Oxygen, a chemical element represented by the symbol O, supports life on Earth. It is a reactive nonmetal that readily forms compounds with most other elements. This element is necessary for the metabolic processes of nearly all living organisms, enabling them to derive energy from food.
Oxygen in Earth’s Atmosphere
Oxygen is present in Earth’s atmosphere primarily as diatomic oxygen (O2), a colorless and odorless gas. This form constitutes approximately 20.95% of dry air by volume. Its presence is directly linked to the aerobic respiration performed by most life forms, including animals and many microorganisms, enabling the release of chemical energy from organic molecules.
Oxygen in Water
Water, a common substance on Earth, is chemically composed of hydrogen and oxygen (H2O). A significant portion of the planet’s oxygen is bound within water molecules.
Beyond this, oxygen also exists as dissolved oxygen (O2) in oceans, rivers, and lakes. This dissolved form is important for aquatic life, as organisms like fish extract it through their gills for respiration. Oxygen dissolves into water from the atmosphere and is also produced by aquatic plants through photosynthesis. The solubility of oxygen in water is influenced by temperature, with colder water holding more dissolved oxygen.
Oxygen in Earth’s Crust and Minerals
Oxygen is the most abundant element in Earth’s crust, accounting for about 46% of its mass. It exists predominantly in solid forms, bonded with other elements to create many minerals and rocks. Common examples include silicates, the largest class of minerals, which make up approximately 90% of the Earth’s crust. Oxygen is also a component of various oxides, such as iron oxides and silicon dioxide.
Oxygen in Living Organisms
Within living biological systems, oxygen is a constituent of the organic molecules that form the building blocks of life. It is found in carbohydrates, proteins, lipids, and nucleic acids. These complex molecules are important for cellular structure and function across all organisms. A large percentage of the body mass in most living beings is also composed of water.
The Global Oxygen Cycle
The global oxygen cycle describes oxygen’s continuous movement through Earth’s atmosphere, hydrosphere, biosphere, and lithosphere. Photosynthesis, primarily carried out by plants, algae, and cyanobacteria, is the primary process that produces free oxygen, releasing it into the atmosphere. This atmospheric oxygen is then consumed by most organisms through respiration, a process that releases energy from organic molecules and produces carbon dioxide.
Other natural processes also contribute. Decomposition, facilitated by bacteria and fungi, consumes oxygen as organic matter breaks down, returning carbon dioxide to the environment. Combustion, such as natural wildfires, also consumes oxygen while releasing carbon dioxide. Geological processes, like the weathering of rocks, further contribute to the cycling of oxygen by incorporating it into new mineral forms.