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42 40 o5 1 0 -m i n u t e s es si G -t on A Closer Look REA DI N ACTIVIT Y OVERVIEW SUMMARY A reading elaborates upon the basic structures common to all cells. The roles of the cell membrane, cytoplasm, and nucleus are emphasized. The relationship between cell biology and disease is presented. KEY CONCEPTS AND PROCESS SKILLS 1. All living things are composed of microscopic units called cells. 2. Cells of different organisms have similar structures, such as the cell membrane. These structures function similarly in different organisms. 3. The function of the cell membrane is to control what can enter or leave the cell. Cell membranes are selectively permeable; some particles pass through but not others. 4. The cell membrane compartmentalizes the cytoplasm from its external environment; the nuclear membrane protects the genetic material in the nucleus. Key Vocabulary cell cytoplasm cell membrane nucleus cross-section organelles Teacher’s Guide C-173 Activity 42 • A Closer Look MATERIALS For the teacher * 1 overhead projector For the Extension For the teacher 2 green peppers or apples different colored clays or soft, chewable candies plastic knives or dental floss * 1 Transparency 42.1, “Same Cell—Different Slices” 1 overhead projector *Not supplied in kit TEACHING SUMMARY Getting Started 1. Review what students have learned so far about cells from their observations and from the cell membrane modeling activity. Doing the Activity 2. Students complete the reading. Follow-Up 3. Discuss the similarities and differences among cells. Extension Students model a cross-section of a cell. BACKGROUND INFORMATION Common Cell Structures Some subcellular structures, such as the cell membrane and the genetic material, are common to all cells due to their indispensable roles. The genetic material is on a chromosome (introduced in the “Our Genes, Our Selves” unit of Science and Life Issues) that is free in a bacterial cell; the chromosomes are enclosed in a nucleus in animals, plants, fungi, and protists. Organisms that have nucleated cells are referred to as eukaryotes, while organisms that lack nuclei are referred to as prokaryotes. Certain other organelles that characterize eukaryotic cells—such as mitochondria, chloroplasts, lysosomes, cilia—are not present in all cells, or are found in dramatically varying numbers, because of the diverse requirements of different cell types. C-174 Science and Life Issues A Closer Look • Activity 42 TEACHING SUGGESTIONS GETTING STARTED 1. Follow up the role of membranes by discussing: Why is the nuclear membrane so important to the nucleus? Membranes within the cell create compartments where specialized activities can occur. Review what students have learned so far The nuclear membrane provides one example, as it about cells from their observations and isolates the genetic material on the chromosomes from the cell membrane modeling activity. from the rest of the cell. In fact, several other major Prepare students for the reading by reviewing concepts introduced in the last few activities. Questions that may provide some review include: What organelles, such as mitochondria, chloroplasts, lysosomes, and Golgi apparatus, are also membrane-bound. organisms are made of cells? All organisms, such as FOLLOW-UP plants, animals, and microbes, are made of cells. What kinds of things do all cells do? How did we investigate this question? In Activity 39, “Cells Alive!” students investigated the ability of cells to respire, demonstrating that cells perform life functions. What does the cell membrane do? All cells have a cell membrane that acts as a barrier, allowing only some substances to enter and leave the cell. DOING THE ACTIVIT Y 2. Students complete the reading. n Teacher’s Note:You may choose to have students complete the reading either as homework or as an 3. Discuss the similarities and differences among cells. Use the reading as the basis for a discussion of the unity and variety of cells. All human cells contain certain common elements, in particular the cell membrane, nucleus, nuclear membrane, and cytoplasm. (One exception is the mature red blood cell in mammals, which lacks a nucleus.) Differences in internal structure and the diversity of cell roles within the human body will be further explored in Activity 46, “Disease Fighters,” when students examine various blood cells. in-class exercise. Develop the idea that different Also introduce the idea that humans and many other cells contain many structures that help them func- multicellular organisms are made of many kinds of tion. The role of specific structures (such as chloro- cells. These different cells have different structures plasts and cell walls in plant cells) within different and perform different functions for the body. groups of organisms is addressed in Unit E, “Ecology,” of Science and Life Issues. A number of websites provide beautiful photos, short movies, and information on cells. The SALI page of the SEPUP website provides links to several of the many sites available. Also discuss the use of electron microscopes to obtain much greater magnifications of cells. In order to view the insides of cells by transmission electron microscopy, very thin sections must be obtained from specimens embedded in plastic and treated with special stains. Transparency 42.1, “Same Cell—Different Slices,” provides a view of a Teacher’s Guide C-175 Activity 42 • A Closer Look generic protist that has a flagellum, but is not pho- intact cells or to view internal structures of broken tosynthetic. Point out that the nucleus is observed cell preparations. only in those sections of the cell that actually pass through the nucleus. You can extend the discussion to consider different shapes and sizes of nuclei in different shapes of cells. Draw students’ attention to the section of the flagellum. Point out that even this tiny structure has substructure visible at high magnification. The tubular structures visible in the cross-section of the flagellum perform a sliding action that is responsible for the motility of the fla- You can demonstrate this idea by first making crosswise and lengthwise slices of two apples or green peppers. Students can see that the structure of the apple or pepper changes depending on whether it is cut through the center or the end. Similarly, when they focus up and down with the light microscope, or when a thin section is made, some structures may not be represented. gellum. While it is not necessary that students Students can make models of cells by using clay or remember this detail, you can use it to make the soft chewable candies of different colors. They point that cells have many structures that carry out should take a pea-sized bit of clay and form a round cell activities. Scanning electron microscopy, as or ellipsoid nucleus. Then they can wrap a larger used to obtain some of the images provided in Fig- piece of clay around the nucleus. Since cells come in ure 2 on page C-62 in the Student Book, provides a many shapes, have different students model differ- view of the surface of the cell. ent shapes: spherical, torpedo-shaped, star-shaped (like an amoeba), cuboidal, or flat pancakes. Stu- Extension dents can then use plastic knives or dental floss to Students model a cross-section of a cell. Many students have difficulty understanding the 3dimensional nature of cells and interpreting what they see through a light microscope or in an elec- slice through their model cells in different locations. Have them note differences between the different slices, in terms of overall shape and whether the nucleus is visible. tron micrograph. Single cells, such as protists or As students explore their models, you may want to human cheek cells, can often be observed through discuss the following question with the class: Some a light microscope as unstained or stained whole microscope slides contain cross–sections of speci- mounts, and need not be cut into thin sections. mens. However, most microbes do not need to be However, for light microscopy of tissues, the cells cross–sectioned to be seen through a classroom must often be embedded in a solid material, such as microscope. What happens to a microbe when it is wax or plastic, and sliced into sections for viewing. pressed between the slide and the coverslip? How The cells shown in the Introduction to the activity would this affect what you see through the micro- on page C-58 in the Student Book were prepared in scope? Use this question to foreshadow the pre- this way. For transmission electron microscopy, pared slides students will observe in Activity 43, very thin sections must be prepared, even when sin- “Microbes Under View.” Since the microbes in that gle cells are to be investigated. Scanning electron activity are so small, many of these single–celled microscopy can be used to view the surfaces of organisms fit onto a single slide. However, not every C-176 Science and Life Issues A Closer Look • Activity 42 specimen looks identical (particularly the Amoeba) b. These flat cells form an even covering on the sur- because of the various angles at which the organ- face of areas like the inside of the mouth. isms have been preserved. You can demonstrate this Cell 3. idea by making additional clay cells in different shapes and then pressing them flat. Point out that c. These round human cells are unusual because the angle at which the cell is preserved will change they do not have a nucleus. They are full of a pro- its appearance on the slide. tein that carries oxygen to all parts of the body. Cell 4. Conclude by emphasizing the point that interpretation of microscopic images of cells requires an d. These cells are able to crawl around the body to understanding of the effect of viewing 3-dimen- attack bacteria and other foreign material. Ruf- sional structures as if they were flat and, in some fles on the cell membrane lead the way as the cases, of viewing only a thin slice of a cell. When cells move. scientists view cells under the microscope, they Cell 1. must consider how the cells were prepared to make the slide when they make conclusions about what they see. 2. Based on its description, which of the four cells described in Question 1 is a nerve cell? Which is a red blood cell? Which is a white blood SUGGESTED ANSWERS TO ANALYSIS QUESTIONS 1. cell? Which is a skin cell? Explain how you were able to match the type of cell with its function. Observe the pictures of cells in Figure 2, Cell 2 is a nerve cell. Signals travel down the “Animal Cells.” These photos of four different ani- long cell process (axon), causing a chemical sig- mal cells were taken with an electron microscope. nal (neurotransmitter) to be released at the end Cells 1, 2, and 4 were taken with a scanning elec- of the cell (synapse). This particular neuron is tron microscope which shows the surface (and not called a neurosecretory cell, because it releases the inside) of the cell. This type of microscope mag- its signaling molecules, called hormones, into nifies the cells much more than the microscopes you the bloodstream. use in class. You can see that the cells have quite different shapes: some are rounded, while others are elongated, flat, or ruffled. These shapes depend on the cells’ functions in the body. Try to match each cell with one of the following descriptions. a. These cells have long branching parts that send signals to distant parts of the body. Cell 2. Cell 4 is a red blood cell. The shape of the cell helps it to squeeze through the tiny capillaries of the circulatory system. (It also increases the surface area available for the diffusion of oxygen.) Hemoglobin, the protein that carries oxygen, contains iron and is responsible for the red color of oxygenated blood. Students will learn about the function of red blood cells in Activity 50, “Fighting Back.” Teacher’s Guide C-177 Activity 42 • A Closer Look Cell 1 is a white blood cell. The cell is moving tance of cellular neurobiology to the study of to the right. The membrane forms ruffles, Alzheimer’s disease, and the unregulated cell which surge ahead in the direction in which the growth characteristic of cancer. cell is moving as the cell probes the surrounding blood plasma for foreign invaders. Students 4. will learn about the functions of white blood The cell membrane separates the cell from its cells in Activity 50, “Fighting Back.” surroundings. Everything that enters or leaves the cell must cross the membrane. In addition, Cell 3 is a skin cell. The technical term for cells the cell membrane mediates communication that line bodily surfaces is “epithelial.” Their between cells. flattened shape is a clear functional adaptation. 3. Explain why membranes are so important to cells. Give one example of how the study of cells helps 5. “Looking for Signs of Micro-Life.” Did you observe treat diseases. any structures within the microbes that you drew? Student answers will vary, but are likely to cen- What do you think these structures are? ter around the examples given in the reading. While student answers will vary, it is possible For example, learning to influence the forma- that students observed the nucleus in some of tion of red blood cells reduces the need for the microbes. blood transfusions, and the study of the normal functioning of the immune system assists in the fight against AIDS. Other possible answers include the role of developmental biology in preventing avoidable birth defects, the impor- C-178 Science and Life Issues Look back at your drawings from Activity 36, 6. Reflection: Which of the questions studied by cell biologists is most interesting to you? Why? Student answers will reflect their own areas of interest. ©2006 The Regents of the University of California Same Cell—Different Slices Science and Life Issues Transparency 42.1 C-179