Oxygen

Oxygen is a colorless, odorless, and tasteless gas that makes up around 21% of the Earth’s atmosphere. Oxygen is a chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds with most other elements. Oxygen can exist in several different forms, including O2, O3 (ozone), and O4. O2, the most common form of oxygen, is a stable molecule consisting of two oxygen atoms bonded together.  Oxygen is Earth’s Most
abundant element, and after hydrogen and helium.  Oxygen makes up almost half of the Earth’s crust in the form of oxides.

All plants, animals, and fungus need oxygen for cellular respiration, which extracts energy by the reaction of oxygen with molecules derived from food and produces carbon dioxide as a waste product. In  tetrapods  breathing brings oxygen into the lungs where gas exchange takes place,
carbon dioxide diffuses out of the blood, and oxygen diffuses into the blood. The body’s circulatory system transports the oxygen to the cells, where cellular respiration takes place.

Many major classes of organic molecules in living organisms contain oxygen atoms, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of life forms. Oxygen is continuously replenished in Earth’s atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form (allotrope) of oxygen, ozone strongly absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of smog and thus a pollutant.