Download THE NITROGEN CYCLE Terms List

THE NITROGEN CYCLE
The nitrogen cycle describes the conversion of nitrogen between different chemical forms. The majority of the earth’s atmosphere (about
78%) is composed of atmospheric nitrogen (N2), but it is not in a form that is usable to living things. Complex species interactions allow
organisms to convert nitrogen to usable forms and exchange it between themselves. Nitrogen is essential for the formation of amino acids
and nucleotides. It is essential for all living things. Described below are five main processes that make up the nitrogen cycle.
Nitrogen Fixation: In order for organisms to use atmospheric nitrogen (N2), it must be “fixed” or converted into ammonia (NH3) and
ammonium (NH4+). This can happen occasionally through a lightning strike, but the bulk of nitrogen fixation is done by free living or
symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen (N2) with hydrogen to produce ammonia
(NH3) and ammonium (NH4+). It is then further converted by the bacteria to make their own organic compounds, like proteins and DNA.
Some nitrogen fixing bacteria, like Rhizobium, live in the root nodules of legumes where they produce ammonia (NH3) in exchange for
sugars. Other nitrogen fixing bacteria live in the soil and produce ammonia (NH3) and ammonium (NH4+).
Ammonification: In ammonification, bacteria or fungi convert the organic nitrogen from dead organisms back into ammonium (NH4+).
Nitrification can also work on ammonium. It can either be cycled back into plants in a usable form or returned to the atmosphere through
denitrification. Bacteria like Bacillus and Proteus are examples of ammonifying bacteria.
Nitrification: Nitrification is the conversion of ammonia (NH3) to nitrate (NO3-). It is usually performed by soil living bacteria, such as
Nitrobacter. This is important because plants can assimilate nitrate into their tissues, and they rely on bacteria to convert it from
ammonia (NH3) and ammonium (NH4+) to a usable form.
Assimilation: In assimilation, plants take nitrogen from the soil by absorption through their root hairs in the form of either nitrate (NO3-)
or ammonium (NH4+). Most nitrogen obtained by terrestrial animals can be traced back to the eating of plants at some stage of the food
chain.
De-nitrification: Nitrogen in its nitrate form (NO3-) is converted back into atmospheric nitrogen gas (N2) by bacterial species such as
Pseudomonas and Clostridium, usually in anaerobic (without oxygen) conditions. These bacteria use nitrate as an electron acceptor
instead of oxygen during respiration.
Use the terms listed below to complete the diagram of the nitrogen cycle on page 2. Note that nitrogen fixation is used twice.
Terms List
Processes: Assimilation by plants, Nitrogen fixation, Ammonification, Nitrification, Nitrogen fixation, De-nitrification
Nitrogen-based substances:
Nitrates (NO3-),
Ammonium (NH4+),
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Nitrogen (N2) in atmosphere
Name _________________________________
THE NITROGEN CYCLE
Period _____ Date ______________ Seat ____
Use the terms list at the bottom of page 1 to complete the nitrogen cycle diagram below.
Atmospheric nitrogen (N2)
De-nitrification
Assimilation
by plants
Nitrogen
Fixation
Nitrate
(NO3-)
Ammonification
Nitrification
Nitrogen Fixation
Ammonium
(NH4+)
2
Name _________________________________
THE NITROGEN CYCLE
Period _____ Date ______________ Seat ____
1. Explain in detail how nitrogen is cycled through the biosphere with the help of nitrogen fixing bacteria and other
organisms; and describe the molecular compounds that the nitrogen is in as it is cycled through the biosphere. Use your
textbook page 71 and the picture above.
Atmospheric nitrogen (N2) is changed into ammonia (NH3) by nitrogen-fixing bacteria (like Rhizobium)
found in the soil and in root nodules of certain plants, like legumes (peanuts, alfalfa). This process is
called nitrogen fixation. Some plants can take up ammonia directly and use it to make amino acids (for
proteins) and nucleotides (for DNA, RNA, and ATP). Ammonia can also be made by soil bacteria (like
Bacillus) that decompose organic matter (from dead plants and animals and animal manure and urine).
This process is called ammonification. Other soil bacteria (like Nitrobacter) change the ammonia into
nitrates (NO3-) that can also be assimilated by plants. Finally, soil bacteria like Clostridium change
nitrates (NO3-) back into nitrogen gas (N2) that is then released back into the atmosphere.
2. How does the way that matter flows through an ecosystem differ from the way that energy flows through an ecosystem?
Most energy that drives life comes from the sun, which is outside the biosphere. This energy is
converted from one form to another as organisms utilize it. For example, electromagnetic energy
(sunlight) is converted by producers into chemical energy (food). This chemical energy is ultimately
converted into thermal energy (heat). In time, thermal energy is radiated back to into outer space.
For the most part, matter (atoms) stays within the biosphere, being part of different molecules as time
goes on. For example, chances are some nitrogen atoms you have breathed in at one time were part of
a molecule (like urea) that passed through the urinary tract of a dinosaur.
3. A farmer plants a field of corn and fertilizes it with a low-nitrogen fertilizer. The corn receives plenty of sunlight and
water, but does not grow well. What might be preventing the corn from growing? Use your textbook page 73.
The corn may not be getting enough nitrogen or some other nutrient (phosphorus, potassium). As a
solution, farmers will plant corn in a field that was planted the year before with a legume (alfalfa,
peanuts, and soybeans). Legumes are able to replenish the soil with nitrogen that the growing corn will
need. In addition, by alternating crops with different nutrient needs, farmers can avoid depleting any
one necessary element present in the soil.
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