Schematic chemical network for a protoplanetary disk, showing the competition of some chemical species (H₂O, CO, CH_3,4, SiO, O₂) for the carbon, silicon, and oxygen, depending on whether there is high OH formation or not. The lines represent reactions between species on the diagram, with arrows pointing at the product(s) of each reaction. The dotted branches correspond to a chemical network where OH is not being efficiently formed. The dashed branches replace the dotted pathways when OH is being efficiently created, thus taking away oxygen from H₂O to form CO, SiO, O₂, and atomic oxygen. The gray arrows represent CRUV photodissociation processes.