The carbon dioxide and oxygen cycle represent a true "win-win" scenario in the earth today. All green plants produce their own food through a process called photosynthesis. The carbon dioxide from the atmosphere is an essential part of that process. Light energy from the sun is combined with water from the soil and carbon dioxide from the air. This produces glucose, a simple sugar, and releases oxygen back into the atmosphere as a waste product.
All animals (and plants, at night) make energy for their cells through a process called cellular respiration. This process takes the glucose that was initially produced by the plants and combines it with oxygen from the atmosphere. This manufactures free energy, in the form of ATP, with water and carbon dioxide as waste products. The water and carbon dioxide are released back into the atmosphere.
Oxygen is the second most plentiful gas in the atmosphere, at 21%. Carbon dioxide has traditionally been under 1%, with other trace gasses. Recent manufacturing processes have stepped up production of carbon dioxide, resulting in a phenomenon known as global warming. Increased levels of carbon dioxide cause a rise in temperatures world-wide.