BSC 314 Lecture Notes - Lecture 64: Lightning, Denitrification, Nitrification
28 Jun 2018
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The Carbon Cycle
Carbon—the element that defines life—enters the biota through photosynthesis while
the oxygen released in the process makes possible aerobic respiration of all living
things. Molecules that contain carbon are the major constituents of living tissues, but the
amount of carbon in active biosphere cycling is minor compared to the amount held in
abiotic reservoirs such as sedimentary rocks, fossil fuel deposits, and deep sea
sediments.
Respiration and photosynthesis are the driving forces of the carbon cycle. Carbon
enters the biosphere as atmospheric carbon dioxide (CO 2), which is incorporated by
photosynthetic organisms into carbohydrates. It leaves, also as CO 2, through the
respiration of organisms. The carbon cycle is thus bound with that of oxygen and,
through oxidation reduction reactions, to other elements of importance to organisms. ‐
Removal of CO 2 from the atmosphere by terrestrial vegetation in photosynthesis is
balanced by the return due to respiration of plants and decomposer organisms in the
soil. Plankton organisms in the surface waters of the oceans also remove and return
atmospheric carbon in about the same proportions, although a minor portion of
dissolved CO 2 is lost in the deep ocean and buried in the sediments. The atmosphere
links all compartments in the cycle, and the changes in atmospheric CO 2 are a
measure of the health of the ecosystem.
A disturbing feature of the current carbon cycle is the net atmospheric increase of CO 2.
In the last 50 years of accurate measurements, annual increases of about 0.4 percent
(1.5 ppm) have occurred consistently—indicating that a cycle in place for the last
100,000 years no longer is in balance. An increase of as much as 30 percent in the last
200 years (since the beginning of the Industrial Revolution) is circumstantial evidence
that the burning of fossil fuels together with human alterations to the natural vegetation
cover of large areas of the globe may be the cause. It is suspected that the increased
levels of CO 2 will trap more reflected radiation in the atmosphere and increase the
temperature on the surface of the Earth causing a greenhouse effect.
The Nitrogen Cycle
The atmosphere holds the greatest reservoir of nitrogen, but, as gaseous, triple bonded ‐
N 2, it is chemically inert and unusable by plants and almost all other organisms. A few
kinds of bacteria that possess the enzyme nitrogenase are the exceptions. They are
able to convert (reduce) N 2 to ammonium ions (NH 4 +), which many organisms,
including plants, are able to metabolize. The process is called nitrogen fixation and
ranks equally with photosynthesis in significance to life. When organisms die, decay
bacteria and fungi release the fixed N of organic compounds. New organisms then
reformulate it into amino acids—hence proteins—together with nucleic acids,
nucleotides, coenzymes, and vitamins all of which are essential to life.
The denitrifying bacteria return some nitrogen to the atmosphere as N 2. This process
— denitrification—is anaerobic and takes place in almost all types of soil.
Microorganisms, therefore, control the major phases of the N cycle.
find more resources at oneclass.com
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Document Summary
Carbon the element that defines life enters the biota through photosynthesis while the oxygen released in the process makes possible aerobic respiration of all living things. Respiration and photosynthesis are the driving forces of the carbon cycle. Carbon enters the biosphere as atmospheric carbon dioxide (co 2), which is incorporated by photosynthetic organisms into carbohydrates. It leaves, also as co 2, through the respiration of organisms. The carbon cycle is thus bound with that of oxygen and, through oxidation reduction reactions, to other elements of importance to organisms. Removal of co 2 from the atmosphere by terrestrial vegetation in photosynthesis is balanced by the return due to respiration of plants and decomposer organisms in the soil. Plankton organisms in the surface waters of the oceans also remove and return atmospheric carbon in about the same proportions, although a minor portion of dissolved co 2 is lost in the deep ocean and buried in the sediments.
