15A Carbon Dioxide and Living Things

Transcription

1 Carbon Dioxide and Living Things Investigation 15A 15A Carbon Dioxide and Living Things How do living things exchange carbon dioxide? The carbon cycle is a series of pathways through which carbon atoms are exchanged and stored. The air you breathe contains carbon in the form of molecules of carbon dioxide. Plants use a process called photosynthesis to convert carbon dioxide into high-energy carbon molecules. Plants and animals break down those carbon molecules and release energy, carbon dioxide, and water in a process called respiration. Both processes also involve oxygen. In this investigation, you will explore how living things exchange carbon dioxide. Materials Safety goggles and apron 4 40-mL Clear plastic containers with screw caps Masking tape Marker 2 5-cm Sprigs of an aquatic plant Bromothymol blue (BTB) solution Straw 500 ml Beakers Funnel Aluminum foil Tweezers Blank white paper Lamp Plant seeds 2 Jars with covers Strainer Container used for filtering Ruler Safety Tip: Wear safety goggles and an apron during this investigation. A Thinking about respiration In respiration, glucose (a carbohydrate) reacts with oxygen to produce carbon dioxide and water. Every cell in your body uses this reaction to release the energy stored in carbohydrates. When you breathe, you take in oxygen, a reactant in respiration. You exhale carbon dioxide, a product of respiration. All types of animal and plant cells use respiration to produce the energy needed for life processes. 1. Obtain 100 ml of bromothymol blue (BTB) solution in a large beaker from your teacher. Bromothymol blue is a chemical that changes color in the presence of carbon dioxide. 2. Using a straw, gently blow bubbles into the solution until it completely changes color. Do not swallow the BTB! Your teacher may have you practice the technique using a beaker of plain water first. a. What was the initial color of the BTB solution? b. What was the color of the solution after you blew bubbles into the beaker? c. What color does BTB change to when carbon dioxide is present? d. What do you think would happen to the color of the solution if the carbon dioxide molecules were removed? 89

2 Investigation 15A Carbon Dioxide and Living Things 1 B Thinking about photosynthesis Plant cells use a chemical reaction called photosynthesis to store energy from the Sun in carbohydrate molecules. In this reaction, carbon dioxide reacts with water to make glucose (a high-energy molecule) and oxygen. The word light above the arrow means that light is required to make the reaction happen. a. Would the presence of plants in water cause the carbon dioxide to increase, decrease, or stay the same? Explain your answer. b. When would you expect photosynthesis to happen, during the day or at night? Explain your answer. C Investigating photosynthesis Experiment A 1. Obtain four clear plastic containers with screw caps. Label each container with a marker and masking tape as follows: 1 Plant: light 2 Plant: dark 3 No plant: light 4 No plant: dark 2. Use the beaker of BTB solution from Part 1 for this experiment. Gently blow bubbles into the solution again to make sure the color has completely changed to yellow. The initial color of the solution should be yellow for this experiment. 3. Record the initial color of the solution for all four containers in Table Use a funnel to pour 25 milliliters of the solution into each of the four test containers. Cap tubes 3 and Place a 5-cm piece of aquatic plant in containers 1 and 2. Make sure each plant is completely submerged in the solution. Cap these containers. 6. Wrap tubes 2 and 4 with aluminum foil to block out any light. 7. Place the four capped tubes upright in front of a lamp. Make sure that the tubes are at least 8 inches away and that light is hitting all tubes equally on the side. Your setup should look similar to the one shown at right. 8. Let the plants sit for 50 minutes. Set up the experiment in Part 5 while you wait. 90

3 D Stop and think Carbon Dioxide and Living Things Investigation 15A a. Why did you need to make sure the BTB solution had completely changed color before starting the experiment? b. Why are tubes 3 and 4 called the control group? Why is a control group necessary in this experiment? c. Predict what you think will happen to the color of the BTB solution in each of the four containers. State your prediction in the form of a hypothesis. E F Investigating respiration Experiment B 1. Obtain two plastic jars with screw caps, and label them with a marker and tape as follows: 1 With seeds, 2 No Seeds. 2. Cover the bottom of jar 1 with plant seeds. 3. Add equal amounts of BTB solution to each jar. Be sure to use enough solution to cover the seeds. The initial color of the solution should be blue for this experiment. 4. Record the initial color of the solution for both jars in Table Cover both jars and place them in similar conditions for at least 45 minutes. 6. While you are waiting, write a hypothesis about what you think will happen to the color of the solution in each container. Recording your data 1. After 45 minutes, carefully unwrap each container from the photosynthesis experiment (Experiment A). 2. Using tweezers, carefully remove the plants from test tubes 1 and Record the final color of the solutions in Table 1. Table 1: Data from photosynthesis experiment (Experiment A) Container Initial color Final color 1 Plant: light 2 Plant: no light 3 No plant: light 4 No plant: no light Now you are ready to collect data from Experiment B. 4. Carefully pour the solution from container 1 into another clear container. Try not to let any seeds come out of the original container. 5. Compare the colors and record your data in Table Follow all cleanup instructions from your teacher. 91

4 Investigation 15A Carbon Dioxide and Living Things 17 Table 2: Data from respiration experiment (Experiment B) Container Initial color Final color 1 With seeds 2 No seeds G Analyzing the results a. From Experiment A, in which container(s) did photosynthesis occur? Use your data and knowledge of photosynthesis to explain your answer. b. Did the results from Experiment A support your hypothesis from step 4c? c. From Experiment B, in which container did respiration occur? Use your data and knowledge of respiration to explain your answer. d. Did the results from Experiment B support your hypothesis from Part 5, step 6? e. Why was light a variable in Experiment A but not a variable in Experiment B? f. Explain the importance of bromothymol blue in both experiments. H Applying your knowledge a. Explain the relationship between photosynthesis and respiration. How are they important pathways of the carbon cycle? b. How is energy involved in the processes of photosynthesis and respiration? Which process stores energy? Which process releases energy? Where does the energy come from? 92

5 Oceans and the Carbon Cycle Investigation 15B 15B Oceans and the Carbon Cycle What role do the oceans play in the carbon cycle? Carbon dioxide is important to living things; it is also a major greenhouse gas that traps energy from the Sun. Together with water vapor and methane, CO 2 helps keep Earth s surface at an average temperature of about 15 C. In the last investigation, you learned that CO 2 is produced during cellular respiration. It is also being produced in larger and larger quantities by the burning of fossil fuels and by deforestation. The oceans absorb CO 2 from the atmosphere, accumulating and storing it for an indefinite period. In this investigation, you will explore how the oceans absorb CO 2 and the chemistry involved in that process. Materials mL Beakers Red cabbage juice solution BTB (bromothymol blue) solution Simulated seawater Distilled water ph paper (range 4.5 to 9) 250-mL Graduated cylinder 2 Straws Safety Tip: Wear safety goggles and an apron during this investigation. A Thinking about the carbon cycle The carbon cycle has carbon sources and carbon sinks. Anything that releases CO 2 into the atmosphere is a carbon source. Anything that absorbs and retains CO 2 is a carbon sink. Study the diagram and then answer the questions. a. List the carbon sources in the diagram. b. List the carbon sinks in the diagram. c. Over time, carbon sources and sinks tend to stay in balance. Today, CO 2 levels are on the rise. What are the sources for this increasing CO 2? d. Based on the diagram, do you think the oceans are a net carbon source or sink? Explain your answer. 93

6 Investigation 15B Oceans and the Carbon Cycle 1 B CO 2 absorption and ph Water and other solutions like seawater absorb CO 2 through a chemical reaction. When CO 2 diffuses into water, carbonic acid forms and then ionizes to form hydrogen, bicarbonate, and carbonate ions. We measure the acidity of a solution using a value called ph. The ph scale is shown below. As you can see, the lower a solution s ph number, the greater its acidity. In this experiment, you will determine the effects of CO 2 absorption on the ph of a solution. What do you think will happen to the acidity of a solution as it absorbs more CO 2. Will it increase, decrease, or stay the same? State your answer in the form of a hypothesis. 1. Put on your safety goggles and apron. 2. Obtain a 500-mL beaker containing 100 ml of red cabbage juice solution. Red cabbage juice changes color as ph changes. 3. Record the initial color and ph of the solution. Use the ph chart on the left side of the table below to find the ph value that matches the color you observe. 4. Choose a bubbler from your group. Other members will observe the beaker and record observations. 5. Using the straw, the bubbler will gently blow bubbles into the solution. Do not inhale the solution. Observe and record each color you observe in the table. Use the chart provided by your teacher to record the ph. Keep recording until the color appears to stop changing. Table 1: Color and ph Red cabbage juice ph scale Data from the experiment color ph Color ph blue 7-8 Initial violet 5-6 dark red 3-4 pink

7 C Analyzing the results a. Was the initial ph of your solution acidic, neutral, or basic? Oceans and the Carbon Cycle Investigation 15B b. Was your hypothesis correct? Explain why or why not, based on your results. c. What happened to the ph of the solution as CO 2 was absorbed? D Comparing CO 2 absorption in fresh water versus seawater BTB turns from dark blue to green to yellow depending on the concentration of ionized carbonic acid in the solution. As the solution absorbs more CO 2, more carbonic acid forms and ionizes, which lowers the ph. The color of the solution gets lighter as the ph gets lower. In this experiment, you will bubble CO 2 into distilled water (representing fresh water) and then into simulated seawater using your breath and a straw. You will then observe and compare the color change in both solutions. 1. Put on your safety goggles and apron. 2. Fill one 500-milliliter beaker with 100 milliliters of distilled water and label it Distilled water. 3. Fill another 500-milliliter beaker with 100 milliliters of seawater and label it Seawater. 4. Add 20 drops of BTB solution to each beaker. 5. Using ph test strips, measure the ph of each beaker. Record the initial color and ph in the first row of Table Place each beaker on a blank white sheet of paper. 7. Choose a bubbler, timer, and observer from your group. 8. Using a straw, the bubbler will gently blow bubbles into the distilled water beaker while the timer measures one-minute time intervals for six minutes and the observer watches for color changes in the beaker. 9. Record your observations in Table Use a ph test strip to measure the final ph. Record the final color and ph in the last row of Table Repeat steps 7 and 8 for the seawater beaker using a clean straw. If the seawater does not change to a pale yellow, record the lightest color change you observe in the six minutes of gentle bubbling. Table 2: Time vs. color change Time (min) Distilled water Seawater Initial color and ph min (final color and ph) 95

8 Investigation 15B Oceans and the Carbon Cycle 1 E Analyzing the data a. Which was the control in this experiment, distilled water or seawater? Why did you need a control? b. Based on your results, which do you think has the ability to absorb more CO 2 before becoming acidic, distilled water or seawater? Explain your answer. c. Might the results of this experiment explain part of the reason why oceans are a carbon sink? Justify your answer. d. What other processes in the oceans store CO 2? What processes release CO 2? e. Based on your results, what effect might continued CO 2 absorption have on the ph of Earth s oceans? f. Based on the results of the experiment, could the release of excess CO 2 from the burning of fossil fuels and deforestation have an effect on the ph of the oceans? Explain your reasoning. F Impact of ocean acidification Although the natural absorption of CO 2 by the oceans helps absorb excess CO 2, many scientists believe the decrease in ph will have a negative effect on marine calcifying organisms. These are organisms that construct shells out of calcium carbonate and include some phytoplankton, coccolithophores, corals, crustaceans, and some molluscs. Calcifying organisms help store carbon in their calcium carbonate shells, and many absorb CO 2 for photosynthesis. When these organisms die, their shells sink to the bottom of the ocean and store carbon in calcium carbonate sediments. In fact, England s famous White Cliffs of Dover are made mostly of the calcium carbonate shells of coccolithophores. a. Conduct research on ocean acidification and its effect on calcifying organisms. Try to find answers to these questions: (1) What is the ph of seawater in the oceans, and has it changed since the Industrial Revolution? (2) How could rising CO 2 levels impact calcifying organisms? (3) What are some steps we can take to lower our negative impact on the ph of oceans? b. Suppose you are a marine biologist who is designing an experiment to test the effects of ocean acidification on a species of coral. Answer these questions: (1) What is your research question and hypothesis? (2) What are the control variables in your experiment? What is your experimental variable? (3) What materials will you need for your experiment? (4) Briefly describe your procedures and the data you will need to collect. 96