Well, lets start with the most obvious one, oxygen. Oxygen makes up a little under twenty-one percent of Earths atmosphere, by percentage, and is necessary for the process of cellular respiration. Animal cells undergo the process of cellular respiration in their mitochondria, which breaks down glucose with oxygen from the atmosphere in order to manufacture energy in the form of adenosine triphosphate. Another form of oxygen, ozone, forms a special layer in the stratosphere that absorbs much of the harmful radiation from the sun that would otherwise make Earth a tough place in which to live. Nitrogen is the most plentiful gas in the atmosphere at a little over seventy-eight percent. Animals can not obtain the nitrogen they need from the air, they must eat plants that absorb the nitrogen from the soil. Many of these plants have bacteria called "nitrogen-fixing bacteria" on their root systems that take the nitrogen from the air and fix it into the ground, where the plants may absorb it. The remaining one percent is composed of carbon dioxide and water vapor, both of which is used also by plants to conduct the process of photosynthesis, the process that manufactures energy for the plant cells.