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Drought deciduous leaves (leaves that fall off during dry periods).
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Leaves that are modified into spines
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Wooly or hairy surface
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Now you're ready to answer questions 1 and 2 on your worksheet.
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This illustration in your textbook show the presence of stomata on the underside of the leaf, but not the top of the leaf. Is this typical? To find out we'll have to look under the microscope, since stomata are very small. However, leaves are too thick and too dense to view directly with our microscopes, especially since the light has to come up through the specimen. So instead of looking at an actual leaf under the microscope, let's look at an impression of the leaf's surface instead! |