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CS_Ch9_Environment 2/28/05 4:42 PM Page 591 Activity 8 Photosynthesis, Respiration, and the Carbon Cycle Activity 8 Photosynthesis, Respiration, and the Carbon Cycle GOALS What Do You Think? In this activity you will: Consider the mass of a seed from a giant redwood tree and the tree itself. It is hard to believe that a giant of a tree began as a small seed. • Learn how oxygen cycles through photosynthesis and respiration. • Practice safe laboratory techniques for using chemicals in a laboratory situation. • Describe the cycling of carbon in an ecosystem. • Speculate how human activities can affect the carbon cycle. • From where do the materials come to make up the mass of a mature tree? Write your answer to this question in your Active Biology log. Be prepared to discuss your ideas with your small group and other members of your class. For You To Do In this activity you will investigate what happens when the exchange of carbon dioxide between a leaf and the atmosphere is blocked. 1. Three days before this activity, one plant was placed in the dark. A second plant of the same species was placed in sunlight. 591 Coordinated Science for the 21st Century CS_Ch9_Environment 2/28/05 2:20 PM Page 592 A Vote for Ecology a) Predict what you will find when you test a leaf from each plant for the presence of starch. Day 1 Isopropyl alcohol is flammable and toxic. Do not expose the liquid or its vapors to heat or flame. Do not ingest; avoid skin/eye contact. In case of spills, flood the area with water and then call your teacher. Make sure you wear goggles, apron, and gloves 2. Remove one leaf from each plant. Use scissors to cut a small notch in the margin of the one placed in sunlight. Using forceps, drop the leaves into a beaker of hot (60°C) tap water. 3. When the leaves are limp, use forceps to transfer the leaves to a screw-cap jar about half full of isopropyl alcohol. Label the jar with your team symbol and store it overnight as directed by your teacher. 4. Select four similar leaves on the plant that has been kept in the dark, but do not remove them from the plant. Using a fingertip, apply a thin film of petroleum jelly to the upper surface of one leaf. Check to be sure the entire surface is covered. (A layer of petroleum jelly, although transparent, is a highly effective barrier across which many gases cannot pass.) Cut one notch in the leaf’s margin. 5. Apply a thin film to the lower surface of a second leaf and cut two notches in its margin. 6. Apply a thin film to both upper and lower surfaces of a third leaf and cut three notches in its margin. 7. Do not apply petroleum jelly to the fourth leaf, but cut four notches in its margin; Place the plant in sunlight. 592 Active Biology a) What is the purpose of the leaf marked with four notches? 8. Wash your hands thoroughly before leaving the laboratory. Day 2 9. Obtain your jar of leaf-containing alcohol from Day 1. Using forceps, carefully remove the leaves from the alcohol and place them in a beaker of room-temperature water. (The alcohol extracts chlorophyll from the leaves but also removes most of the water, making them brittle.) Recap the jar of alcohol and return it to your teacher. 10. When the leaves have softened, place them in a screw-cap jar about half full of Lugol’s iodine solution. Lugol’s iodine solution is used to test for the presence of small amounts of starch. Starch gives a blue-black color. 11. After several minutes, use forceps to remove both leaves, rinse them in a beaker of water, and spread them out in open Petri dishes of water placed on a sheet of white paper. a) Record the color of each leaf. Recap the jar of Lugol’s iodine solution and return it to your teacher. b) What was the purpose of the iodine test on Day 2? c) If you use these tests as an indication of photosynthetic activity, what are you assuming? Lugol's iodine solution is poisonous if ingested, irritating to skin and eyes, and can stain clothing. Should a spill or splash occur, call your teacher immediately; flush the area with water for 15 minutes; rinse mouth with water. CS_Ch9_Environment 2/28/05 2:20 PM Page 593 Activity 8 Photosynthesis, Respiration, and the Carbon Cycle 12. Wash your hands thoroughly before leaving the laboratory. 14. Repeat Steps 10 and 11. a) Compare the color reactions of the four leaves and record your observations. Day 4 13. Remove from the plant the four notched leaves prepared on Day 1 and place them on paper towels. To remove the petroleum jelly, dip a swab applicator in the HistoclearTM and gently rub it over the surface of the film once or twice. Then gently use a paper towel to remove any residue of petroleum jelly. Discard the swab applicator and the paper towel in the waste bag. b) In which of the leaves coated with petroleum jelly did photosynthetic activity appear to have been greatest? Least? 15. Wash your hands thoroughly before leaving the laboratory. Histoclear is a combustible liquid. Do not expose to heat or flame. Do not ingest; avoid skin/eye contact. Should a spill or splash occur, call your teacher immediately; wash skin area with soap and water. The Carbon Cycle You take in carbon in all the foods you eat.You return carbon dioxide to the air every time you exhale. A plant also returns carbon dioxide to the air when it uses its own sugars as a source of energy.When another plant takes in the carbon dioxide during photosynthesis, the cycle of carbon through the community is complete. In this activity you observed what happens when this exchange of carbon dioxide does not take place. However, when the exchange does take place, the plant can use the carbon from the carbon dioxide to live and grow. Carbon dioxide is also returned to the air by decomposers.When producers or consumers die, decomposers begin their work. As its source of energy, a decomposer uses the energy locked in the bodies of dead organisms. It uses the carbon from the bodies to build its own body. Carbon that is not used is returned to the air as carbon dioxide. Eventually, almost all the carbon that is taken in by plants during photosynthesis is returned to the air by the activity of decomposers. 593 Coordinated Science for the 21st Century CS_Ch9_Environment 2/28/05 2:20 PM Page 594 A Vote for Ecology OXYGEN CYCLE THROUGH PHOTOSYNTHESIS AND RESPIRATION oxygen photosynthesis (green plants) respiration (plants and animals animals) respiration ((decomposers)) carbon dioxide Hundreds of millions of years ago, many energy-rich plant bodies were buried before decomposers could get to them.When that happened, the bodies slowly changed during long periods of time.They became a source of fuels like coal, oil, and natural gas.Today, when these fuels are burned, energy is released.The carbon in the fuels is returned to the air as carbon dioxide.You can see that even the energy obtained from fuels is a result of photosynthesis.The process in which carbon is passed from one organism to another, then to the abiotic community, and finally back to the plants is called the carbon cycle. The Cycling of Matter The energy from the Sun flows through the ecosystem in the form of carbon-carbon bonds in organic matter.When respiration occurs, the carbon-carbon bonds are broken and energy is released.The carbon is 594 Active Biology CS_Ch9_Environment 2/28/05 2:20 PM Page 595 Activity 8 Photosynthesis, Respiration, and the Carbon Cycle combined with oxygen to form carbon dioxide.The energy that is released is either used by the organism (to move, digest food, excrete wastes, etc.) or the energy may be lost as heat. In photosynthesis energy is used to combine the carbon molecules from the carbon dioxide, and oxygen is released.This is illustrated in the diagram. All the energy comes from the Sun.The ultimate fate of all energy in ecosystems is to be lost as heat. Energy does not recycle! Bio Words carbon cycle: the process in which carbon is passed from one organism to another, then to the abiotic community, and finally back to the plants However, inorganic nutrients do recycle.They are inorganic because they do not contain carbon-carbon bonds.These inorganic nutrients include the phosphorous in your teeth, bones, and cell membranes. Also, nitrogen is found in your amino acids (the building blocks of protein). Iron is in your blood.These are just a few of the inorganic nutrients found in your body. Autotrophs obtain these inorganic nutrients from the inorganic nutrient pool.These nutrients can usually be found in the soil or water surrounding the plants or algae.These inorganic nutrients are then passed THE CARBON from organism to CYCLE atmospheric carbon dioxide organism as one photosynthesis animal respiration organism is consumed by another. Ultimately, plan exchange of CO2 respiration all organisms die. between atmosph and water They become detritus, food for burn the decomposers. At this stage, the last of the energy is extracted (and lost as heat).The inorganic nutrients decay are returned to the decay soil or water to be taken up again.The inorganic nutrients carbon are recycled; the compounds energy is not. 595 Coordinated Science for the 21st Century CS_Ch9_Environment 2/28/05 2:20 PM Page 596 A Vote for Ecology Reflecting on the Activity and the Challenge In this activity you learned that carbon is the key element in all organic matter. You investigated the process of photosynthesis and then related this process to respiration in the carbon- oxygen cycle. The cycling of matter like carbon is essential to the survival of any ecosystem. You will need to explain this cycle in your booklet. 1. Explain why photosynthesis and cellular respiration are considered to be paired processes. 2. What is the importance of decomposers in the carbon cycle? 3. What effect does the burning of fossil fuels have on the carbon cycle? 4. Scientists have expressed concerns about the burning of the rainforests to clear the land for the planting of crops. a) Explain how the burning of the forests could change oxygen levels. b) What impact would the change in oxygen levels have on living things? Inquiring Further The greenhouse effect The term greenhouse effect was coined in the 1930s to describe the heatblocking action of atmospheric gases. Research and report the connection between the greenhouse effect and the carbon cycle. 596 Active Biology