Do you enjoy breathing air? You should spare a thought once in a while for heme, an iron-containing molecule essential to all organisms engaged in an air-breathing lifestyle. Heme molecules are most famously part of hemoglobin, the oxygen-transporting protein in blood, but they are also components of numerous other proteins involved in gas transport and fundamental chemistry in cells. On its own, heme is toxic and reactive, but when slotted correctly into certain proteins, it’s absolutely essential.