KEY CONCEPTS AND PROCESS SKILLS

Transcription

1 Cells Alive! to 1 50-minute session ACTIVITY OVERVIEW L A B O R ATO R Y SUMMARY In this activity, the idea that cells are alive and perform life functions (such as respiration) is explored. Students are introduced to yeast as they investigate the ability of yeast cells to respire. Comparisons of human and yeast cells reinforce the cellular nature of all life. KEY CONCEPTS AND PROCESS SKILLS 1. All living things are composed of microscopic units called cells. 2. Controlling variables and thoroughly describing procedures helps scientists (and students) to ensure that their results are reproducible. 3. Cells are alive. The functions of living organisms, including respiration, are performed by cells. KEY VOCABULARY cell control hypothesis microbe multicellular respiration magnify/magnification C-145

2 Activity 39 Cells Alive! MATERIALS AND ADVANCE PREPARATION For the teacher 1 measuring cup * several packets of fresh quick-rise dry yeast (one 1-oz. packet per class) * warm water Demoslide tubes (optional) For the class * microscopes (optional) slides (optional) coverslips (optional) Lugol s solution (optional) For each group of four students 1 Chemplate with gas delivery tube 1 dropper bottle of bromthymol blue (BTB) indicator 1 large plastic cup containing water 1 2 packet of sugar 2 droppers 2 stir sticks 1 magnifier * 1 sheet of unlined paper * 75 drops (approximately 3 ml) yeast suspension * tape, transparent (optional) C-146 Science and Life Issues *Not supplied in kit Teacher s Note: Whenever living organisms are used in the laboratory, many variables are involved. Because the freshness of the yeast affects the time required to observe results, yeast packets are not provided in the laboratory materials. Instead, it is recommended that fresh yeast packets be purchased immediately prior to doing the experiment. You may observe results faster if you use heat lamps or place the Chemplates on radiators, particularly if your classroom is relatively cool. However, do not overheat them. Preparing Yeast Suspensions In areas with extremely soft water (in which the addition of the bromthymol blue results in a yellow-green color rather than a blue color), use distilled water instead of tap water.

3 Cells Alive! Activity 39 You will have much better lab results if you prepare a fresh yeast suspension for each class period about 10 minutes prior to use. Do this by adding 1 packet (1/4 oz or 7 grams) of fresh yeast to 1/3-1/2 cup of warm water (about 30 C) and stir thoroughly. Note that if the water is too hot, it can kill the yeast. Be sure the suspension is mixed and has not settled out when provided to students (if it has settled out, stir again before distributing). Setting up the Microscope Demonstration of Yeast Set up several microscopes with yeast slides. To make the yeast slides, add 5 10 drops of the yeast suspension to a half-full cup of water to dilute it before using it for the slides. Look at the slide under a microscope to be sure the density of the yeast cells allows students to easily find the yeast and see individual cells. Adjust with more yeast or water as needed. Use either regular slides or the Demoslides provided to set up several microscopes for student observations. TEACHING SUMMARY Getting Started 1. Introduce yeast as a single-celled organism. 2. Review the use of BTB as an indicator. Doing the Activity 3. Students conduct an experiment to collect evidence that yeast cells respire and they observe yeast cells through the microscope. Follow-Up 4. The class shares experimental results and discusses respiration. BACKGROUND INFORMATION Yeast Yeasts are single-celled fungi that can reproduce both asexually and sexually. The most well-known and commercially significant yeasts (such as brewer s yeast and baker s yeast) are varieties of the species Saccharomyces cerevisiae. Brewer s yeast is used to ferment the sugars of grains or fruit to produce alcoholic beverages. The carbon dioxide gas produced by the fermentation of baker s yeast forms tiny bubbles that are trapped in bread dough. As these bubbles form, the dough rises. Most bread recipes recommend letting the dough rise in a warm place, because warmer temperatures promote more rapid carbon dioxide production. The high temperatures of baking kill the yeast C-147

4 Activity 39 Cells Alive! cells and also cause the small gas bubbles to expand, thus creating the airy texture of breads. Yeasts have been used significantly in genetic studies because they have a relatively small number of genes. The basic cellular mechanisms of replication, recombination, cell division, and metabolism are generally conserved between yeasts and larger eukaryotes, including mammals. Based on molecular genetics and cellular structure, yeasts are much more similar to humans than they are to any bacteria. Cellular Respiration All organisms use chemical reactions to convert energy stored in food to forms of energy that can be used by cells. Cells extract energy from food molecules through the process of cellular respiration, which can be either aerobic or anaerobic. Aerobic respiration requires oxygen, while anaerobic respiration does not. Yeast cells respire both aerobically and anaerobically. The way in which yeast cells respire anaerobically is known as fermentation, a process in which sugar and water produce carbon dioxide and ethanol. The following equation represents the fermentation of sucrose (table sugar) by yeast: C 12 H 22 O 11 + H 2 O 4C 2 H 5 OH + 4CO 2 + energy sucrose + water produces ethanol + carbon dioxide + energy Aerobic respiration is a complete breakdown of sugar to carbon dioxide and water. The equation for aerobic respiration, assuming the starting sugar is sucrose (table sugar) is: C 12 H 22 O O 2 12 CO H 2 O + energy sucrose + oxygen produces carbon dioxide + water + energy Bromthymol Blue Indicator Bromthymol blue (BTB) is a chemical indicator that is yellow in acidic solution and blue in basic or neutral solution. As a solution of BTB gradually changes color, it appears to go through an intermediate green stage. The green color results from some of the BTB molecules being in the blue state and some in the yellow state. Since carbon dioxide in solution produces carbonic acid, BTB is yellow in the presence of sufficient carbon dioxide. C-148 Science and Life Issues

5 Cells Alive! Activity 39 TEACHING SUGGESTIONS GETTING STARTED 1. Introduce yeast as a single-celled organism. Show students a small sample of yeast in a clear plastic bag or cup and ask if they know what it is. You may wish to pass around samples and have students see if they can identify it by smell. If no one identifies it, tell them that it is yeast. Ask, How many of you have used yeast? What is it used for? It is used for making bread, beer, wine, etc. (If necessary, distinguish between commercial yeast and other forms of yeast that can cause infections.) Ask students if they know what yeasts are: Yeasts are single-celled organisms. Ask students, What happens when large, many-celled organisms like people breathe? Students should indicate that we take in oxygen and release carbon dioxide. Remind them that the oxygen travels to the body s tissues, where it helps to break down nutrients for energy. Carbon dioxide is released as waste. (In Activity 18, The Circulation Game, in the Body Works unit, students modeled the role of the body s tissues using nutrients and oxygen and releasing carbon dioxide.) Ask, Do yeasts perform the same functions as larger organisms, such as taking in nutrients, breaking them down to produce energy through respiration, and releasing wastes? You may want to quickly review the process of respiration, in which organisms break down sugar (in the presence or absence of oxygen) and release carbon dioxide. Some students may become confused about which gas is taken in vs. which is released. At this point, have students read the Introduction to Activity 39, Cells Alive! on page C-47 in the Student Book. Review the idea that all living organisms are made up of cells and that yeasts are living organisms made up of a single cell. In this activity, students will collect evidence that yeast cells perform the function of cellular respiration. This will provide evidence for the idea that all cells perform common functions. 2. Review the use of BTB as an indicator. Teacher s Note: This activity builds on concepts first introduced in the Body Works unit of Science and Life Issues. If your class did not complete Activity 17, Gas Exchange, (in which students used BTB to measure relative amounts of carbon dioxide in their exhaled breath), they may need significantly more assistance in understanding the major ideas presented in this activity. In addition, they may not be prepared to be assessed on Analysis Question 2. If necessary, review the use of the bromthymol blue (BTB) as an indicator (see below). BTB as an Indicator BTB Color In the Presence Of: blue air (little carbon dioxide) yellow more carbon dioxide DOING THE ACTIVITY 3. Students conduct an experiment to collect evidence that yeast cells respire and they observe yeast cells through the microscope. Demonstrate how to set up the Chemplate, as shown on page C-49 in the Student Book. Ask students to make sure the rubber tubes are thoroughly C-149

6 Activity 39 Cells Alive! cleaned, since residual liquids, such as yeast suspension, remaining on the apparatus may cause the BTB indicator to change color. Remind students to share one packet of sugar between two groups of four students. This will reduce waste as well as the number of sugar packets that could accidentally spill. Note that Step 8 requires students to write a prediction. The reason for placing this step later in the Procedure is so that students are familiar with the lab set-up and have some knowledge with which to develop the prediction. Instruct students to be as specific as possible. For example, instead of saying, I predict a color change, students should be able to predict that, if yeast cells respire like people, the BTB solution will turn from blue to yellow. Steps 6 10 provide students with tasks to complete while waiting the minutes required to see changes in the solutions. Point out that Procedure Step 7 requires students to create a data table. Depending on your student population, you may want to provide students with some guidance on how to construct the table prior to beginning the activity. If your students are proficient at constructing data tables, you may wish to assess them on the Organizing Data element of the DESIGN- ING AND CONDUCTING INVESTIGATIONS (DCI) variable. A sample data table (Table 1) is provided in the Follow-Up section of this activity. Point out the microscopes set up with yeast suspension so that students can examine yeast cells during the activity. Remind students to turn the microscope lights off between viewings to avoid drying out the yeast suspension on the slides. To focus on individual yeast organisms, they will need to view the yeast cells at a minimum of 100x magnification. They will see the cell wall and unremarkable cytoplasm of the yeast cells. The nuclei and cell membrane are not visible. If you add a small amount of Lugol s solution, the cytoplasm will stain and there will be a faint suggestion of some substructure at 400X. Despite the lack of detail visible, the slides will help to establish the cellular structure of yeast. FOLLOW-UP 4. The class shares experimental results and discusses respiration. Students should have observed that the gas produced by the yeast cells caused the BTB solution to turn yellow or yellow-green, and thus they may infer the presence of carbon dioxide (CO 2 ) gas. The longer the tube is left in place, the more carbon dioxide (indicated by yellow color) spreads through the solution. Teacher s Note: If yellow BTB solution is left exposed to air, it will eventually return to its original blue color. This means that solutions that are left out overnight may become blue (as the yeast cells stop producing carbon dioxide and carbon dioxide dissolved in the solution disperses to the air). You may want to discuss the lab results before having students respond to the Analysis Questions (sample results are described below in Table 1). C-150 Science and Life Issues

7 Cells Alive! Activity 39 Table 1: Sample Data Table Observations Cup 2 Cup 3 Cup 4 Large Oval Cup 8 (yeast + (yeast + (yeast + Cup (BTB + water) water) water + sugar) water + sugar) (BTB + water) Initial beige mixture beige mixture beige mixture blue blue (some sugar may (some sugar may be visible) be visible) Final beige suspension bubbles, bubbles, blue-green blue on bottom; clear frothy beige frothy beige or blue with liquid on top liquid liquid streaks of yellow Discuss the purpose of each cup within the context of the experiment. Cup 2 acted as the control; Cup 3 allowed students to observe what was happening; Cup 4 and the large oval cup allowed the gas production by the yeast cells to bubble through the BTB solution. Cup 8 allowed a standard for comparison of the BTB color. This discussion can help prepare students to be assessed on responses to Analysis Question 2 using the Analyzing and Interpreting Data element of the DESIGNING AND CON- DUCTING INVESTIGATIONS (DCI) variable. If students are having difficulty with Analysis Question 2, review the composition of inhaled vs. exhaled breath in humans, which was first introduced in Activity 17, Gas Exchange, (in the Body Works unit) on Transparency 17.2, Composition of Breath, and appears below. Components of Earth s Atmosphere Composition of Air Breathed In (%) Composition of Air Breathed Out (%) Nitrogen Oxygen Argon Carbon dioxide Water vapor C-151

8 Activity 39 Cells Alive! Remind students that many organisms, including people, are multicellular (many-celled). Point out that the concept that cells are alive and perform life functions is true for both single-celled organisms and multicellular organisms like humans. While students know that humans breathe, they may not have made the connection that the oxygen in the lungs is then carried by the blood to cells in different parts of the body. These individual cells respire and produce wastes, including carbon dioxide. Remind students of Activity 18, The Circulation Game, when oxygen was transported to the tissues by capillaries and carbon dioxide was released. Explain now that this exchange actually occurs at the level of cells. Differentiate between breathing (the mechanical process by which air is taken into the lungs and wastes are exhaled) and the process of respiration (a cellular process). Explain that in all organisms, respiration, the breakdown of nutrients to produce energy and carbon dioxide, occurs within the cells. SUGGESTED ANSWERS TO ANALYSIS QUESTIONS 1. Compare your experimental results to your prediction. Was your prediction correct? Explain. Students may predict that nothing will happen (i.e. the BTB remains blue) because the yeast cells will do nothing. The only other explanation for the BTB remaining the same color would be that the yeast cells produce a gas that is identical to the composition of air and/or is not the same as the gas given off by humans. Students may also predict that the BTB will turn yellow from carbon dioxide production as a result of respiration, which is what begins to happen. 2. Describe your results. Explain how your results do or do not provide evidence that yeast cells respire. DCI aid Students should observe the BTB solution beginning to turn yellow near the end of the tube. After stirring, the solution in the cup appears blue-green; the difference in color is particularly noticeable when compared to the standard (especially if the tray is placed on a white sheet of paper). In addition, students should have observed significant bubbling of the solution in Cup 3 (which was identical to Cup 4 but was not covered). The solution in Cup 2 should not have bubbled unless sugar was accidentally dropped into that cup. A complete and correct level 3 response follows: The BTB solution began to show streaks of yellow. After it was stirred, it was still blue, but a lighter or greener blue. This means that the yeast cells began to produce carbon dioxide, but not too much. BTB turns yellow when there is a lot of carbon dioxide. Carbon dioxide is what people exhale when they breathe. The yeast cells also produced carbon dioxide. This is the evidence that yeast cells respire. 3. Think about the needs of multicellular organisms such as humans. What purpose did the sugar serve for the yeast? The sugar provided food (nutrients) for the yeast. C-152 Science and Life Issues

9 Cells Alive! Activity a. What was the purpose of Cup 2? It was the control, to show that the yeast cells produce waste only if they have food. b. Imagine that you had more materials available to you. Design another control for this experiment. Students may suggest that the yeast without sugar should have been set up identically to the yeast with sugar. This could have been done if another cup cover and Chemplate had been available. If you have time and/or additional materials available, consider having students conduct this experiment with this control. 5. Based on your observations of the yeast cells under the microscope, your investigation of the gas produced by the yeast cells, and the picture of yeast cells at high magnification in Figure 2, what do yeasts have in common with humans? Students should cite the cellular structure of the yeast and the production of carbon dioxide gas as evidence that yeast and human cells have common structures and functions. The production of carbon dioxide gas only when sugar is present provides evidence that the yeast cells are using the sugar for energy and producing carbon dioxide wastes in a manner similar to that used by humans and other organisms. Teacher s Note: Some students may think that the yeast cells are not alive because they do not move. Discuss examples that show that movement is not a good criterion for life, as many living organisms do not seem to move. Examples include plants, sponges, and corals. However, all of these creatures grow and reproduce. This can provide an opportunity to review the needs and functions of living organisms, such as respiration, reproduction, utilizing food sources, and producing wastes. (In high school, students may extend the activity by doing microscopic yeast counts to demonstrate that yeast cells reproduce.) Students may observe the outer boundary of a yeast cell and refer to it as a cell membrane. While yeast cells do, in fact, have cell membranes, students are observing the cell walls of the yeast cells. C-153