For ${\mathrm{C}}_3$ plants, ${\mathrm{C}\mathrm{O}}_2$ fixation is carried out by enzyme RUBISCO which is highly sensitive to ${\mathrm{C}\mathrm{O}}_2$ concentration. Increases in ${\mathrm{C}\mathrm{O}}_2$ concentration increases the rate of photosynthesis and decreases in ${\mathrm{C}\mathrm{O}}_2$ concentration decreases the rate of photosynthesis.