The question is, why is there this difference? What accounts for this difference in the first place inside the red blood cells? Why is it that inside the red blood cells, the affinity of hemoglobin for oxygen is so much lower than in the pure hemoglobin case? Well, the answer lies in this molecule here. The molecule is known as two three BPG or two three biphosphoglycerate. This is a naturally occurring molecule that is found inside our cells.