Engage
- To introduce the topic of the carbon cycle, ask students to brainstorm what they already know about carbon. If needed, remind students that carbon is an atom on the periodic table, that it is the fourth most abundant element, and that it can be found in different forms everywhere we look. Use these questions for class discussion or to assess students’ prior knowledge:
- Is there a fixed amount of carbon on the planet? (Like water, there is a relatively constant amount of carbon on the planet. Also like water, carbon constantly moves through different states and places.)
- Do you have any carbon in you? (Yes, in the carbohydrates, fats, proteins, enzymes, and tissues of your body. These are called organic molecules).
- What is photosynthesis? How is it related to the carbon cycle? (During photosynthesis, plants convert carbon dioxide and water into sugar in the presence of sunlight. This process moves carbon from the atmosphere to sugar in plant cells. Then, the sugars are either converted to energy through the process of respiration or used as building blocks to form starch, cellulose, lignin, wood, leaves, or roots. Energy is used to construct these complex molecules. This process, where carbon is removed from the atmosphere and stored in biomass, is known as sequestration.)
Explore
- Explain to students that they will model a carbon atom moving through the carbon cycle. Make sure they understand the terms carbon pool, carbon flux, carbon cycle, and carbon sequestration (see Background). If you choose, you may use the Global Carbon Cycle teacher page in your explanation.
- Give each student a copy of the Carbon Pathway student page and provide instructions for how to use this worksheet to record the journey through the carbon cycle. Ask students to count off by six and move to their first station according to their number.
- Students should write their first station on the first row in the table, roll the die, read the outcome from the card at that station, write down the next destination, and move to that station. Even if they end up staying at the same location, they should write down the station name and describe what happened at it.
Students measure the density of wood.
Photo credit: Yvonne Krowka
- Have students complete 10 or more rounds and record their journeys through the carbon cycle. If you have a small class, you may wish to have students complete additional rounds to increase the chance that they will experience diverse outcomes and visit all the stations; this will lead to richer discussion after the activity.
- When everyone is finished, discuss the following questions as a class:
- At which station (carbon pool) did you spend the most time? At which did you spend the least time?
- Did anyone get stuck in one station or between two stations? Why do you think that happened?
- While each of your journeys was different, was there anything similar about them?
- At which pools can carbon be stored? At which is carbon released into the atmosphere?
- Point out that there are many different ways to enter and leave most carbon pools. The time carbon atoms spend in each pool varies. For example, some atoms might cycle very quickly between the atmosphere and forests through photosynthesis and respiration, while others may get sequestered in a tree for hundreds of years.
- Remind students that the activity focused on the biological portion of the carbon cycle. Ask students if there are other places that carbon is stored and other ways that carbon moves. (Carbon moves through organisms in the ocean. It also moves through a geological cycle, in which it is stored in rocks, fossil fuels, and sediments on the ocean floor and includes fluxes like weathering, compaction, and volcanic eruptions.)
- Explain to students that they will work in groups to draw a carbon cycle diagram on a large sheet of paper that represents the places each member in their group visited.
- When the groups finish, have the students place their diagram on the wall. Direct students to walk around the room and use sticky notes to write their observations on the various diagrams. They should be looking for similarities and differences in the other diagrams compared with their group’s diagram.
Explain
- After the students have observed each group’s carbon cycle diagram, have them retrieve their own diagram and return to their seats. Let them make any changes they feel they would now like to incorporate into their own diagrams. Discuss:
Student poster example for their journey as a carbon atom.
Photo Credit: Mary Servino
- What were some of the ways your diagram was like other diagrams and different from them? (Answers will vary.)
- According to your diagram, how does carbon enter the atmosphere? (Both biologic and human-caused processes.)
- How do trees and other plants help remove carbon from the atmosphere? (Trees take in carbon [sequester it] through photosynthesis and store it in their tissues. In this way, forests can act as carbon sinks.)
Elaborate
- Ask your students how they might calculate how much carbon a tree can store. (The amount of carbon an individual tree can store depends on the species of tree, how large the tree is, and how old it is. In general, the larger the tree, the more carbon it can store. The denser the wood, the more carbon it stores. Also, the faster growing the tree, the more quickly it will add carbon to its stores.)
- Optional: To explore the concept of density, give each group a sample of wood (see Getting Ready), and have them use the balance and the graduated cylinder to determine the density. Density is found by taking the mass (weighing) and dividing it by the volume (using the graduated cylinder to get the volume of an irregular shaped object). Ask groups to share their findings with the class. A simpler variation on this idea would be to select one wood sample and model the experiment for the whole class. Lead a discussion, asking such questions as:
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- Why might different samples have a different density? (Some of the answers might include measurement error and or that different types of wood have different densities. Students may want to say that size is an issue, but this would be a misconception. Remind them that density is a ratio of mass divided by volume. If a piece of wood is denser it has more matter per volume than a less dense piece of wood, but it is not necessarily larger.)
- What elements do you think are present in a piece of wood? (It is mostly carbon, but also includes hydrogen, oxygen, and other elements.)
- What does density tell us about how much carbon is in the wood? (Wood with higher density has more carbon.)
- Explain to students that they will select a specific tree and will determine the approximate amount of carbon stored in it. Point out that foresters use a formula for calculating carbon in a tree that takes into account four measurements: its diameter at breast height, its height, its wood density, and the amount of water in the tree. Since wood density and the amount of water in a tree are difficult to measure in a living tree, students will use just two of these measurements to approximate the amount of carbon.
- Divide the class into small groups, and have each group select a tree to measure. Using the directions on the Tree Data student page, have students measure the diameter and height of the tree using either metric or English units.
- Have students use either the How Much Carbon Is in a Tree? (Metric Units) or How Much Carbon Is in a Tree? (English Units) student page to estimate the amount of carbon in the tree they measured.
- Lead a class discussion about the carbon storage results that students calculated.
- What was the greatest amount of carbon that was stored by a single tree? Was it clearly the biggest tree? What was the least amount of carbon stored by a tree?
- What was the average amount of carbon stored by the sample of trees we measured?
- For how long will the carbon in each of those trees be stored? (As long as the tree is alive or the wood exists in a product.) What is likely to kill trees in this area? (Possible answers include development, road widening and construction, lightning, old age, insects, disease, etc.)
- Where does the carbon go after it leaves the tree? (Some will be stored in soil; some will move to the atmosphere; some will be eaten and become part of animal biomass.)
- How does carbon storage relate to growth? (The bigger the tree grows, the more carbon it stores.)
- What does carbon content tell us about the age of the tree? (More carbon means an older tree.)
