Biological role of oxygen - Photosynthesis and respiration

A simplified overall formula for photosynthesis is: (Brown 2003, 958) ::6CO³0_{1³ + 6H2O + photons → C6H12O³89³ + 6O³1011³ (or simply Carbon dioxide + water + sunlight → glucose + dioxygen)}

Photolytic Oxygen evolution occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons. (Thylakoid membranes are part of chloroplasts in algae and plants while they simply are one of many membrane structures in cyanobacteria. In fact, chloroplasts are thought to have evolved from cyanobacteria that were once symbiotic partners with the progenerators of plants and algae.) Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize ATP via photophosphorylation. (Raven 2005, 115–27) The O³12_{13³ remaining after oxidation of the water molecule is released into the atmosphere. (Water oxidation is catalyzed by a manganese-containing enzyme complex known as the Oxygen evolving complex (OEC) or water-splitting complex found associated with the lumenal side of thylakoid membranes. Manganese is an important cofactor, and Calcium and chloride are also required for the reaction to occur.(Raven 2005))}

Molecular dioxygen, O³14_{15³, is essential for cellular respiration in all aerobic organisms. Oxygen is used in mitochondria to help generate adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as: ::C³1617³H12O³2021³ + 6O³2223³ → 6CO³2425³ + 6H³2627³O + 2880 kJ·mol^-1}

In vertebrates, O³28_{29³ is diffused through membranes in the lungs and into red blood cells. Hemoglobin binds O³3031³, changing its color from bluish red to bright red. (CO2 is released from another part of hemoglobin (see Bohr effect)) Other animals use hemocyanin (molluscs and some arthropods) or hemerythrin (spiders and lobsters). A liter of blood can dissolve 200 cc of O³3233³.}

Reactive oxygen species, such as superoxide ion (O₂⁻) and Hydrogen peroxide (H³34_{35³O³3637³), are dangerous by-products of oxygen use in organisms. Parts of the immune system of higher organisms, however, 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.}

An adult human in rest inhales 1.8 to 2.4 grams of oxygen per minute. ( "For humans, the normal volume is 6-8 liters per minute." http://www.patentstorm.us/patents/6224560-description.html) This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year. ((1.8 grams)*(60 minutes)*(24 hours)*(365 days)*(6.6 billion people)/1,000,000=6.24 billion tonnes)

See also:

• Biological role of oxygen
• oxygen