concept

Carbon fixation

Taking carbon that exists as a gas — CO₂ in the air — and attaching it to an organic molecule, turning gas into matter. This is the step that makes a planet with an atmosphere into a planet with life: every carbon atom in your body was once a CO₂ molecule that some leaf caught. On Earth most of this work is done by the enzyme Rubisco, inside the Calvin-Benson cycle. The word "fixed" means held: carbon that was flying free in the atmosphere is now anchored into a scaffold that living things can use, pass along, and eventually return.