Studies of oxygen transport in pregnant mammals have shown that the O2-saturation curves of fetal and maternal blood are markedly different when measured under the same conditions. Fetal erythrocytes contain a structural variant of hemoglobin, HbF, consisting of two γ and two β subunits (γ2β2), whereas maternal erythrocytes contain HbA (α2β2).

When all the BPG is carefully removed from samples of HbA and HbF, the measured O2-saturation curves (and consequently the O2 affinities) are displaced to the left. However, HbA now has a greater affinity for oxygen than does HbF. When BPG is reintroduced, the O2-saturation curves return to normal.
What is the effect of BPG on the O2 affinity of hemoglobin and why are HbA and HbF impacted differently by BPG?

a) HbF or fetal haemoglobin has higher affinity for oxygen under physiological conditions. This is shown by the left shift of the oxy-hemoglobin disassociation curve. This enables HbF to take up oxygen from the mothers blood and transfer it to foetus under oxygen deficient conditions.

b) BPG stabilises the T state configuration of haemoglobin making it harder to find oxygen. BPG is produced in the cells during normal cellular respiration process. So increase in BPG shifts the oxyhemoglobin dissociation to the right and when BPG is removed, the oxyhaemogloxy disassociation curve is shifted to the left. This explains the different oxygen affinities of HbA and HbF.

the overall balanced equation

6co2 + 6h2o > c6h12o6 + 6o2

sunlight energy

where: co2 = carbon dioxide

h2o = water

light energy is required

c6h12o6 = glucose

o2 = oxygen