The reactive enodiolate species is also sensitive to oxygen (O), which results in the dual carboxylase / oxygenase activity of RuBisCO. This reaction is considered wasteful as it produces products (3-phosphoglycerate and 2-phosphoglycolate) that must be catabolized through photorespiration. This process requires energy and is a missed-opportunity for CO fixation, which results in the net loss of carbon fixation efficiency for the organism. The dual carboxylase / oxygenase activity of RuBisCO is exacerbated by the fact that O and CO are small, relatively indistinguishable molecules that can bind only weakly, if at all, in Michaelis-Menten complexes. There are four forms of RuBisCO (Form I, II, III, and IV), with Form I being the most abundantly used form. Form I is used extensively by higher plants, eukaryotic algae, cyanobacteria, and Pseudomonadota (formerly proteobacteria). Form II is also used but much less widespread, and can be found in some species of Pseudomonadota and in dinoflagellates. RuBisCOs from different photosynthetic organisms display varying abilities to distinguish between CO and O. This property can be quantified and is termed "specificity" (S). A higher value of S means that a RuBisCO's carboxylase activity is greater than its oxygenase activity.