Microbes, Plants, and You 38 40- to 2 50-minute sessions ACTIVITY OVERVIEW L A B O R AT O R Y Students observe stained Amoeba proteus cells, green plant cells, human cheek cells, and stained and unstained onion cells under a microscope. Students make observations of structures that are shared and that differ among the cells. The importance of staining for the visualization of cell structures is highlighted. KEY CONCEPTS AND PROCESS SKILLS (with correlation to NSE 5 8 Content Standards) 1. Students use appropriate tools (microscopes) to gather, analyze, and interpret data. (Inquiry: 1) 2. Living systems at all levels of organization demonstrate the complementary nature of structure and function. (Life Science: 1) 3. All living things are composed of microscopic units called cells. (Life Science: 1) 4. Cells of different organisms have similar structures, such as the cell membrane. These structures function similarly in different organisms. (Life Science: 1) KEY VOCABULARY cell cell membrane cytoplasm field of view (optional) magnify/magnification nuclear membrane nucleus organelles C-129
Activity 38 Microbes, Plants, and You MATERIALS AND ADVANCE PREPARATION For the teacher 1 Transparency 38.1, Inside Animal and Plant Cells * 1 overhead projector * water For the class * 1 onion (one piece for each group of 4) (Use a red onion if you plan to do the extension) * 1 microscope video camera (optional) For each group of four students 1 prepared slide of Amoeba proteus 1 bottle of Lugol s solution 1 prepared slide of cheek cells 1 small piece of elodea leaf, fresh spinach leaf, or similar plant leaf 1 toothpick * 1 pair of forceps (optional) * 1 cup of water dropper forceps For each pair of students 2 microscope slides 2 coverslips * 1 paper towel * 1 microscope For each student 1 Student Sheet 35.3, Microscopy Drawings 1 pair of safety goggles * compass *Not supplied in kit SAFETY NOTE Wear safety goggles while working with chemicals. Do not touch the chemicals or bring them into contact with your eyes or mouth. Wash you hands after completing the activity. C-130
Microbes, Plants, and You Activity 38 Many varieties of leaves will work for viewing cell structure, but avoid waxy leaves. Elodea leaves work especially well. Another widely available aquarium plant, Sagittaria subulata (dwarf sagittaria), or spinach are good substitutes. Either have several varieties or experiment with the type of leaf you want to use beforehand. If one is available, set up a microscope video camera to use as you review and discuss students observations. TEACHING SUMMARY Getting Started 1. Review what students have learned about cells in Activities 36 and 37. Doing the Activity 2. Students use a microscope to view onion cells, cheek cells, and microbe cells. Follow-Up 3. Discuss student observations of cells, introduce the names for key cell structures and allow students to add labels to their drawings. 4. Students answer the Analysis Questions. C-131
Activity 38 Microbes, Plants, and You TEACHING SUGGESTIONS GETTING STARTED 1. Review what students have learned about cells in Activities 36 and 37. Begin by projecting a student s timeline or Transparency 37.1, Key to the Timeline for the Germ Theory of Disease. Ask, In Activity 36, you observed some organisms in water. Which one of these scientists observations were you investigating in Activity 36? They should realize that they were having an experience similar to Leeuwenhoek s. Point out that most of the organisms students observed were single cells. Tell students that they will now have a chance to use their microscope skills to observe several types of cells and see what they have in common and how they differ. Have students do Procedure Step 1. The diagrams provide an opportunity for you to elicit your students ideas about plant and animal cells; you can build on those ideas or plan ways to correct them if necessary. For more information on identifying and addressing students ideas see Eliciting and Addressing Students Ideas in Teacher Resources II: Diverse Students. You might also want to remind students how to use the microscope and point out to them that basic instructions have been included again in the Procedure. If you have a microscope video camera at your disposal, use it to demonstrate how to locate and focus on cells. An overhead projector is useful for demonstrating the staining procedure, illustrated in Figure 1 of Activity 38, in the Student Book. DOING THE ACTIVIT Y 2. Students use a microscope to view onion cells, cheek cells, and microbe cells. In this activity, one pair of students begins by preparing and observing a slide of the onion cells, and then they view the prepared slide of the amoeba cells. The other pair prepares and observes a slide of the plant cells, and then they view the prepared slide of the human cheek cells. If you have the time, students should wash off the slides they made and prepare and observe their own slide of the other plant (onion or green plant). If you are short of time, once each pair has prepared and observed either the plant or onion slide, have them exchange the slides they made, and observe the slide prepared by the other pair. Have students use Student Sheet, 35.3, Microscopy Drawings, for recording their observations. Alternatively, you may have students make their microscopy drawings in their science notebooks. To prepare a wet mount of the onion, students must lift a layer of the onion and peel away a sample of the very thin, transparent inner epidermis that lies between layers. The sample must be placed on the slide carefully so that it remains flat and smooth, with as few wrinkles as possible. For both wet mounts, demonstrate how to lower the coverslip gradually so that air bubbles do not get trapped below it. One edge of the coverslip should touch the slide at one edge of the water drop, while the other edge of the coverslip is held up at an angle from the slide, then gradually lowered, as shown in Figure 1 of Activity 36, Looking for Signs of Micro- Life, in the Student Book. Have students stain the onion cells. Staining is best accomplished by placing a drop of Lugol s solution to one side of the coverslip and then using the edge of a paper towel to wick the stain under the coverslip toward the other side, as shown in Figure 1 in the Student Book. An alternate procedure, if you have enough slides, and coverslips, is to allow one student in each pair to prepare an unstained slide and the other student to prepare a stained slide. This facilitates direct comparison of stained and unstained versions. In this case, the student preparing the stained slide will add one drop of Lugol s solution to the drop of water containing the cells before dropping the coverslip into place. n Teacher s Note: Circulate around the room, monitoring students use of the microscopes, their ability to locate and focus on the cells, and their drawing/recording of their observations. If they are having difficulties interpreting what they see, allow some time for exploration. C-132
Microbes, Plants, and You Activity 38 Some information about each of the cells follows: The cheek cells usually have a clearly defined cell membrane and nucleus, which can be seen in stained slides. Because they are individual cells, and fairly flat, they are relatively easy to view. The onion cells are packed together and more rectangular in shape. The cell walls are visible, but the cell membranes are so close to the cell walls that they cannot easily be observed. Because the cells are in layers in the tissues, students can focus up and down to see different layers of cells. The nuclei will also go in and out of focus. You can use this observation to introduce the idea that not all cells are flat, pancake-like structures. Interpreting microscopic images requires an understanding of the three-dimensional structure of cells and tissues. The Amoeba proteus cells usually have a clearly defined nucleus. However, because of the highly irregular surface of the cell, the edge and cell membrane may not be very easy to observe. FOLLOW-UP 3. Discuss student observations of cells, introducing the names for key cell structures and allowing students to add labels to their drawings. n Teacher s Note: The terms introduced in the following discussion of cells will appear in the Student Book in Activities 40, A Cell Model, and 42, A Closer Look. Discuss student observations before they answer the Analysis Questions. Discuss each slide, encouraging students to describe some of the main features they observed in each cell type. As students describe the cells, introduce the words cell membrane and cell nucleus to describe these structures in the cells. Ask if they saw any structure separating the nucleus from the rest of the cell. This structure may be visible in the stained cheek and onion cells and is the nuclear membrane. If it is not visible, students may infer that one is likely to be present, separating the nucleus from the rest of the cell. Point out the cytoplasm all the material within the cell, but outside the nucleus. Introduce organelles, or little organs, as any tiny structures observed within the cells. Students are likely to have observed them in the cheek cells. Transparency 38.1, Inside Animal and Plant Cells, provides schematic drawings of generalized cells. Project them and introduce or review the names of these structures. Give students an opportunity to label these structures of the cells in their drawings (when they were observed in the cells). Point out that the borders of the onion cells are actually cell walls, which are not present in animal cells. These cell walls usually obscure the cell membranes of plants. 4. Students answer the Analysis Questions. Before students answer the questions, discuss the fact that each cell they observed had some structures in common with the other cells, but also had some unique structures, a unique shape, or other properties that distinguished it from the other cells. Explain that each cell has a structure that suits its function. SUGGESTED ANSWERS TO QUESTIONS 1. Compare the three kinds of cells you have just observed. a. What structures do they have in common? Explain. All of the cells observed have a cell membrane and nucleus. However, what appears to be a membrane around the onion cell is actually a cell wall. The cell membrane is inside this wall, but is probably not visible. The cheek and amoeba cells have cytoplasm that stains. You may wish to tell students that there are cells that lack nuclei, but all cells have a membrane and some type of contents inside. C-133
Activity 38 Microbes, Plants, and You b. How are the cells different? Explain. The cells have different shapes. In addition, the cheek cell appears to have tiny structures in the cytoplasm. The onion cell looks empty. (This is due to a large water vacuole that pushes the cytoplasm to the edge of the cell.) 2. Look at the Venn diagram you drew at the beginning of the activity. Make any additions or changes now that you have observed plant and animal cells with a microscope. Plant cell Animal cell 5. Do you think there are any small structures (organelles) inside your cheek cell other than the nucleus and cytoplasm? What evidence do you have to support your answer? Tiny structures in the cytoplasm that look like dots or very small bubbles ( cytoplasmic granules ) are commonly observed, especially in cheek cells. Students might also argue that the cell must have other subcellular structures, due to the cell s many functions; however, this reasoning is not yet supported by observational evidence. 6. Do you think there are any small structures (organelles) inside the plant cells, either the onion or Square shape Some are green because of chloroplasts Will not move Has membrane Has a nucleus Both very small Irregular shape Cytoplasm shows stain May move the green plant cell, other than the nucleus? What evidence do you have to support your answer? The green plant cell has tiny green structures that appear to be contained and are likely to be organelles. 3. Did you find evidence in this activity that the human body is made up of cells? Explain. Based on the observations of cheek cells, there is evidence that the human body is made of cells. 4. You stained the onion cell. How did the cell look before and after staining? Explain the purpose of the stain. The cell looked almost empty before staining. The stain made it much easier to see the nucleus. C-134
Inside Animal and Plant Cells cell wall cell membrane mitochondria cytoplasm nucleus vacuole chloroplasts PLANT CELL cell membrane cytoplasm nucleus nuclear membrane 2009 The Regents of the University of California mitochondria ANIMAL CELL Issues and Life Science Transparency 38.1 C-135