The Carbon Cycle

In the process of "feeding" (acquiring the energy they need to maintain themselves and grow, i.e., respire), bacteria process carbon (and nitrogen) in a way that allows these elements to be recycled (Fig. 25.7). Bacteria "eat" organic matter (carbohydrates), obtaining the energy present in the molecules. As a result of this activity (respiration), they convert carbohydrates to carbon dioxide. One of the two raw materials of photosynthesis, carbon dioxide constitutes 0.03% of our atmosphere. The combined plant life of the oceans and the land masses is estimated to use about 14.5 billion metric tons (16 billion

Water in the

Water in the

Co2 Fixation Photosynthesis
Figure 25.6 The water cycle.

496 Chapter 25

burning of forests, fuel wood, and organic debris

- carbon dioxide (CO2) in air combustion of fossil fuels for vehicles, electricity, and heat burning of forests, fuel wood, and organic debris

- carbon dioxide (CO2) in air combustion of fossil fuels for vehicles, electricity, and heat

Carbon Obtained

calcium-containing sediments

Figure 25.7 The carbon cycle.

L O'Keefe calcium-containing sediments

Figure 25.7 The carbon cycle.

L O'Keefe tons) of carbon obtained from carbon dioxide every year. Respiration by all living organisms constantly replaces carbon dioxide, with perhaps as much as 90% or more of it being produced by the incredible numbers of decay bacteria and fungi as they decompose tissues.

The burning of fossil fuels by the internal combustion engines of industry and transportation releases lesser but significantly increasing amounts of carbon dioxide into the air, and a small amount originates with fires and volcanic activity. At the present rate of use by photosynthetic plants, it has been calculated that all of the carbon dioxide of our atmosphere would be used up in about 22 years if it were not constantly being recycled.

Scientists have found that plant growth initially can be accelerated by increasing levels of carbon dioxide in the air, and some commercial nurseries pump carbon dioxide along rows of bedding plants as a "fertilizer." However, the accelerated growth soon becomes limited by other factors in the environment and ultimately does not necessarily result in crop yield increases. Recent studies indicate that while C3 plants may respond to increased carbon dioxide levels, C4 plants may not, suggesting that in nature, the response of C3 plants to higher levels of carbon dioxide may give them a competitive edge over C4 plants.

Anaerobic bacteria produce large volumes of carbon-containing methane gas, which is discussed on page 504.

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Essentials of Human Physiology

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  • ermias
    How is carbon obtained?
    8 years ago

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