Hemoglobin, also spelled haemoglobin, is an iron-containing protein in red blood cells that is responsible for oxygen transport. Oxygen binding is mediated by the heme group embedded in the protein (Figure 1).

Figure 1 is made of two halves. The left hand side shows an illustration of eight red blood cells. These cells are round and discoid in shape. This means that they have a thicker rim and narrower center. There is an arrow which leads to the right hand side of the image which shows a 3D model of the protein Hemoglobin. This protein is shown as a tightly gathered structure made up of coils. These coils make up four Heme subunits, each of which is shown as having bound to oxygen.

Figure 1: Red blood cells (a) and structure of hemoglobin (b).