Objectives
Describe the path of carbon in the carbon cycle.
Explain how photosynthesis is related to climate.
Key Terms
carbon cycle
greenhouse effect
For the past several chapters, you have been navigating the
microscopic world of cells as though you were a miniature explorer.
Now imagine you could zoom out into space and look at planet Earth as
a whole. Keep reading to see how these cellular processes fit into your
new perspective of life on Earth.
The Carbon Cycle
Some of the processes that occur on a global scale on Earth depend on
the metabolism of tiny chloroplasts and mitochondria. An example is
the carbon cycle, the process by which carbon moves from inorganic to
organic compounds and back. Through photosynthesis, producers such
as grass convert inorganic carbon dioxide to organic compounds.
Consumers such as a Cape buffalo obtain the organic compounds by
eating the producers. Cape buffalo may in turn be eaten by a lioness or
another consumer. Ultimately, cellular respiration by both producers
and consumers returns carbon dioxide to the atmosphere.
No other chemical process on the planet matches the output of
photosynthesis. Earth's plants and other photosynthetic organisms
make about 160 billion metric tons of organic material per year. That's
about equal to 80 trillion copies of this book—25 stacks of books
reaching from Earth to the sun!
Photosynthesis and Global Climate
As you have just read, a key element of the carbon cycle is carbon
dioxide. Plants use carbon dioxide to make sugars in photosynthesis,
and most organisms give off carbon dioxide as waste from cellular
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respiration. Though any one organism may use or produce relatively
small amounts of carbon dioxide, the total effect of all the organisms on
Earth has a very large effect on the amount of carbon dioxide in the
atmosphere.
Before this century, carbon dioxide made up about 0.03 percent (300
parts per million) of Earth's atmosphere. This amount of carbon dioxide
is enough to provide plants with plenty of carbon for photosynthesis.
Carbon dioxide in the atmosphere also traps heat from the sun that
would otherwise escape from Earth back into space (Figure 8-16). This
important property, known as the greenhouse effect, keeps the world
climate warm enough for living things. The greenhouse effect keeps the
average temperature on Earth some 10°C warmer than it would be
otherwise.
Figure 8-16
Some heat radiating from Earth's surface back out
toward space is trapped by carbon dioxide (along with
some other types of gases) in the atmosphere. This
greenhouse effect keeps Earth warm enough for living
things.
In the past century, the amount of atmospheric carbon dioxide has been
rising, reaching more than 360 parts per million. In Chapter 36, you'll
read more about this change and about its possible effects.
Concept Check 8.4
1. Give an example of carbon moving from an inorganic compound to
an organic compound in the carbon cycle. Give an example of carbon
moving from an organic to an inorganic compound.
2. How is carbon dioxide important to Earth's climate?
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Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights
reserved.
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Wednesday, October 19, 2011 9:21:04 AM CT