At the current low levels of atmospheric carbon dioxide, rubisco in C3 plants can be fooled into substituting oxygen molecules for carbon-dioxide molecules.
Most plants use an enzyme called rubisco to convert carbon dioxide (CO2) into sugars containing three carbon atoms a process known as C3 photosynthesis.
To get around the problem, a small number of plants have evolved a way to enrich the carbon-dioxide concentration around the rubisco enzyme, and to suppress the oxygen concentration.
As a result, certain plants in warm climates have evolved a different mechanism, called C4 photosynthesis, in which other enzymes help to concentrate CO2 around the rubisco, and the initial result is a four-carbon sugar.