An understanding of the mechanism of CO₂ fixation requires knowledge of the physical and chemical properties of CO₂, particularly those related to its interaction with water. The amount of any gas dissolved in water is proportional to its partial pressure (P_gas) above the solution (Henry's law) and its Bunsen absorption coefficient (α). The Bunsen absorption coefficient is the volume of gas absorbed by one volume of water at a pressure of 1 atmosphere and is temperature dependent, decreasing as the temperature rises.

The solubility of a gas therefore decreases with increasing temperature. Thus, for a given temperature,

where V₀ is the normal volume of an ideal gas at standard temperature and pressure (V₀ = 22.4 L mol^–1).

We can calculate the partial pressure of a gas by multiplying the mole fraction of the gas by the total pressure. The mole fraction of a gas is its partial volume divided by the total volume of all the gases present. Thus, the mole fractions of CO₂ and O₂ in air are 0.0345% and 20.95%, respectively. At sea level, atmospheric pressure is about 0.1 MPa, so the partial pressures of CO₂ and O₂ are 3.4 × 10^–5 and 2.1 × 10^–2 MPa, respectively.

From these values and those of α, the corresponding solution concentrations of CO₂ and O₂ can be computed by the equation given above. The table below presents values for these concentrations at different temperatures.

These values place considerable constraints on carboxylation. As a carboxylase, rubisco must be capable of operating efficiently even at the rather low concentrations of CO₂ available to it. Rubisco also functions as an oxygenase in photorespiration, so the solution concentration of O₂ is also important.

Because of the different temperature dependences of the Bunsen absorption coefficients α(CO₂) and α(O₂), the concentrations of these two gases vary with temperature such that the ratio of CO₂ to O₂ decreases as the temperature increases. This effect is important biologically, because as the temperature increases, the ratio of carboxylation to oxygenation catalyzed by rubisco decreases and the ratio of photorespiration to photosynthesis thus increases.

Web Table 8.8.A   Solubility of CO₂ and O₂ as a function of temperature.

Strictly, the incorporation of atmospheric CO₂ into a solution comprise two reactions:

The solubilization CO₂ (gas) → CO₂ (aq)

and the hydration CO₂ (aq) + H₂O → H₂CO₃, K = 2 × 10^–3.

Both the low proportion of H₂CO₃ relative to CO₂ (aq) and the difficulty to distinguish experimentally these species leads to lumping their concentrations together in Henry’s law. Further descriptions of inorganic carbon speciation in oceans and freshwater can be obtained with the computer program developed by Lewis and Wallace (1998).