Natural Selection Image Map

Oxygen Likes all Elements  

Oxygen is one of the most reactive elements, and this makes tracking its exchange through an ecosystem very complex.  The percentage of oxygen in the atmosphere has greatly increased over millions of years.  Can you explain why?  The atmosphere presently contains about 20% oxygen.  How can we account for the increase in oxygen if matter cannot be created?  It has come about through the different processes of recycling oxygen.  


Without oxygen, fire cannot burn.

If you have ever tried to hold your breath for a long time, you know that it is not possible to hold it for very long.  The cells in your body, especially your brain cells, will die if they donít get oxygen.  Your body needs a constant supply of oxygen because it is critical for the process of breaking down glucose for energy.  In the process of getting energy from glucose, oxygen is combined with hydrogen to form a water molecule. 

During photosynthesis (See Story Time), a water molecule is split by sunlight in the energy-forming step. This releases oxygen into the atmosphere.  Animals consume oxygen just as fast as plants can provide it, so it seems that the amount of oxygen in the atmosphere would remain  constant.  However, according to fossil records, plants existed on the earth long before the animals.  This means that an excess amount of oxygen was released to the atmosphere from the photosynthetic plants with no animals to consume it through respiration.  Therefore, until animals began to evolve on the earth, the oxygen content of air continued to increase.

The exchange of oxygen seems simple at first:

   Animals use oxygen to get energy and give it back to the ecosystem in the form of water.

   Sunlight splits water during photosynthesis, and oxygen is released into the atmosphere for animals to breathe.

Oxygen is present in many other compounds besides water.  It gets involved with the recycling of elements discussed below because it reacts  with all of them.  It forms compounds such as nitrate (NO3), sulfur dioxide (SO2), phosphate (PO4), carbon dioxide (CO2), and water (H2O).  

Ozone hole over Antarctica in 2001.For last three years, the size has remained about the same. Source: NASA.

Oxygen also occurs in air as a compound called ozone (O3).  

Ozone in the lower atmosphere is harmful because it hurts the eyes and respiratory system and kills plant life.  

Ozone that is high up  (in the stratosphere) protects the earth from the sun's harmful ultraviolet radiation emitted. Pollutants put in the air by human activity reduce ozone formation up in the stratosphere.    

In the lower atmosphere, pollutants such as nitrous oxide and volatile hydrocarbons contribute to the production of ozone.  This creates the ugly smog that is present in big cities like New York, Los Angeles, and Houston.

Thinning of the ozone in the stratosphere could cause many problems that include altering DNA, increasing the chance of getting skin cancer, and contributing to a global warming of the earth.  

 

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