·   Water precipitates to form clouds and falls to the ground.

·   The high energy exerted by the sun excites water molecules and enables them to escape into the atmosphere as vapor.

As water recycles, it helps promote other cycles in several ways: 

·   Water acts as a solvent for chemical reactions.

·   Materials contained in water get to interact with all parts of the ecosystem.

·   Water is necessary for plants to turn solar energy into chemical energy.

Almost nothing could survive on earth without water.  Water stays on the earth because earth's gravity is strong enough to keep the water from escaping.  In addition, the sun provides enough heat to keep water in its liquid form, yet is not so hot that all of the water totally evaporates.

Astronauts have discovered canyons on Mars that suggest water once existed there.  Water is gone now because Mar's small amount of gravity allows water escape into space.  If water existed on Mars, vegetation would begin to grow and life could exist.  

Nitrogen:  A Double-Edged Sword

  Recycling of water and carbon take care of our needs for making carbohydrates and fats, but what about the proteins and nucleic acids contained in our RNA and DNA?  All of these chemicals contain nitrogen, another chemical that has to be recycled in the environment.  The air you breathe contains roughly 70% nitrogen gas, but this gas form of nitrogen provides a small contribution to the total nitrogen exchanged in the environment.  In order for nitrogen to be used by organisms, it must be converted to a compound called ammonia (NH₃).  The majority of the ammonia available for exchange in an ecosystem is formed from two different methods, both involving the bacteria:

·   Molecular nitrogen (N₂) is fixed by bacteria to make ammonia (consumes lots of energy).

·   Fungi and bacteria decompose the nitrogen contained in dead organic matter and release nitrates (NO₃) and ammonia (releases energy).

Adding excess nitrogen to these cycles from outside sources disrupts the balance of the cycle, and the extra nitrogen is released into the atmosphere in a form called nitrous oxide.  What element does nitrogen combine with to form this compound?  This form of nitrogen is a pollutant because it falls back to land with rain in the form of nitric acid (HNO₃).  Human actions create most of this problem.  The sources that contribute the most to the excess nitrogen include:

• Agriculture – deforestation and breaking up the soil

• Industry – combustion (burning gas)

• Automobile exhaust – combustion (burning gas)

 

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