Haemoglobin and oxygen transport

Many students find the topic interesting because ligand exchange reactions are often visually quite striking. This engagement with the topic can be strengthened by pointing out some of the contexts in which they play a major part. Students who study biology as well as chemistry will be particularly interested in the role of haemoglobin in oxygen transfer in the body and the dangers of carbon monoxide in stopping this transport.

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Transport of oxygen in the blood

Oxygen can form a dative bond with an iron atom at the centre of the complex haemoglobin molecule (Hb) in blood cells to form oxyhaemoglobin:

Hb(aq) + O2(aq)  HbO2(aq)

                                                     haemoglobin             oxyhaemoglobin

In the lungs the concentration of oxygen is relatively high. How will this affect the position of the equilibrium?

The position of the equilibriuim lies to the right.

The oxyhaemoglobin is carried round the body in the blood, to cells where the concentration of oxygen is lower.

What will happen when the oxyhaemoglobin reaches these cells?

The equilibrium position moves to the left and oxygen is released.

Carbon monoxide poisoning

Carbon monoxide can form a dative bond with iron in the same way that oxygen can so it too can form an equilibrium system.

Hb(aq) + CO(aq)  HbCO(aq)

                                                    haemoglobin               carboxyhaemoglobin

The equilibrium constant for this equilibrium is about 200 times greater than for the haemoglobin/oxygen equilibrium so carbon monoxide will bind to the haemoglobin in preference to the oxygen.

Why is carbon monoxide poisonous?

Cells are deprived of oxygen because the haemoglobin is transporting carbon monoxide rather than oxygen.

In heavy smokers, up to 20% of the iron atoms can be bound to carbon monoxide rather than oxygen. Breathing air containing as little as 500 ppm of carbon monoxide can prove fatal.

The function of haemoglobin as oxygen carrier and the poisonous effect of carbon monoxide is explored in molecule of the month.

 
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