Environmental Engineering Reference
In-Depth Information
Oxygen Cycle
The oxygen cycle is not described in most textbooks but is nevertheless very important for the living world.
Oxygen (O
2
) is the byproduct of photosynthesis in plants and a reactant in the cellular respiration of plants and
animals. Oxygen is the second major component of the atmosphere after nitrogen gas (N
2
) and composes ap-
proximately 21 percent of the atmosphere.
Oxygen is a reactive molecule. Besides involvement in the photosynthetic and cellular respiration reactions,
oxygen is removed from the atmosphere during the weathering process of rock and minerals. As new rock and
minerals are exposed to the atmosphere during weathering, oxygen combines with them in a process called ox-
idation and is, thus, removed from the atmosphere.
Oxygen is also important in the formation of atmospheric ozone. Sunlight breaks water (H
2
O) vapor into hy-
drogen gas and oxygen, and the hydrogen escapes Earth's atmosphere and travels into outer space. Ozone (O
3
)
is a naturally occurring reaction product in the atmosphere, where it forms a layer. The ozone layer is important
