So the oxygen binding curve for hemoglobin that we spoke about previously was actually the curve for hemoglobin when it is present inside red blood cells. So why is that important? Well, it turns out that if we isolate and peer purify the hemoglobin and then we examine the oxygen binding curve for pure hemoglobin, there will be a tremendous difference between the binding curve for the pure hemoglobin and the binding curve for that hemoglobin found inside red blood cells. So, the oxygen binding curve of hemoglobin inside red blood cells differs considerably from the pure hemoglobin curve, as we can see from the following graph. So, in the graph, the y axis is the fractional saturation of hemoglobin. It ranges from zero to one.