The Carbon Cycle

Carbon atoms are found in the molecules that make up living organisms. Carbon is an important part of soil humus, which is formed when dead organisms decay, and it is found in the atmosphere as carbon dioxide gas (CO2). The carbon cycle describes how carbon molecules move between the living and nonliving world, as shown in Figure 13.

The carbon cycle begins when producers remove CO2 from the air during photosynthesis. They use CO2, water, and sunlight to produce energy-rich sugar molecules. Energy is released from these molecules during respiration—the chemical process that provides energy for cells. Respiration uses oxygen and releases CO2. Photosynthesis uses CO2 and releases oxygen. These two processes help recycle carbon on Earth.

Reading Check

How does carbon dioxide enter the atmosphere?

Human activities also release CO2 into the atmosphere. Fossil fuels such as gasoline, coal, and heating oil are the remains of organisms that lived millions of years ago. These fuels are made of energy-rich, carbon-based molecules. When people burn these fuels, CO2 is released into the atmosphere as a waste product. People also use wood for construction and for fuel. Trees that are harvested for these purposes no longer remove CO2 from the atmosphere during photosynthesis. The amount of CO2 in the atmosphere is increasing. Extra CO2 could trap more heat from the Sun and cause average temperatures on Earth to rise.


Topic: Life Processes

Visit for Web links to information about chemical equations that describe photosynthesis and respiration.

Activity Use these equations to explain how respiration is the reverse of photosynthesis.

I review


The Cycles of Matter

• Earth's biosphere contains a fixed amount of water, carbon, nitrogen, oxygen, and other materials that cycle through the environment.

The Water Cycle

• Water cycles through the environment using several pathways.

The Nitrogen Cycle

• Some types of bacteria can form nitrogen compounds that plants and animals can use.

The Carbon Cycle

• Producers remove CO2 from the air during photosynthesis and produce O2.

• Consumers remove O2 and produce CO2.

I review

Self Check

1. Describe the water cycle.

2. Infer how burning fossil fuels might affect the makeup of gases in the atmosphere.

3. Explain why plants, animals, and other organisms need nitrogen.

4. Think Critically Most chemical fertilizers contain nitrogen, phosphorous, and potassium. If they do not contain carbon, how do plants obtain carbon?

Applying Skills

5. Identify and Manipulate Variables and Controls

Describe an experiment that would determine whether extra carbon dioxide enhances the growth of tomato plants.

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SECTION 2 Cycles in Nature E ♦ 49

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