Answers
THE CARBON CYCLE
Sunlight
Burning of
Fossil Fuels
Atmospheric
carbon
Photosynthesis
Plant
Respiration
Animal
Respiration
Decomposing
Matter
Dissolved
CO2
Fossil Fuels
1. Fill in the diagram of the carbon cycle above with the following words or phases:
 Animal respiration
 Fossil fuels
 Atmospheric carbon
 Photosynthesis
 Burning of Fossil Fuels
 Plant respiration
 Decomposing matter
 Sunlight
 Dissolved CO2
2. Name the process in which plants convert CO2 into carbon compounds. Write out the
chemical equation that describes this process.
Photosynthesis, 6CO2 + 6H2O
(sunlight)
C6H12O6 + 6O2
(sugar)
Energy is stored in the chemical bonds of molecules, specifically sugar.
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Answers
THE CARBON CYCLE
3. How do plants and animals help to maintain a balance of carbon dioxide in the atmosphere?
(Hint: Think back to when we talked about photosynthesis and cellular respiration)
When plants photosynthesize, they remove CO2 from the atmosphere. When
animals consume plants, the food obtained is broken down and “burned” by
mitochondria. This process is called cellular respiration. When plants and
animals undergo cellular respiration, CO2 is returned back into the atmosphere.
Through photosynthesis and cellular respiration, CO 2 is continually cycled
between producers and consumers.
4. Explain in detail how carbon is cycled through the atmosphere, the ocean, and the land; and
describe the molecular compounds that the carbon is in as it is cycled through the biosphere.
Use your textbook page 70-71 and the picture above.
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Through photosynthesis, atmospheric CO2 is taken in by producers to
make food. Producers then become food for consumers. Food is
made up of carbon-rich molecules like carbohydrates, proteins,
lipids, and nucleotides. Organisms use cellular respiration to
“extract” energy from food. This process releases CO2 back into the
atmosphere.
Some plant and animal remains become buried. Decomposition,
pressure, and heat turn this organic matter into fossil fuels (coal and
oil) over millions of years. The burning fossil fuels, forests, and
volcanic activity releases CO2 back into the atmosphere.
Some of the CO2 dissolves in rainwater and is then absorbed by the
sea where marine organisms like zooplankton and coral use it to
build their shells. When these animals die, their shells collect on the
seafloor. Over millions of years, heat and pressure turn layers of
shell sediment into calcium carbonate (limestone). The weathering
of limestone returns CO2 to the atmosphere.
5. Atmospheric carbon dioxide produces a “greenhouse effect” by trapping heat near Earth’s
surface. What human activities might tend to increase the greenhouse effect?
1. Burning fossil fuels releases carbon dioxide into the atmosphere
a. Coal-fire plants for electricity to heat homes, operate factories, etc.
b. Driving automobiles (burning gasoline)
2. Agricultural field burning
3. Raising cattle (methane from manure)
4. Disposing of waste (landfills)
5. Industrial processes
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