The CO compensation point (Γ) is the CO concentration at which the rate of photosynthesis exactly matches the rate of respiration. There is a significant difference in Γ between  plants and  plants: on land, the typical value for Γ in a  plant ranges from 40&ndash;100 μmol/mol, while in  plants the values are lower at 3&ndash;10 μmol/mol. Plants with a weaker CCM, such as C2 photosynthesis, may display an intermediate value at 25 μmol/mol.
The μmol/mol unit may alternatively be expressed as the partial pressure of CO in pascals; for atmospheric conditions, 1μmol/mol = 1 ppm ≈ 0.1 Pa. For modeling of photosynthesis, the more important variable is the CO compensation point in the absence of mitochondrial respiration, also known as the CO photocompensation point (Γ*), the biochemical CO compensation point of Rubisco. It may be measured by whole-leaf isotopic gas exchange, or be estimated in the Laisk method using an intermediate "apparent" value of C* with correction. C* approximates Γ* in the absence of carbon refixation, i.e. carbon fixation from photorespiration products. In  plants, both values are lower than their  counterparts. In C2 plants that operate by refixation, only C* is significantly lower.
As it is not yet common to routinely change the  concentration of air, the concentration points are largely theoretical derived from modeling and extrapolation, though they do hold up well in these applications. Both Γ and Γ* are linearly related to the partial pressure of oxygen (p(O)) due to the side reaction of Rubisco. Γ is also related to temperature due to the temperature-dependence of respiration rates. It is also related to irradiation, as light is required to produce RuBP (ribulose-1,5-bisphosphate), the electron acceptor for Rubisco. At normal irradiation, there would almost always be enough RuBP; but at low irradiation, lack of RuBP decreases the photosynthetic activity and therefore affects Γ.