Carbon Cycle
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: January 4, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Carbon Cycle
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Carbon Cycle
• Give an overview of the carbon cycle.
• Discuss the roles of photosynthesis and cellular respiration in the carbon cycle.
• Describe processes that have led to increased atmospheric carbon dioxide levels.
How could releasing this much pollution into the atmosphere not be a poor idea?
Burning of fossil fuels, such as oil, releases carbon into the atmosphere. This carbon must be cycled - removed
from the atmosphere - back into living organisms, or it stays in the atmosphere. Increased carbon in the atmosphere
contributes to the greenhouse effect on Earth.
The Carbon Cycle
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 Figure 1.1.
The carbon cycle is discussed in the following video: http://www.youtube.com/watch?v=0Vwa6qtEih8 (1:56).
Why is recycling carbon important? Recall that 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.
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
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FIGURE 1.1
The Carbon Cycle. Carbon moves from
one reservoir to another in the carbon
cycle. What role do organisms play in this
cycle?
cycle. Some of the different forms in which carbon appears are described in Table 1.1. KEY: C = Carbon, O =
Oxygen, H = Hydrogen
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Chapter 1. Carbon Cycle
TABLE 1.1: Forms of Carbon in the Carbon Cycle
Form of Carbon
Carbon Dioxide
Carbonic Acid
Bicarbonate Ion
Organic Compounds
Other
Compounds
Carbon
Chemical Formula
CO2
H2 CO3
HCO3 −
Examples:
C6 H12 O6
(Glucose),
CH4
(Methane)
Examples:
CaCO3
(Calcium
Carbonate),
CaMg(CO3 )2 (Calcium
Magnesium Carbonate)
State
Gas
Liquid
Liquid(dissolved ion)
Solid Gas
Main Reservoir
Atmosphere
Ocean
Ocean
Biosphere Organic Sediments (Fossil Fuels)
Solid Solid
Sedimentary
Rock,
Shells, Sedimentary Rock
Carbon in Carbon Dioxide
Carbon cycles quickly between organisms and the atmosphere. 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.
Summary
• Carbon must be recycled through living organisms or it stays in the atmosphere.
• Carbon cycles quickly between organisms and the atmosphere.
• Due to human activities, there is more carbon dioxide in the atmosphere today than in the past hundreds of
thousands of years.
Explore More
Use this resource to answer the questions that follow.
• The Carbon Cycle at http://www.physicalgeography.net/fundamentals/9r.html .
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1.
2.
3.
4.
What are three ways carbon is stored on Earth?
Where on Earth is most carbon stored?
How do ecosystems gain most of the carbon?
How is carbon released from ecosystems?
Review
1.
2.
3.
4.
What is the role of the carbon cycle?
Why is cycling carbon important?
Describe a major method that carbon is cycled.
How have human activities increased atmospheric carbon dioxide levels?
References
1. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. An illustration of the carbon cycle . CC BY-NC
3.0
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