Nitrogen Cycle, natural cyclic process in the course of which atmospheric nitrogen enters the soil and becomes part of living organisms, before returning to the atmosphere. Nitrogen, an essential part of the amino acids, is a basic element of life. It also makes up 78 percent of the Earth’s atmosphere, but gaseous nitrogen must be converted to a chemically usable form before it can be used by living organisms. This is accomplished through the nitrogen cycle, in which gaseous nitrogen is converted to ammonia or nitrates. The high energies provided by lightning and cosmic radiation serve to combine atmospheric nitrogen and oxygen into nitrates, which are carried to the Earth’s surface in precipitation. Biological fixation (Nitrogen Fixation), which accounts for the bulk of the nitrogen-conversion process, is accomplished by free-living, nitrogen-fixing bacteria; symbiotic bacteria living on the roots of plants (mostly legumes and alders); cyanobacteria (formerly known as blue-green algae); archaebacteria (also known as archaea) in deep-sea hydrothermal vents and other geothermal environments; certain lichens; and epiphytes in tropical forests.

Nitrogen “fixed” as ammonia and nitrates is taken up directly by plants and incorporated in their tissues as plant proteins. The nitrogen then passes through the food chain from plants to herbivores to carnivores (Food Web). When plants and animals die, the nitrogenous compounds are broken down by decomposing into ammonia, a process called ammonification. Some of this ammonia is taken up by plants; the rest is dissolved in water or held in the soil, where microorganisms convert it into nitrates and nitrites in a process called nitrification. Nitrates may be stored in decomposing humus or leached from the soil and carried to streams and lakes. They may also be converted to free nitrogen through denitrification and returned to the atmosphere.
In natural systems, nitrogen lost by denitrification, leaching, erosion, and similar processes is replaced by fixation and other nitrogen sources. Human intrusion in the nitrogen cycle, however, can result in less nitrogen being cycled, or in an overload of the system. For example, the cultivation of croplands, harvesting of crops, and cutting of forests all have caused a steady decline of nitrogen in the soil. (Some of the losses on agricultural lands are replaced only by applying energy-expensive nitrogenous fertilizers manufactured by artificial fixation.) On the other hand, the leaching of nitrogen from overfertilized croplands, cutover forestland, and animal wastes and sewage has added too much nitrogen to aquatic ecosystems, resulting in reduced water quality and the stimulation of excessive algal growth. In addition, nitrogen dioxide poured into the atmosphere from automobile exhausts and power plants breaks down to form ozone and reacts with other atmospheric pollutants to form photochemical smog



regards

The following link may also be helpful in understanding the three biogeochemical cycles
http://www.cssforum.com.pk/115179-post8.html

Regards

__________________
When you have to shoot, shoot. Don't talk. ~ The Good, The Bad, and the Ugly

Assalam O Alaikum

Dear Dr.Atif, Mohsin, Afrms & all.



EDS papers from 1999 to 2004 are still to be solved. While papers from 2005 to 2009 have been solved under the concerned past paper. You are requested to please solve the remaining papers (1999 to 2004) as soon as possible to complete the task and start another.


regards,

__________________
No matter how fast i run or how far i go it wont escape me, pain, misery, emptiness.

Inshallah i will try to contribute where required actually i want to complete the zoology notes.75% of the notes have still to be made.

You (Zia),Mohsin and Dr Atif Rana are doing a fantastic job.

Dear Zia

I had already made my plan for this job
Insha Allah in a very short time you will see the solutions there in your thread. Just give me a few days

Regards

__________________
When you have to shoot, shoot. Don't talk. ~ The Good, The Bad, and the Ugly

Reaction time:

Reaction time (RT) is the elapsed time between the presentation of a sensory stimulus and the subsequent behavioral response. RT is often used in experimental psychology to measure the duration of mental operations, an area of research known as mental chronometry. The behavioral response is typically a button press but can also be an eye movement, a vocal response, or some other observable behavior
Reaction time is quickest for young adults and gradually slows down with age. It can be improved with practice, up to a point, and it declines under conditions of fatigue and distractions

rgds

Photosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea. Photosynthetic organisms are called photoautotrophs, since it allows them to create their own food. However, not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon. In plants, algae and cyanobacteria photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is crucially important for life on Earth, since as well as it maintaining the normal level of oxygen in the atmosphere, nearly all life either depends on it directly as a source of energy, or indirectly as the ultimate source of the energy in their food.



rgds

from Paper 1998

1.“Earthquakes have helped a great deal in deciphering the internal structure of the earth”. Comment on this statement

A: Earthquakes are very useful to humans because they provide a picture of what’s going on underground. This can make oil and gas extraction more efficient, and allows scientists to monitor the progress of water during geothermal energy extraction.
They also tell you what is happening in the structure of the earth, for example, magma chambers, and allow scientists to monitor volcanoes and the threat of eruption.
Earthquakes can also tell us about the internal structure of the Earth. By measuring the time it takes the seismic waves to travel through the Earth we can map out the structure of the Earth down to the core.
On a global scale, earthquakes are just a symptom of the movements of the plates going on all the time, and they occur because the material at the surface at the earth is changing due to convection within the Earth’s mantle – mountains are being created, minerals from below are being brought up and new sea floors are being made.
Tectonic activity is essential to sustaining life on Earth. Matter is constantly being recycled between the atmosphere and the crust. We have continents because of tectonic activity. Mid ocean ridges support a huge amount of life, and may have been important in the origin of life, and the atmosphere is reliant on volcanic eruptions for it’s composition.

(note: simmilar questions may be
Earthquakes have always been part of the earth's geology. What are the positive effects of earthquakes? Would mankind have evolved on earth without its restless tectonic activity? )

rgds

what's the unit of viscosity.........if it is eeta
then how we could define eeta?
what is called dimensional unit or a simple unit?
is viscosity have dimensonal unit?

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jab bhi kaam aaya mera PARVARIGAAR kaam aaya

Viscosity
A measure of the resistance of a fluid to deform under either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.

Eta is not the unit of viscosity. It is the symbol for it.
The SI physical unit of dynamic viscosity is the pascal-second (Pa·s) which has no special name.

The most common unit of viscosity is the dyne second per square centimeter [dyne·s/cm^2], which is given the name poise [P] after the French physiologist Jean Louis Poiseuille


No need to define eta, it's just a symbol to represent viscosity
what needs to be defined is Pa.s (Pascal.second) and poise

pascal.second
If a fluid with a viscosity of one Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one pascal, it moves a distance equal to the thickness of the layer between the plates in one second.

poise
1 P = 1 g·cm^-1·s^-1
1 pascal second = 10 poise = 1,000 millipascal second
1 centipoise = 1 millipascal second


Every unit has a dimension i.e the parameter required to describe the position and relevant characteristics of any object within a conceptual space

See the below article for more detail on dimensions and units (under the heading "units as dimensions")
http://en.wikipedia.org/wiki/Physical_units

I don't think there is something like a "simple unit".


The units of viscosity are derived units. They are derived from the SI base units like the units of pressure, length and time. As all the SI base units are dimensional units, hence, units of viscosity are also dimensional


For more details on viscosity and related concepts see the following link
http://hypertextbook.com/physics/matter/viscosity/

Hope this helps

__________________
When you have to shoot, shoot. Don't talk. ~ The Good, The Bad, and the Ugly