Carbon dioxide basic water illustrates this property, in which case hydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided by Grignard reagents and organolithium compounds react with to give carboxylates: In metal carbon dioxide complexes, serves as a ligand, which can facilitate the conversion of to other chemicals. The reduction of to CO is ordinarily a difficult and slow reaction: Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars from absorbed from the air and water: The redox potential for this reaction near pH 7 is about −0.53 V "versus" the
Carbon dioxide global average concentration of 409 parts per million by volume (or 622 parts per million by mass). Atmospheric concentrations of carbon dioxide fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. Concentrations also vary on a regional basis, most strongly near the ground with much smaller variations aloft. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. The concentration of carbon dioxide has risen due to human activities. Combustion