Respiration is one of the most important processes required of all living things. Animals take in oxygen and expel carbon dioxide. Plants need carbon dioxide and give off oxygen as a waste product. However, neither of these gases are most plentiful in Earth’s atmosphere. The air is predominantly made up of nitrogen.
TL;DR (Too Long; Didn't Read)
Nitrogen makes up approximately 78 percent of the air in the Earth's atmosphere.
What Air Is Made Of
Nitrogen is the most abundant gas in the atmosphere. Air is composed of 78 percent nitrogen. Oxygen accounts for 21 percent, and the inert noble gas argon makes up 0.9 percent of the air. The remaining 0.1 percent consists of several trace gases. Most of the 0.1 percent is carbon dioxide. Other traces gases include neon, helium, methane (CH4), nitrous oxide (N2O) and ozone (O3.)
Chemistry of the Atmosphere
Nitrogen gas is not highly reactive with other molecules in the atmosphere and is mainly present in air as N2. Nitrogen’s unreactive behavior results from the powerful triple bonds that form between the three pairs of electrons shared between two nitrogen atoms. These bonds have relatively short radii, which requires more energy to break. Nitrogen becomes more reactive at higher temperatures. At lower temperatures, the presence of certain catalysts causes nitrogen to become more reactive with other molecules. One common nitrogen-based reaction that occurs in the atmosphere is the formation of NO, nitrogen oxide, during storms when lightning strikes.
Nitrogen is important to all organisms because it forms the basis of many compounds necessary for life. Proteins, enzymes, hormones and chlorophyll all contain nitrogen. Nucleic acids also contain nitrogen and form the long chains of nucleotides that make up the backbone of DNA and RNA. However, living things cannot use N2 in its gaseous form in the atmosphere. Nitrogen gas found in air pockets within soil is converted into a form usable by plants through a process called nitrogen fixation. Nitrogen-fixing organisms include certain types of bacteria and other micro-organisms that live on the roots of legumes such as soybeans, alfalfa and red clover. The micro-organisms convert N2 into other compounds such as ammonium and nitrate, which are taken up by plant roots. Consumers eat the plants and later deposit nitrogen compounds back into the soil through elimination or decomposition. Plants also return nitrogen to the soil when they decompose. The nitrogen-fixing micro-organisms in soil break down these compounds, and the nitrogen cycle continues.
Because nitrogen can be highly reactive at high temperatures, nitrogen oxide compounds form when fuel is burned. One of these compounds, nitrogen dioxide (NO2), is a byproduct of combustion and is present in emissions from automobiles and factories. In gaseous form, NO2 is a respiratory irritant. In the presence of water in the atmosphere, it can react to form acid rain.