Lesson 5: Carbon in the Biosphere


You've been studying the carbon cycle and how this relates to climate. In the last activity, the CO2 spike model showed you one way that scientists study how climate responds to greenhouse gases like CO2. You also investigated CO2 moving between air and water. This was a good model for the ocean and atmosphere. The color change of the BTB indicator helped you see that.

In Lesson 2, you saw in the carbon cycle game that the biosphere was part of the carbon cycle. The biosphere is plants and animals — including organisms like you! But you didn't really look at how or why the carbon moved between the atmosphere and the biosphere. That's the topic of this lesson. It will include some words that might be new to you. The big ideas of Lesson 5 are:

5a. Do You Have Carbon Breath?!

In Lesson 2 you used BTB indicator to "see" whether carbon was moving in or out of water. You know carbon is in your body, but what about your breath? Listen to your teacher regarding an activity that you will do in a team.

  1. Set-up for the investigation with these steps.
    1. Get one test tube or small beaker for your team. Get 1 straw per person.
    2. Your teacher will have a container with water and BTB solution. It should be bright blue. Have one person on your team add about 2 inches of solution in your container.
    3. Gently swirl the blue solution with one end of your straw. Without splashing gently mix in some air.
    4. Did you see any changes with the stirring? Write in your notebook what you saw.
  2. Select one person in your team. You need to test whether they have "carbon-breath." Have them gently blow through the straw into the solution. What do you observe?
  3. The table below summarizes carbon movement from lesson 2. From that table, what do you think happened in step 2 above?
Experiment Color change Carbon movement
Seltzer water Yellow → blue (or blue'ish green) Carbon out of water
Dry ice (solid CO2) above water Blue → yellow Carbon in to water
  1. Did your team observe this?
    • No color change when gently stirring in air
    • Yes, color change when teammate breathed into the solution.

If you got the result above, try to explain this as a team: the air that you breathe out (exhale) has more carbon than the air that you breathe in (inhale). How can that be? How can your breath have more carbon than the air you just breathed in?

5b. Photosynthesis and Respiration

You've been learning that CO2 relates to climate change. To counter that, perhaps you've heard someone say, "Plant a tree!" To understand this better, look at the picture of a pinyon pine seed. Then look at the grown pine tree. The mass of the tree is much greater than the seed. How does the tree gain mass and get large?

Seed and Tree

Very little of the tree comes from soil. The mass actually comes from CO2 in the air. Though it doesn't seem like CO2 gas can produce a tree, it does! Photosynthesis is the process where plants use carbon dioxide (CO2), water (H2O), and sunlight (Energy) to make glucose (C6H12O6) and oxygen (O2). Next, the plant uses glucose from the reaction to make the cells and tissues of bark, wood, or pine needles. The reaction is shown as:

Photosynthesis: 6CO2 + 6H2O + Energy → C6H12O6 + 6O2

Thus, photosynthesis is the process that moves carbon from the atmosphere to the biosphere. However, only some organisms are able to use light ("photo-") to make, or synthesize, their cells and tissues. These are called primary producers. Primary producers include plants, algae, and some bacteria. They are the first step in food webs that all other organisms rely on. For example, a horse is not a primary producer, but it does need to eat grass, which is a primary producer.

It may seem hard to believe that organisms of the biosphere rely on carbon in a gas in the air. But they do. To help you with this, watch the time-lapse movie of a plant growing from a seed. It is not in soil, but only a wet paper towel. The seed and new leaves convert CO2 to mass as the young plant grows.

Because your computer is not online, go to the video folder and open TimeLapseRadish.mp4 in your computer's media player.

Organisms grow as they add carbon. But what happens when they die? You've probably seen plants or foods that are rotten or decayed. Decomposers include the bacteria, fungi, and worms that cause that rot and decay. When decomposers do this, they get their energy from dead, organic matter. In fact, nearly all organisms get their energy this way, including you when you eat! The process is called cellular respiration because it happens in the cells of your body. For an organism breathing in oxygen (O2), you can show "food" as the sugar glucose (C6H12O6). When cells of an organism produce energy, they also generate carbon dioxide (CO2) and water (H2O). The reaction is then:

Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Discuss the questions below with your reading team. Get ready to share your ideas in a class discussion.

  1. Review the diagram you made of the carbon cycle in lesson 2. As organic matter dies and decomposes, what reservoir does the carbon flow to?
  2. Look at the reactions for photosynthesis and respiration.
    1. List a way that they are similar.
    2. List a way that they are different.
  3. You excrete some of the food that you eat. Other food you eat is used as energy in your cells. How do you think you are able to get rid of that carbon? Explain your evidence.
    (See question 4 as a hint).

Climate Effect: Bark Beetle Infestations
Drought kills trees, and recent warming has made the die-off even worse, with massive swaths of dead ponderosa and pinyon trees all over the Southwest. Too little water and a warmer climate are the root causes, but the final cause of death is often bark beetles. Healthy trees can stave off their attacks, but trees weakened by drought succumb and perish.

5c. The Breathing Biosphere

In the biosphere, photosynthesis and respiration tend to balance. In photosynthesis, carbon moves from the atmosphere to the plants. In respiration, carbon goes from the biosphere back to the atmosphere (respiration). There tends to be no net change in the average CO2 in the atmosphere. This is why people talk about "the breathing biosphere." But just like the seasons on the Colorado Plateau, the breathing biosphere also has seasons. You can watch the video below. You can see the see-saw of seasons as Earth breathes.

Because your computer is not online, go to the video folder and open Net_Primary_Product.mov in your computer's media player.

A famous record of atmospheric CO2 was begun in 1958 by Charles David Keeling. He collected data on the top of Mauna Loa volcano in Hawaii. There, as air mixed in the northern hemisphere, he could sample the breathing biosphere. His data are in the graph are below.

Atmospheric Carbon Dioxide

Discuss the following questions with your class.

  1. What are the axes for the graph?
  2. What trend with time do you see for CO2 ?
  3. What pattern do you see annually with carbon entering and leaving the air at Mauna Loa? Look at the annual cycle, giving "wiggles" in the data.
  4. A member of your team had "carbon breath." How would you and the class answer this question — Does Earth have carbon breath? What might that mean?
    (HINT: Think about how respiration might add CO2 to air more quickly than photosynthesis can remove CO2)

For the northern hemisphere, Keeling realized that he was not only looking at the breathing biosphere. The breathing biosphere explained the yearly ups-and-downs due to photosynthesis and respiration. However, that didn't explain the rise in CO2 from 1958 to 2010. Over time, the rise was large compared with the response of the biosphere.

You can see the data in a bit more detail. The graph below shows CO2 data measured at Mauna Loa for the period 2004-2010. The green curve shows changes only from natural processes. The black curve shows the actual data — instead of just going up and down each year, the slope of the curve increased over time. Only a human part explains the slope. You'll investigate this more in the interactive below.

Carbon Dioxide, Mauna Loa

You can analyze the Keeling data yourself in the model below. You will adjust values for photosynthesis, respiration, and human inputs. This helps you see how combinations of these factors each month can explain the observed data.

Launch the model below. Note the red and green bars. The red bars show how much CO2 is going into the atmosphere from respiration. The green bars show how much CO2 is taken out of the air from photosynthesis. You need to do a bunch of clicking to try to make the amount of photosynthesis and respiration match the observed data. To make the curve change for a given month:

A group of researchers has estimated the rates of photosynthesis (P) and respiration (R) for air at Mauna Loa. To get their data in the model, click on "best fit." You can also return to values of P & R of 0.0 or 0.5. You can also adjust the human input with the slidebar at the bottom of the model. As you adjust it, test large and small growth rates in CO2 (in ppm per year) due to humans.

Do these steps with your team. Be prepared to discuss this with your class.

  1. Answer these questions about photosynthesis and respiration at Mauna Loa.
    1. What is a month when respiration is greater than photosynthesis?
    2. What is a month when photosynthesis is greater than respiration?
    3. Are there times when respiration (i.e., carbon to the atmosphere) does not occur?
  2. Which value for human contribution best matches the measured data?
  3. Say that the human rate of CO2 increase goes from 1.9 to 2.5 ppm per year. You need to model how to counteract that increase.
    1. Do you counteract that increase by changing photosynthesis, respiration, or both in the model? Describe your thinking in your notebook.
    2. What did you find?
  4. What sources do you think relate to the human part of this curve? Is there a source you can think of from your community?