Unit A

Module 1 ~ Lesson 4

The Oxygen Cycle

Most living things cannot survive without a source of oxygen

Photosynthesis is an important process to sustain oxygen in the atmosphere.

Oxygen reacts so intensely with other elements in chemical reactions that significant amounts of energy are released. Organisms can then use the energy released in these reactions. Given the reactive nature of oxygen, it is natural to wonder why there is any oxygen left in the atmosphere. Photosynthesis provides a method for oxygen to re-enter the atmosphere in its elemental form. 

The carbon and oxygen cycle are closely related

Photosynthesis and cellular respiration are the main processes responsible for the balance of oxygen and carbon dioxide

The diagram (left) demonstrates the relationship between the oxygen and carbon dioxide cycles. Carbon dioxide comprises only about 0.03% of the atmosphere, whereas oxygen accounts for 21%. This means that the dynamic relationship between carbon and oxygen is only a small part of the total oxygen system. The total oxygen system also involves the cycling of other nutrients, such as sulfur, phosphorus, and nitrogen.


Read "The Carbon and Oxygen Cycles" and "The Slow Cycling of Carbon" on pages 43 to 46 in your textbook.



Biotic components directly available as nutrients are living or recently living organisms. The atmosphere, soil, and water are abiotic components directly available as nutrients.             


Fossil fuels and fossils are previously living components where nutrients are not readily available. Rocks, large rock formations, and sediments that are deep underground are also unavailable.        


a. Deforestation reduces the amount of photosynthesis that occurs, which results in less carbon dioxide being removed from the atmosphere. Additionally, it will decrease the amount of oxygen released by photosynthesis. It would also decrease the available energy to consumers. Finally, the decomposition of the debris will release additional carbon dioxide into the atmosphere.
 b. Burning fossil fuels releases carbon from slow cycling and quickly increases the amount of carbon in the atmosphere.
c. Agriculture creates crops that remove carbon dioxide from the atmosphere through photosynthesis. However, agriculture often relies on deforestation for land, and trees remove more carbon (and add more oxygen) than crops would.