Observing Respiration

Transcription

1 Chapter 9 Cellular Respiration Design an Experiment Observing Respiration Introduction Cellular respiration occurs in all living things. During this process, animals take in oxygen and release carbon dioxide by breathing. Is it possible to observe an animal release carbon dioxide? Plants are different from animals in that they do not breathe in the same way animals do. Do they carry out cellular respiration? How could the release of carbon dioxide by plants be observed? In this investigation, you will first observe the release of carbon dioxide by humans. Then you will design and conduct an experiment to investigate whether plants also release carbon dioxide. Problem How can you observe the release of carbon dioxide by an animal? Do plants also release carbon dioxide as a product of cellular respiration? Pre-Lab Discussion Read through the entire investigation. Then, work with a partner to answer the following questions. 1. Identify the manipulated and responding variables in Part A. The manipulated variable will be exhaling into the test tube and the responding variable will be the color of the indicator. You may want to refer students to Chapter 9 in the textbook for a discussion of cellular respiration and its reactants and products. Time required: Part A: 40 minutes for set-up; 10 minutes to observe 24 hours later. Part B will depend on students experiments 2. What is an acid indicator? How and why is one being used in this experiment? An indicator is a substance that changes color with a change in acidity. When carbon dioxide is exhaled into the test tube containing the cabbage indicator, a color change from purplish-blue to reddish-blue is observed, indicating the presence of an acid. This is because of the presence of carbon dioxide, which forms a weak acid called carbonic acid when combined with water. 3. Apply your understanding of Part A to an experiment with a plant such as a radish or bean seedling instead of a person. Why couldn t an identical experiment be used to demonstrate the release of carbon dioxide by plants? A plant seedling cannot blow through a straw into the test tube. Biology Laboratory Manual A/Chapter 9 95

2 4. How could a new experiment be designed to demonstrate the release of carbon dioxide by a seedling? A seedling could be placed into the test tube of indicator, left over time, and compared to a control test tube of indicator with no seedling. 5. Discuss the possible outcomes of an experiment with a seedling and explain the meaning of each. If the indicator in the test tube containing the seedling turns reddish-blue, the seedling released carbon dioxide. If the indicator in the test tube containing the seedling does not change color, the seedling did not release carbon dioxide. Suggested Materials (per group) distilled water 10 radish seedlings hot plate test tubes mL beakers stoppers purple cabbage leaves cotton ball large slotted spoon forceps straw Safety Put on safety goggles. Put on a laboratory apron and plastic gloves. Be careful to avoid breakage when working with glassware. Use extreme care when working with heated equipment or materials to avoid burns. Observe proper laboratory procedures when using electrical equipment. Always use special caution when working with laboratory chemicals, as they may irritate the skin or stain skin or clothing. Never touch or taste any chemical unless instructed to do so. Note all safety alert symbols next to the steps in Design Your Experiment and review the meaning of each symbol by referring to Safety Symbols on page 8. Pea or bean seedlings also can be used. Design Your Experiment Part A. Using Cabbage Indicator 1. Tear the purple cabbage leaves into small pieces. Place the cabbage pieces into one of the beakers. 2. Pour about 250 ml of distilled water into the other beaker. Using the hot plate, heat the water until it boils. CAUTION: Put on safety goggles. Be careful when working with the hot plate. 3. Pour the hot distilled water into the bowl that contains the cabbage. CAUTION: Be careful when working with heated materials to avoid burns. Allow the water to cool. The water will turn purplish-blue in color when mixed with the cabbage. 96 Biology Laboratory Manual A/Chapter 9

3 4. Using the slotted spoon, scoop out the cabbage pieces. Discard the cabbage, saving the liquid. The liquid will serve as an acid indicator. The color of the cabbage indicator will change from purplish-blue to reddish-blue when the indicator is mixed with an acid. When carbon dioxide combines with water, it forms a weak acid called carbonic acid. 5. Pour the cabbage indicator into 2 test tubes so that they are half full. Put a rubber stopper into one test tube. 6. Exhale a few times into the uncovered test tube, as shown in Figure 1. CAUTION: Be sure not to inhale any of the cabbage indicator. Observe any changes in the color of the cabbage indicator in both test tubes. Record your observations. Straw Test tube Cabbage indicator Design Your Experiment Part B. Carbon Dioxide and Plants 1. In the space provided below, design an experiment to determine whether plants release carbon dioxide. Write your hypothesis, identify the variables, and write out the procedure in the space below. Be sure to include a control in your experimental plan. Hypothesis: Acceptable hypotheses: Plants release carbon dioxide as a product of cellular respiration or Plants do not release carbon dioxide. Manipulated variables: Addition of radish seedlings to test tube It is vital that before students conduct their self-designed experiments, the procedure be carefully examined and approved by the instructor. Biology Laboratory Manual A/Chapter 9 97

4 Responding variables: Presence of carbon dioxide as indicated by color of indicator in test tube. Controlled variables: Time, volume of indicator, size of seedlings, temperature, etc. Procedure: 1. Make indicator as described in Part A Add cabbage indicator to two test tubes so they are one quarter full. Place a cotton ball and 10 radish seedlings in one test tube. Put a rubber stopper in each of the two test tubes. Wait 1 day. Make observations after 1 day and record. 7. Safety Precautions: 2. Submit your written experimental design to your teacher for approval. Once your design has been approved, carry out your experiment. Wash your hands with soap and warm water before leaving the lab. 3. Record your data in a table such as the data table shown or create your own data table. If you need more space, attach additional paper. You may choose to create a class data table so that the class data Data Table can be posted and analyzed. Test Tube Description Color of Cabbage Indicator 1 Test tube with human breath Reddish-blue 2 Test tube covered with foil Purplish-blue 3 Test tube with radish seedlings Reddish-blue 4 Test tube with no radish seedlings Purplish-blue If students struggle in designing this experiment, ask leading questions about photosynthesis and nutrition in adult plants and seedlings. Analysis and Conclusions 1. Analyzing Data Describe the color change after exhalation into the test tube of cabbage indicator as compared to the test tube of indicator that was left alone? How can you explain what occurred? The indicator that was exhaled into changed from purplish-blue to reddish-blue while the test tube that was left alone remained purplish-blue. This indicates that carbon dioxide was exhaled and when it combined with water, it formed carbonic acid. 98 Biology Laboratory Manual A/Chapter 9

5 2. Analyzing Data In the experiment that you designed, describe any color changes that occurred and provide an explanation. What happened in the control test tube and what does it mean? Answers will depend on student designs and results. For the test tube containing radish seedlings, the indicator turned to reddish-blue indicating that carbon dioxide was available to combine with water to form carbonic acid. The test tube with no radish seedlings was the control because it had no source of carbon dioxide and it did not change color. 3. Drawing Conclusions What conclusions can be drawn from the data collected in both experiments? Humans and plant seedlings both produce carbon dioxide. Since carbon dioxide is a product of cellular respiration, both must be undergoing cellular respiration. 4. Designing Experiments What argument could be made against your conclusions if a control had not been used? If controls had not been used in either case, it could be argued that the cabbage indicator turns from purplish-blue to reddish-blue spontaneously as a function of time. 5. Predicting If the experiment is done again but more seedlings are placed in one of the test tubes, what do you predict would occur? Explain the reason for your prediction. The color of the water would change more rapidly with more seedlings because more cellular respiration would have occurred and more carbon dioxide produced as a result. 6. Drawing Conclusions Did the color of the cabbage indicator change when you exhaled into the test tube? Explain your answer. Yes. The carbon dioxide that was exhaled formed carbonic acid when it combined with the water in the cabbage indicator. Going Further What question(s) did the results of your experiment raise? Design an experiment that would address one such question or that would logically follow this experiment. Biology Laboratory Manual A/Chapter 9 99