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Transcript
7– 2 Eukaryotic Cell Structure Section 7–2 1 FOCUS A t first glance, a factory is a puzzling place. A bewildering variety of machines buzz and clatter, people move quickly in different directions, and the sheer diversity of so much activity can be confusing. However, if you take your time and watch carefully, before long you will begin to identify patterns. What might at first have seemed like chaos begins to make sense. Objectives 7.2.1 Describe the function of the cell nucleus. 7.2.2 Describe the functions of the major cell organelles. 7.2.3 Identify the main roles of the cytoskeleton. Key Concept • What are the functions of the major cell structures? Vocabulary organelle cytoplasm nuclear envelope chromatin chromosome nucleolus ribosome endoplasmic reticulum Golgi apparatus lysosome vacuole mitochondrion chloroplast cytoskeleton centriole Vocabulary Preview Pronounce each Vocabulary word and have students repeat the pronunciation as a class. Pay special attention to words that are difficult for English language learners. Reading Strategy Comparing the Cell to a Factory In some respects, the eukaryotic cell is like a factory. The first time you look at a microscope image of a cell, such as the one in Figure 7–5, the cell seems impossibly complex. Look closely at a eukaryotic cell, however, and patterns begin to emerge. To see those patterns more clearly, we’ll look at some structures that are common to eukaryotic cells, shown in Figure 7– 6. Because many of these structures act as if they are specialized organs, these structures are known as organelles, literally “little organs.” Cell biologists divide the eukaryotic cell into two major parts: the nucleus and the cytoplasm. The cytoplasm is the portion of the cell outside the nucleus. As you will see, the nucleus and cytoplasm work together in the business of life. Reading Strategy: Building Vocabulary To help students begin their understanding of the differences between plant cells and animal cells, have them preview Figure 7–6 and answer the caption question. Before you read, preview the vocabulary by skimming the section and making a list of the highlighted boldface terms. Leave space to make notes as you read. 2 INSTRUCT Comparing the Cell to a Factory Build Science Skills Using Models Divide the class into small groups, and have groups make a labeled, two-dimensional drawing of a typical cell. First, have groups meet before reading the section to discuss what the inside of a cell might contain. Then, ask groups to meet again after learning about the structures of a cell to make the labeled drawing. 왘 Figure 7– 5 This electron micrograph of a plant cell shows many of the different types of structures that are found in eukaryotic cells. The cell has been artificially colored so that you can distinguish one structure from another. (magnification: 1500⫻) SECTION RESOURCES • iText, Section 7–2 • Transparencies Plus, Section 7–2 Tim Technology: • Teaching Resources, Section Review 7–2 • Reading and Study Workbook A, Save Section 7–2 e • Adapted Reading and Study Workbook B, Section 7–2 • Lesson Plans, Section 7–2 174 Chapter 7 r Print: PLANT AND ANIMAL CELLS Figure 7–6 Both plant and animal cells contain a variety of organelles. Some structures are specific to either plant cells or animal cells only. Interpreting Graphics What structures do plant cells have that animal cells do not? For: Cell Structure activity Visit: PHSchool.com Web Code: cbp-3072 Plant Cell Nucleus Nucleolus Nuclear envelope Ribosome (free) Smooth endoplasmic reticulum Rough endoplasmic reticulum Ribosome (attached) Cell wall Golgi apparatus Cell membrane Chloroplast Mitochondrion Vacuole Smooth endoplasmic reticulum Nucleolus Nuclear envelope Build Science Skills Predicting Ask students what specific functions a unicellular organism would need to carry out in order to live. Then, divide the class into small groups, and ask each group to make a table of predictions about what structures would likely be found inside a unicellular organism. The table should have two columns: Necessary Function and Structure Needed to Carry Out Function. Use Visuals Animal Cell Nucleus For: Cell Structure activity Visit: PHSchool.com Web Code: cbe-3072 Students can interact with the cell art online. Ribosome (free) Cell membrane Figure 7– 6 Encourage students to make copies of these labeled illustrations in their notebooks. As they learn about the various structures that make up a cell, they can add definitions and descriptions of functions for each of the labels. Point out that when they have completed this task, they will have made the best possible tool for review. Build Science Skills Rough endoplasmic reticulum Ribosome (attached) Centrioles Golgi apparatus Comparing and Contrasting Set up microscope stations at several locations around the room, and provide prepared slides of an animal cell and a plant cell at each location. Have students make labeled drawings of each and write a paragraph comparing and contrasting the two types of cells. Mitochondrion Less Proficient Readers To reinforce students’ understanding of cell structures, draw an animal cell on the board. Include and label the nucleus, the cell membrane, and the cytoplasm. Have students make a copy of the drawing on a sheet of paper. Then, as each organelle is studied and discussed, add labeled structures to the cell on the board, and have students add these structures to their own drawings. English Language Learners When students read about the cytoskeleton on page 181, explain that cyto- means “cell,” and thus cytoskeleton can be thought of as the “skeleton of the cell.” Explain that this is an analogy, since the cytoskeleton is not like an animal skeleton. Also, explain that a filament is a threadlike material and a tubule is a “very slender tube.” Thus, the cytoskeleton can be thought of as composed of threads and slender tubes. Answer to . . . Figure 7– 6 Plant cells have a cell wall and chloroplasts. Many plant cells also have a large, central vacuole. Cell Structure and Function 175 7–2 (continued) FIGURE 7–7 THE NUCLEUS Nucleus The nucleus controls most cell processes and contains the hereditary information of DNA. The DNA combines with protein to form chromatin, which is found throughout the nucleus. The small, dense region in the nucleus is the nucleolus. Use Visuals Figure 7–7 Ask students: What is the nucleolus? (It is a small, dense region of the nucleus where the assembly of ribosomes begins.) Where is the DNA that a nucleus contains? (The DNA is part of the chromatin, which is spread throughout the nucleus most of the time.) Why is DNA important? (It holds coded instructions for making proteins and other important molecules.) Point out that the genetic information is the coded instructions for making molecules. Build Science Skills Nucleolus Nuclear envelope Chromatin Inferring Remind students that prokaryotes do not contain a nucleus. Then, ask: If the nucleus controls most cell processes in eukaryotes, how can prokaryotes live without a nucleus? (Some students might suggest that the lives of prokaryotes aren’t as complex as those of eukaryotes. Others might correctly infer that the most important part of a nucleus is the DNA it contains, and prokaryotes have DNA without having a nucleus.) Nuclear pores Nucleus In the same way that the main office controls a large factory, the nucleus is the control center of the cell. The nucleus contains nearly all the cell’s DNA and with it the coded instructions for making proteins and other important molecules. The structure of the nucleus is shown in Figure 7–7. The nucleus is surrounded by a nuclear envelope composed of two membranes. The nuclear envelope is dotted with thousands of nuclear pores, which allow material to move into and out of the nucleus. Like messages, instructions, and blueprints moving in and out of a main office, a steady stream of proteins, RNA, and other molecules move through the nuclear pores to and from the rest of the cell. The granular material you can see in the nucleus is called chromatin. Chromatin consists of DNA bound to protein. Most of the time, chromatin is spread throughout the nucleus. When a cell divides, however, chromatin condenses to form chromosomes (KROH-muh-sohms). These distinct, threadlike structures contain the genetic information that is passed from one generation of cells to the next. You will learn more about chromosomes in later chapters. Most nuclei also contain a small, dense region known as the nucleolus (noo-KLEE-uh-lus). The nucleolus is where the assembly of ribosomes begins. What kind of information is contained in chromosomes? HISTORY OF SCIENCE The nucleus controls the cell During the 1930s and 1940s, researchers performed a series of experiments that demonstrated the link between a cell’s nucleus and the physical characteristics of the cell. Two species of Acetabularia algae were used in the experiments. This marine alga, though 5 cm long, consists of a single cell. Each cell includes a holdfast at the bottom, a stalk, and a cuplike cap at the top, and the cell’s nucleus is in the 176 Chapter 7 holdfast. The two species that were used had different-shaped caps. Researchers cut the cap off one cell, removed the nucleus from its holdfast, and transplanted a nucleus from a cell of the second species into the holdfast of the first cell. The cell regenerated a new cap, and researchers cut off that one. Eventually, the cap that grew was the shape of the cap from the second species, from which the transplanted nucleus came, and not the shape of the first cap. Ribosomes FIGURE 7–8 ENDOPLASMIC RETICULUM Build Science Skills The endoplasmic reticulum synthesizes proteins for export from the cell. The rough endoplasmic reticulum, shown here, gets its name from the “rough” appearance of the ribosomes on its surface. Ribosomes (magnification: 160,000⫻) Ribosomes Endoplasmic reticulum (magnification: about 40,000⫻) Using Analogies Read aloud the sentence in the text that compares a ribosome to a machine. Use this comparison to discuss how a eukaryotic cell is like a factory. Then, encourage students who need an extra challenge to work together in writing a short play based on the analogy of the cell as a factory. Explain that a good play needs some conflict or danger. The “factory” might be under economic threat or some environmental threat. Advise students to include the functions of as many parts of the factory—cell organelles—as possible. Once the play has been written, encourage the “playwrights” to recruit class members to act out the drama. Endoplasmic Reticulum Ribosomes One of the most important jobs carried out in the cellular “factory” is making proteins. Proteins are assembled on ribosomes. Ribosomes are small particles of RNA and protein found throughout the cytoplasm. They produce proteins by following coded instructions that come from the nucleus. Each ribosome, in its own way, is like a small machine in a factory, turning out proteins on orders that come from its “boss”—the cell nucleus. Cells that are active in protein synthesis are often packed with ribosomes. Endoplasmic Reticulum Eukaryotic cells also contain an internal membrane system known as the endoplasmic reticulum (en-doh-PLAZ-mik rih-TIK-yuh-lum), or ER. The endoplasmic reticulum is the site where lipid components of the cell membrane are assembled, along with proteins and other materials that are exported from the cell. The portion of the ER involved in the synthesis of proteins is called rough endoplasmic reticulum, or rough ER. It is given this name because of the ribosomes found on its surface. Newly made proteins leave these ribosomes and are inserted into the rough ER, where they may be chemically modified. Use Visuals Figure 7–8 Ask students: What are ribosomes composed of? (RNA and protein) Where are ribosomes produced? (In the nucleolus) What do ribosomes produce? (Proteins) What happens to these proteins after they’re produced by ribosomes? (Membrane proteins are inserted directly into the ER membrane. Many of the proteins produced on the rough ER are released or secreted from the cell.) If this were an illustration of smooth ER, how would it be different? (The ER would not have ribosomes on its surface.) What is the function of smooth ER? (The smooth ER contains enzymes that help synthesize lipids, such as steroids. Smooth ER also helps to detoxify and process chemicals.) HISTORY OF SCIENCE Learning from sea urchin nuclei The German cytologist Theodor Boveri (1862–1915) performed an experiment before the invention of microdissection that demonstrated the importance of the nucleus. By vigorous shaking, Boveri removed the nuclei from the eggs of sea urchins of the genus Sphaerechinus. He then fertilized the eggs (which had no nuclei) with sperm from sea urchins of the genus Echinus. In a practical sense, fertilization resulted in the substitution of one nucleus for another. The larvae that developed had only the traits of Echinus, even though the sperm contributed little more than a tiny bit of nucleus to the developing organism. Answer to . . . Chromosomes contain the genetic information that is passed from one generation to the next. Cell Structure and Function 177 FIGURE 7–9 GOLGI APPARATUS 7–2 (continued) Golgi Apparatus The Golgi apparatus modifies, sorts, and packages proteins. Notice the stacklike membranes that make up the Golgi apparatus in this transmission electron micrograph. Build Science Skills Comparing and Contrasting Students often confuse the Golgi apparatus with the endoplasmic reticulum, because both are usually represented as folded membranes within the cytoplasm. Have students compare the illustrations in Figure 7–8 with those in Figure 7–9. Then, call on students at random to explain the differences in functions between ER and the Golgi apparatus. Lysosomes (magnification: about 45,700⫻) Build Science Skills Observing Divide the class into small groups, and give each group access to a paramecium culture and a yeast suspension, as well as to a microscope slide, coverslip, toothpick, dropper pipette, and microscope. (Prepare the yeast suspension by adding a pinch of Congo red indicator to a thick mixture of yeast and water. Then, bring it to a gentle boil for 5 minutes. Cool before using. Transfer some paramecium culture from the stock culture at least a day ahead of time, and then limit the food supply to the transferred culture.) Have each group prepare a slide of live paramecia using the dropper pipette. Students should focus the slide under the low-power objective of the microscope. They should then obtain a small sample of the yeast solution. The indicator in the solution is red above pH 5 and blue below pH 3. The next step is to use a toothpick to transfer a small drop of yeast suspension to the edge of the slide and observe the paramecia under the microscope for 5 minutes. (Students should observe that the paramecia sweep the yeast through their oral grooves and form vacuoles to enclose it. The vacuoles become blue at first and eventually red, as lysosomes fuse with the vacuole and release acids that digest the yeast.) 178 Chapter 7 Proteins that are released, or exported, from the cell are synthesized on the rough ER, as are many membrane proteins. Rough ER is abundant in cells that produce large amounts of protein for export. Other cellular proteins are made on “free” ribosomes, which are not attached to membranes. The other portion of the ER is known as smooth endoplasmic reticulum (smooth ER) because ribosomes are not found on its surface. In many cells, the smooth ER contains collections of enzymes that perform specialized tasks, including the synthesis of membrane lipids and the detoxification of drugs. Liver cells, which play a key role in detoxifying drugs, often contain large amounts of smooth ER. Golgi Apparatus Proteins produced in the rough ER move next into an organelle called the Golgi apparatus, discovered by the Italian scientist Camillo Golgi. As you can see in Figure 7–9, Golgi appears as a stack of closely apposed membranes. The function of the Golgi apparatus is to modify, sort, and package proteins and other materials from the endoplasmic reticulum for storage in the cell or secretion outside the cell. The Golgi apparatus is somewhat like a customization shop, where the finishing touches are put on proteins before they are ready to leave the “factory.” From the Golgi apparatus, proteins are then “shipped” to their final destinations throughout the cell or outside of the cell. FACTS AND FIGURES Important products of the Golgi apparatus One of the most important cell components packaged and distributed by the Golgi apparatus is material for the membranes of the cell and its organelles. Lysosomes, which are essentially membranous bags filled with enzymes, are products of the Golgi apparatuses. These enzymes would destroy the cell if they were not surrounded by membrane. An example of how lysosomes function in cells can be seen in the way paramecia digest their food. Upon contact with a food organism or some other particle, the paramecium envelops the food in a vacuole. Lysosomes then fuse with the vacuole and release acids. The acids quickly digest the contents of the vacuole. Lysosomes Even the neatest, cleanest factory needs a cleanup crew, and that’s what lysosomes (LY-suh-sohmz) are. Lysosomes are small organelles filled with enzymes. One function of lysosomes is the digestion, or breakdown, of lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell. Lysosomes are also involved in breaking down organelles that have outlived their usefulness. Lysosomes perform the vital function of removing “junk” that might otherwise accumulate and clutter up the cell. A number of serious human diseases, including Tay-Sachs disease, can be traced to lysosomes that fail to function properly. Figure 7–10 Vacuoles have a variety of functions. In the Coleus plant cell (top), the large blue structure is the central vacuole that stores salts, proteins, and carbohydrates. The paramecium (bottom) contains contractile vacuoles that fill with water and then pump the water out of the cell. Applying Concepts How do vacuoles help support plant structures? Vacuole What is the role of lysosomes? Vacuoles Every factory needs a place to store things, and cells contain places for storage as well. Some kinds of cells contain saclike structures called vacuoles (VAK-yoo-ohlz) that store materials such as water, salts, proteins, and carbohydrates. In many plant cells there is a single, large central vacuole filled with liquid. The pressure of the central vacuole in these cells makes it possible for plants to support heavy structures such as leaves and flowers. Vacuoles are also found in some single-celled organisms and in some animals. The paramecium in Figure 7–10 contains a vacuole called a contractile vacuole. By contracting rhythmically, this specialized vacuole pumps excess water out of the cell. The control of water content within the cell is just one example of an important process known as homeostasis. Homeostasis is the maintenance of a controlled internal environment. Mitochondria and Chloroplasts All living things require a source of energy. Factories are hooked up to the local power company, but what about cells? Most cells get energy in one of two ways—from food molecules or from the sun. Vacuoles Use Visuals Figure 7–10 After students have studied the figure and read the caption, explain that the Coleus cell is from a multicellular, leafy plant, whereas the paramecium is a microscopic unicellular organism that is part of the kingdom Protista, which students will learn about in Chapter 20. Then, ask: How is the function of a vacuole in a plant cell different from that in a unicellular organism? (A vacuole in a plant cell stores materials such as water, salts, proteins, and carbohydrates. It also helps support plant structures. A vacuole in a unicellular organism is specialized to pump water out of the cell.) Mitochondria and Chloroplasts (magnification: about 3000⫻) Contractile vacuole Mitochondria Nearly all eukaryotic cells, including plants, contain mitochondria (myt-oh-KAHN-dree-uh; singular: mitochondrion). Mitochondria are organelles that convert the chemical energy stored in food into compounds that are more convenient for the cell to use. Mitochondria are enclosed by two membranes—an outer membrane and an inner membrane. The inner membrane is folded up inside the organelle. One of the most interesting aspects of mitochondria is the way in which they are inherited. In humans, all or nearly all of our mitochondria come from the cytoplasm of the ovum, or egg cell. This means that when your relatives are discussing which side of the family should take credit for your best characteristics, you can tell them that you got your mitchondria from Mom! Build Science Skills Using Analogies Explain to students that mitochondria have long been called the “powerhouses” of cells. Ask: What is a “powerhouse”? (A powerhouse is another name for a power plant, which produces electricity for cities and regions.) How is a mitochondrion like a powerhouse? Powerhouses convert one source of energy to another more useful form. For example, energy from coal, oil, or gas is often converted to electricity, a more useful form for homes and industry. A mitochondrion also converts energy to a more useful form. It uses energy from food to make high-energy compounds that the cell can use in growth, development, and movement. TEACHER TO TEACHER When I introduce the structure of the cell, I try to analogize the cell with the students’ city. Taking this analogy a step further, I organize a cooperative learning activity in which I ask teams of students to “create” an imaginary city that correlates cell structures with city components. Teams should include most of the organelles in their city. For example, they might use a mitochondrion as the local power plant or microtubules as major thoroughfares. For this activity, each team will need a large sheet of paper or poster board for drawing the city, as well as colored pencils or similar materials. If possible, have teams display their work and explain their “creations” to the class. —Jorge E. Sanchez Biology Teacher Green Valley High School Henderson, NV Answers to . . . Lysosomes break down lipids, carbohydrates, and proteins. They also break down organelles that have outlived their usefulness in the cell. Figure 7–10 The pressure exerted by the liquid in the vacuole makes it possible for plants to support heavy structures. Cell Structure and Function 179 7–2 (continued) Objective Students will make models of cell organelles and a large class model of a cell. Skill Focus Using models Materials craft supplies, index cards Time 20 minutes Advance Prep Collect a variety of craft supplies, including scissors, construction paper, cardboard tubes, plastic bags, yarn, glue, and beads. Safety Caution students about the use of pins and about standing on chairs as they hang up their models. Supervise them as they do so. Strategies • You may want students to build a model of a different kind of cell. A model of a plant cell is suggested because plant cells have a great variety of structures and organelles. • Make sure at least one group is working on these major structures: cell wall, cell membrane, nucleus, microtubules, microfilaments, ribosomes, smooth ER, rough ER, Golgi apparatus, lysosomes, vacuoles, mitochondria, and chloroplasts. Expected Outcomes Students will make models of plant-cell structures and arrange them to form a complete cell. Analyze and Conclude 1. Students’ answers should reflect an understanding of the functions of plant cell organelles. 2. Scales will vary depending on the size of the cell model. A typical scale, assuming that the classroom is 5 m across, would be 5/0.00005 (50 micrometers 0.00005 meters), or 100,000 : 1. 3. The model should be similar in shape and structure to a real cell part. The model is different in that it is much larger, is made of different materials, and does not function. 4. Students should explain how their model would be an improvement on their previous model. 180 Chapter 7 How can you make a model of a cell? Materials variety of craft supplies, index cards Procedure 1. Your class is going to make a model of a plant cell using the whole classroom. Work with a partner or in a small group to decide what cell part or organelle you would like to model. (Use Figure 7–6 as a starting point. It will give you an idea of the relative sizes of various cell parts and their possible positions. Figures 7–7 through 7–10 can provide additional information.) 2. Using materials of your choice, make a threedimensional model of the cell part or organelle you chose. Make the model as complete and as accurate as you can. 3. Label an index card with the name of your cell part or organelle and list its main features and functions. Attach the card to your model. 4. Attach your model to an appropriate place in the room. If possible, attach your model to another related cell part or organelle. Analyze and Conclude 1. Inferring What are the functions of the different organelles in plant cells? 2. Calculating Assume that a typical plant cell is 50 micrometers wide. Calculate the scale of your classroom cell model. (Hint: Divide the width of the classroom by the width of a cell, making sure to use the same units.) 3. Comparing and Contrasting How is your model cell part or organelle similar to the real cell part or organelle? How is it different? 4. Evaluating Based on your work with this model, describe how you could make a better model. Specify what new information the improved model would demonstrate. Chloroplasts Plants and some other organisms contain For: Cell structure activity Visit: PHSchool.com Web Code: cbd-3072 chloroplasts. Chloroplasts are organelles that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis. Chloroplasts are the biological equivalents of solar power plants. Like mitochondria, chloroplasts are surrounded by two membranes. Inside the organelle are large stacks of other membranes, which contain the green pigment chlorophyll. Organelle DNA Unlike other organelles that contain no DNA, chloroplasts and mitochondria contain their own genetic information in the form of small DNA molecules. Lynn Margulis, an American biologist, has suggested that mitochondria and chloroplasts are actually the descendants of ancient prokaryotes. Margulis suggests that the prokaryotic ancestors of these organelles evolved a symbiotic relationship with early eukaryotes, taking up residence within the eukaryotic cell. One group of prokaryotes had the ability to use oxygen to generate ATP. These prokaryotes evolved into mitochondria. Other prokaryotes that carried out photosynthesis evolved into chloroplasts. This idea is called the endosymbiotic theory. FACTS AND FIGURES The origin of eukaryotes? The idea that chloroplasts and mitochondria originated in symbiotic relationships with prokaryotic cells is called the endosymbiont hypothesis. According to this hypothesis, the ancestors of eukaryotic cells were smaller species of prokaryotes living within larger species of prokaryotes. Chloroplasts possibly originated when cyanobacteria became established in larger prokaryotes either as parasites or as prey that were not digested. Mitochondria were once possibly anaerobic heterotrophs that found “safe harbor” inside larger prokaryotes as the world became increasingly aerobic. As the host and symbionts over time became more and more interdependent, the organisms merged to become a single organism. Cytoskeleton A supporting structure and a transportation system complete our picture of the cell as a factory. As you know, a factory building is supported by steel or cement beams and by columns that support its walls and roof. Eukaryotic cells have a structure—the cytoskeleton —that helps support the cell. The cytoskeleton is a network of protein filaments that helps the cell to maintain its shape. The cytoskeleton is also involved in movement. Microfilaments and microtubules are two of the principal protein filaments that make up the cytoskeleton. Microfilaments are threadlike structures made of a protein called actin. They form extensive networks in some cells and produce a tough, flexible framework that supports the cell. Microfilaments also help cells move. Microfilament assembly and disassembly is responsible for the cytoplasmic movements that allow cells, such as amoebas, to crawl along surfaces. Microtubules, as shown in Figure 7–11, are hollow structures made up of proteins known as tubulins. In many cells, they play critical roles in maintaining cell shape. Microtubules are also important in cell division, where they form a structure known as the mitotic spindle, which helps to separate chromosomes. In animal cells, tubulin is also used to form a pair of structures known as centrioles. Centrioles are located near the nucleus and help to organize cell division. Centrioles are not found in plant cells. Microtubules also help to build projections from the cell surface, which are known as cilia (singular: cilium) and flagella (singular: flagellum), that enable cells to swim rapidly through liquids. Cilia and flagella can produce considerable force; and in some cells they move almost like the oars of a boat, pulling or pushing cells through the water. You will learn more about cilia and flagella in later chapters. Your students can extend their knowledge of cell structure through this online experience. Cytoskeleton Build Science Skills (magnification: 1000⫻) Cell membrane Endoplasmic reticulum Microtubule Microfilament Ribosomes 3 ASSESS Mitochondrion 왖 Figure 7–11 The cytoskeleton is a network of protein filaments that helps the cell to maintain its shape and is involved in many forms of cell movement. The micrograph shows the microtubules of kidney cells. Microtubules are part of the cytoskeleton that help maintain cell shape. Key Concept Describe the functions of the endoplasmic reticulum, Golgi apparatus, chloroplast, and mitochondrion. 2. Describe the role of the nucleus in the cell. 3. What are two functions of the cytoskeleton? Have students make a Venn diagram to show organelles that are found only in prokaryotic cells, those that are found only in eukaryotic cells, and those that are found in both types of cells. Ask students to make a compare/ contrast table that lists all the parts of a typical cell. Column heads might include Name, Structure, and Function. 4. How is a cell like a factory? 5. Critical Thinking Inferring You examine an unknown cell under the microscope and discover that the cell contains chloroplasts. What type of organism could you infer that the cell came from? Persuasive Writing Image that you are Lynn Margulis. Write a persuasive letter to the editor of a magazine, explaining your idea. Your explanation should be clear to people who do not have a biology background. Hint: Review the concept of symbiosis in Section 4–2. 7–2 Section Assessment 1. Rough ER makes membranes and secretory proteins. Smooth ER makes lipids and helps in detoxification. The Golgi apparatus modifies, sorts, and packages proteins and other materials from the ER for storage or secretion. Chloroplasts capture the energy of sunlight and convert it into chemical energy. Mitochondria convert stored chemical energy into compounds that the cell can use. 2. It is the control center of the cell. Evaluate Understanding Reteach 7–2 Section Assessment 1. Using Analogies Show students a photo of a house that’s being built, with only the foundation laid and the basic frame constructed. Builders call this initial stage “framing” the house. Ask: How is this house frame like a cell’s cytoskeleton? (Just as a cytoskeleton is a network of protein filaments that helps a cell maintain its shape, the frame of a house is a network of boards and timbers that forms the shape of the house.) 3. It helps the cell maintain its shape and also is involved in movement. 4. Answers may vary. A typical response will compare ribosomes to factory machines and the cytoskeleton to a supporting structure. Students should also compare other organelles to various parts of a factory. 5. Students should infer that the organism would either be a plant or some other organism that carries out photosynthesis. Students’ letters may vary. The focus of all letters, though, should be to explain the endosymbiotic theory. Because this explanation must be clear to people without a biology background, students should explain in simple but accurate terms what mitochondria, chloroplasts, and prokaryotes are, as well as how such symbiotic relationships could evolve. If your class subscribes to the iText, use it to review the Key Concepts in Section 7–2. Cell Structure and Function 181