Background
You may not even be aware of it, but you contribute to the carbon cycle every day. If you ride in a car, take a train, or fly in an airplane, you contribute to the production of carbon dioxide. The fruits and vegetables you eat were probably shipped in a vehicle that produced carbon dioxide. The meat you eat comes from animals that produced carbon dioxide. The electricity that powers your home and your school quite possibly comes from a coal-burning power plant that produces carbon dioxide. After all that carbon dioxide is produced, where does it go?
Biogeochemical Cycles
Biogeochemical cycles describe how compounds are recycled and reused in an ecosystem. Each step of a biogeochemical cycle involves a chemical reaction or change in state from one form of compound into a new form of compound. The water cycle is an example of a biogeochemical cycle. Water evaporates from the ground or from living organisms, transforming from a liquid into a gas. The water vapor then condenses into tiny liquid droplets, forming clouds. The water falls back to earth as rain, or if the temperature of the atmosphere is below freezing, as hail or snow.
Biogeochemical cycles involve cycling compounds through living, geological, and chemical processes. Therefore, both living and nonliving factors are necessary to reuse matter in the ecosystem. In the water cycle, organisms consume water because their tissues require water to carry out life processes. Animals release water through many processes, and plants transpire water through their leaves, returning this water to the environment to be recycled. Other biogeochemical cycles include the carbon cycle, nitrogen cycle, the oxygen cycle, the phosphorous cycle, and the sulfur cycle.
The Carbon Cycle
Carbon, due in part to its unique ability to form covalent bonds with up to 4 other atoms, is the most abundant element in organic compounds. All organisms are composed, to some extent, of carbon. Lipids, proteins, carbohydrates, and nucleic acids are all carbon compounds. Even coal and petroleum, the ancient remains of plants and sea life, are composed primarily of carbon. Diamonds are carbon compounds formed under extremely high heat and pressure. So, how is carbon recycled from one form into another in the ecosystem?
Photosynthesis and respiration are interdependent processes in the carbon cycle. Through the process of photosynthesis, plants and other autotrophs take up carbon dioxide from the atmosphere and trap energy from the sun to produce organic compounds that help them grow and maintain life. While through the process of cellular respiration, organisms utilize organic compounds from food for energy, and they release carbon dioxide back into the atmosphere as a byproduct of respiration.
Humans influence the amount of carbon dioxide that is released into the atmosphere. Many human activities contribute to increased carbon dioxide emissions, including the burning of petroleum and fossil fuels and the clearing of forests and vegetation. Many scientists theorize that carbon dioxide emissions are leading to higher concentrations of atmospheric carbon dioxide, which has many consequences for ecosystems.
Carbon compounds are stored in several different places on Earth called carbon sinks. Forests, coal, and oil are all important carbon sinks, but the ocean is the world’s largest carbon sink. The ocean can and does absorb large quantities of carbon dioxide from the atmosphere. However, as the amount of carbon dioxide in ocean water increases, the acidity of the ocean water increases. This is because when carbon dioxide is dissolved in water, it forms carbonic acid. Marine organisms with shells, such as mollusks and corals, are very sensitive to increased acidity. Their shells are composed of a carbon compound called calcium carbonate which can dissolve in acidic environments.
Bromothymol Blue and Carbon Dioxide
Bromothymol blue is a pH indicator. In conditions that are acidic, bromothymol blue appears yellow; however, in basic conditions it is blue. In this set of activities, bromothymol blue is used to determine the presence of added carbon dioxide. When carbon dioxide is dissolved in water, carbonic acid forms, forming an acidic compound. The added carbon dioxide causes a solution containing bromothymol blue to turn yellow. When the dissolved carbon dioxide is removed, the solution will turn blue.
In this set of activities, students can investigate the different organismal contributions to the carbon cycle.
Standards
This activity addresses the following AP® Environmental Science concepts:
- ERT-1 Ecosystems are the result of biotic and abiotic interactions.
- ERT-1.D Explain the steps and reservoir interactions in the carbon cycle.
- ERT-1.D.3 Carbon cycles between photosynthesis and cellular respiration in living things.
Materials
Activity 1
- Test tube, 34 mL
- Bromothymol blue, 0.04%
- Distilled water
- Drinking straw
Activity 2
- 6 test tubes, 34 mL
- 6 rubber stoppers, #2
- 4 ramshorn snails
- 2 sprigs of Elodea, Chara, or similar species
- Distilled water
- Bromothymol blue, 0.04%
- Aluminum foil
- Light source
- Masking tape
- Marker
Preparation and Procedure
Activity 1
Introduction
In this activity, students determine how cellular respiration impacts bromothymol blue. Students exhale into the bromothymol blue solution, adding carbon dioxide.
Teacher tip: Test your distilled water with the bromothymol blue. If the solution is yellow and not green, you may need add a small amount of baking soda to increase the pH. Make sure to add very small amounts of baking soda until the pH of the solution is approximately 7 and appears green when bromothymol blue is added.
Procedures
- Add 25 mL of water to a test tube.
- Add 7 drops of 0.04% bromothymol blue to the water in the test tube. Record the color of the water.
- Insert a straw into the solution in the test tube.
- Carefully exhale through the straw into the test tube for one minute, and then observe the color of the water. Take caution not to inhale the solution through the straw.
Results
Students should see the solution turn yellow because the exhaled carbon dioxide reacts with the water to produce carbonic acid.
Activity 2
Introduction
In this activity, students will examine the role that autotrophs and heterotrophs play in the carbon cycle.
Teacher tip: Test your distilled water with the bromothymol blue. If the solution is yellow and not green, you may need to add a small amount of baking soda to increase the pH. Make sure to add very small amounts of baking soda until the pH of the solution is approximately 7 and appears green when bromothymol blue is added.
Procedure
- Label your test tubes 1A, 1B, 2A, 2B, 3A, and 3B using the masking tape and marker. Make sure to place the label near the top of the test tube.
- Add 25 mL of water to each tube. As soon as you add water, plug each tube with a rubber stopper.
- Remove the rubber stopper from test tube 1 and add 7 drops of 0.04% bromothymol blue.
- Replace the rubber stopper, place your finger over the top of the stopper, and shake the test tube gently.
- Repeat steps 2 to 4 for the remaining tubes.
- Remove the rubber stopper from tubes 1A and 1B and add 2 snails to each tube. Quickly replace the rubber stopper, making sure the tubes are sealed tightly.
- Repeat step 6 for tubes 2A and 2B, but instead of snails, place a 10-cm sprig of Elodea into each tube.
- Do not place any living organisms in test tubes 3A and 3B.
- Observe the 6 tubes. In your data table, record the initial color of the solution in each.
- Wrap test tubes 1B, 2B, and 3B in aluminum foil so that no light can enter the test tubes.
- Place your test tubes in a test tube rack near a sunny window.
- Wait 24 hours.
- Remove the foil from test tubes 1B, 2B, and 3B.
- Examine all the tubes again and record the final color of the solution in each tube.
Results
Students should see the following results:
Safety
Safety Requirements
- Goggles
- Gloves
- Lab apron or coat
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