section_path: Biological production and role of O/Photosynthesis and respiration
section_name: Photosynthesis and respiration
paragraph_text: Photosynthesis and respiration
  
  In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.
  
  A simplified overall formula for photosynthesis is
  
  or simply
  
  Photolytic oxygen evolution occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons. Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize adenosine triphosphate (ATP) via photophosphorylation. The O remaining (after production of the water molecule) is released into the atmosphere.
  
  Oxygen is used in mitochondria in the generation of ATP during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as
  
  In vertebrates, O diffuses through membranes in the lungs and into red blood cells. Hemoglobin binds O, changing color from bluish red to bright red (CO is released from another part of hemoglobin through the Bohr effect). Other animals use hemocyanin (molluscs and some arthropods) or hemerythrin (spiders and lobsters). A liter of blood can dissolve 200 cm of O.
  
  Until the discovery of anaerobic metazoa, oxygen was thought to be a requirement for all complex life.
  
  Reactive oxygen species, such as superoxide ion (O) and hydrogen peroxide (H O), are reactive by-products of oxygen use in organisms. Parts of the immune system of higher organisms create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in the hypersensitive response of plants against pathogen attack. Oxygen is damaging to obligately anaerobic organisms, which were the dominant form of early life on Earth until O began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event, about a billion years after the first appearance of these organisms.
  
  An adult human at rest inhales 1.8 to 2.4 grams of oxygen per minute. This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.