Carbon Cycle Study Guide

1.3 Carbon Cycle (Bio.1.3)

Explore this Phenomenon

1. What do you notice about the amount of atmospheric Carbon Dioxide Concentration over the past six decades?
2. What do you think is causing the trend that is being observed with Carbon Dioxide Concentration?

Bio.1.3 Carbon Cycle

Analyze and interpret data to determine the effects of photosynthesis and cellular respiration on the scale and proportion of carbon reservoirs in the carbon cycle. Emphasize the cycling of carbon through the biosphere, atmosphere, hydrosphere, and geosphere and how changes to various reservoirs impact ecosystems. Examples of changes to the scale and proportion of reservoirs could include deforestation, fossil fuel combustion, or ocean uptake of carbon dioxide. (PS3.D, LS1.C, LS2.B)

It is important to understand how photosynthesis and cellular respiration affect the scale and proportion of carbon in different reservoirs. As you read this chapter, look for ways that photosynthesis and cellular respiration move carbon from one reservoir to another, and how this affects the proportion of carbon in each reservoir.

Matter Cycles During Photosynthesis and Respiration

The Carbon Cycle

How does carbon move through different ecosystems and what are some of those effects? Flowing water can slowly dissolve carbon in sedimentary rock. Most of this carbon ends up in the ocean. The deep ocean can store carbon for thousands of years or more. Sedimentary rock and the ocean are major reservoirs of stored carbon. Carbon is also stored for varying lengths of time in the atmosphere, in living organisms, and as fossil fuel deposits. These are all parts of the carbon cycle, which is shown in the next figure.

The Carbon Cycle. Carbon moves from one reservoir to another in the carbon cycle. What role do organisms play in this cycle?

Why is recycling carbon important? Carbon is the cornerstone of organic compounds, the compounds necessary for life. But do organisms make their own carbon? Do they have the genes that encode proteins necessary to make carbon? No. In fact, there are no such genes. Carbon must be recycled from other living organisms, from carbon in the atmosphere, and from carbon in other parts of the biosphere.

Photosynthesis and cellular respiration recycle carbon. Autotrophs take carbon from the atmosphere and convert it to organic molecules. Protein, carbohydrates, and fats are all parts of heterotrophs and all contain carbon. When organisms break down food to produce energy, they break down protein, carbohydrates, and fat, and release carbon dioxide.

Carbon in the Atmosphere

Though carbon can be found in ocean water, rocks and sediment and other parts of the biosphere, the atmosphere may be the most recognizable reservoir of carbon. Carbon occurs in various forms in different parts of the carbon cycle. Some of the different forms in which carbon appears are described in the Table below.

 Carbon in Carbon Dioxide

Carbon cycles quickly between organisms and the atmosphere via photosynthesis and respiration. In the atmosphere, carbon exists primarily as carbon dioxide (CO2). Carbon dioxide cycles through the atmosphere by several different processes, including those listed below.

• Living organisms release carbon dioxide as a byproduct of cellular respiration.
• Photosynthesis removes carbon dioxide from the atmosphere and uses it to make organic compounds .
• Carbon dioxide is given off when dead organisms and other organic materials decompose.
• Burning organic material, such as fossil fuels, releases carbon dioxide.
• Carbon cycles far more slowly through geological processes such as sedimentation. Carbon may be stored in sedimentary rock for millions of years.
• When volcanoes erupt, they give off carbon dioxide that is stored in the mantle.
• Carbon dioxide is released when limestone is heated during the production of cement.
• Ocean water releases dissolved carbon dioxide into the atmosphere when water temperature rises.
• Carbon dioxide is also removed when ocean water cools and dissolves more carbon dioxide from the air.

Because of human activities, there is more carbon dioxide in the atmosphere today than in the past hundreds of thousands of years. Burning fossil fuels and has released great quantities of carbon dioxide into the atmosphere. Cutting forests and clearing land has also increased carbon dioxide into the atmosphere because these activities reduce the number of autotrophic organisms that use up carbon dioxide in photosynthesis . In addition, clearing often involves burning, which releases carbon dioxide that was previously stored in autotrophs. Carbon cycles through the atmosphere relatively quickly.

Human Actions Impact the Carbon Cycle

Humans have changed the natural balance of the carbon cycle because we use coal, oil, and natural gas to supply our energy demands. Fossil fuels are a sink for CO2 when they form, but they are a source for CO2 when they are burned. The equation for combustion of propane, which is a simple hydrocarbon looks like this: The equation shows that when propane burns, it uses oxygen and produces carbon dioxide and water. So when a car burns a tank of gas, the amount of CO2 in the atmosphere increases just a little. Added over millions of tanks of gas and coal burned for electricity in power plants and all of the other sources of CO2, the result is the increase in atmospheric CO2 seen in the Figure above.

The second largest source of atmospheric CO2 is deforestation (Figure below ). Trees naturally absorb CO2 while they are alive. Trees that are cut down lose their ability to absorb CO2. If the tree is burned or decomposes, it becomes a source of CO2. A forest can go from being a carbon sink to being a carbon source.

This forest in Mexico has been cut down and burned to clear forested land for agriculture .

Effects of Carbon in the Oceans on Ecosystems

As the oceans absorb CO2, the minerals in the seawater change. Normally, the ocean has a lot of calcium carbonate available. Shellfish and coral use this mineral to build and repair their shells. When an area of the ocean experiences an increase in carbon dioxide, a chemical reaction makes the calcium carbonate unavailable. Shells thin. Coral stops growing. Animals die. Many sea animals spend at least part of their life cycles on the reefs. As these ecosystems disappear, species will die out, the food web will experience disruptions, and our oceans will become less diverse.

Many oceanographers say that this threat to the ecosystem is actually more urgent than the threat posed by global warming. Coral reefs take thousands of years to grow, but can deteriorate in only a few decades. Without some sort of a change, we may be looking at a future where the seas are a little more acidic, and a lot less lively.

See for Yourself

• https://www.youtube.com/watch?v=t1AyIs1xNCk

Putting It Together

1. What do you notice about the amount of atmospheric Carbon Dioxide Concentration over the past six decades?
2. What do you think is causing the trend that is being observed with Carbon Dioxide Concentration?
3. How is this increase in Carbon affecting different ecosystems on the Earth?