3.  Plants are the main source of breathable oxygen for all animals that depend on it.  During photosynthesis plants absorb carbon dioxide and release oxygen.

4.  99.5% of all oxygen is located in the crust and mantle of the earth. .36% of the earth’s oxygen is in the atmosphere which is the oxygen we breathe.  Although oxygen is the most abundant element on the planet we can only use a small portion of it.

5.  The chemical formula in photosynthesis is 6CO 2 + 6H 2 O + energy → C 6 H 12 O 6 + 6O 2.  The chemical formula in chemical weathering for oxidized iron rocks is 4FeO + O 2 → 2Fe 2 O 3.  These are the most important chemical formulas having to do with oxygen.

6.  Humans along with every other animal completely depend on oxygen to make possible chemical reactions in their body's.  The worst thing humans can do is to destroy forests which they are doing.  Without any plants or forests the oxygen cycle will cease to occur killing all humans.