International A Level Biology / Mammalian transport systems

The Bohr effect and fetal haemoglobin revision notes

Edexcel International A Level Biology (YBI11), chapter 2: Mammalian transport systems, specification point 1.9. Free lesson.

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The Bohr effect explains how active tissues make haemoglobin release more oxygen. Fetal haemoglobin has a higher affinity than adult haemoglobin, so oxygen can move from the mother's blood to the fetus at the placenta.

How carbon dioxide causes the Bohr effect

When a tissue respires faster, it produces more carbon dioxide. In red blood cells, carbonic anhydrase helps convert this carbon dioxide into carbonic acid. The acid releases hydrogen ions, lowering the pH. These ions bind to haemoglobin and change its shape slightly, so it has a lower affinity, meaning it holds oxygen less tightly. More oxygen then dissociates from oxyhaemoglobin. Bohr effect is the fall in haemoglobin's affinity for oxygen when carbon dioxide increases. The effect is named after Christian Bohr. Spell it B, O, H, R.

How the Bohr shift unloads oxygen

A graph of haemoglobin saturation against the partial pressure of oxygen shows the change. With higher carbon dioxide, the adult haemoglobin curve shifts to the right. At the same partial pressure, haemoglobin is less saturated, so more oxygen has been unloaded. In the lungs, carbon dioxide is low, so haemoglobin can still load oxygen fully. At 4 kPa, saturation is 57% with lower carbon dioxide and 36% with higher carbon dioxide.

How to calculate the extra oxygen unloaded

To compare how much oxygen is unloaded, subtract the saturation at the muscle from the saturation in the lungs. Then compare the two amounts unloaded. The readings are already percentages, so subtract them directly.

Why fetal haemoglobin has a higher affinity

Fetal haemoglobin has slightly different polypeptide chains from adult haemoglobin. Its curve lies to the left, which means it has a higher affinity for oxygen and is more saturated at the same partial pressure. At the placenta, the mother's and fetus's blood are at about the same partial pressure, around 4 kPa. Fetal haemoglobin's higher affinity means oxygen dissociates from the mother's haemoglobin, crosses the placenta and binds to the fetus's haemoglobin. At about 4 kPa, adult haemoglobin is 57% saturated and fetal haemoglobin is 77% saturated. The oxygen transfer is caused by…

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