RuBisCO is an enzyme that acts as an oxygenase in low carbon dioxide concentration and acts as a carboxylase under high concentration of carbon dioxide. RuBisCO binds with oxygen at high temperatures and low CO2:O2 ratio and binds with carbon dioxide at low temperatures and high CO2:O2 ratio. Yeasts poison themselves when the concentration of alcohol reaches around 13%. The Calvin pathway occurs in the bundle sheath cells of the C4 plants. Oxygen is the final hydrogen ion and electron acceptor. The oxygen combines with the hydrogen ions and electrons to form water.