Transport of gases in the bloodEdexcel A-Level Biology B: Revision notes

Haemoglobin is a globular protein found in erythrocytes. It has a quaternary structure: four polypeptide chains (two alpha and two beta in adults), each folded into its own tertiary structure. Each chain holds a haem group, a non-protein (prosthetic) group containing an iron(II) ion (Fe²⁺). Each haem group binds one O₂ molecule, so one haemoglobin carries up to four oxygen molecules, forming oxyhaemoglobin.

Because it is a protein, its shape and its affinity for oxygen can be changed by the binding of oxygen and by pH. This allows it to load oxygen where oxygen is plentiful and unload it where it is scarce.

The oxygen dissociation curve plots percentage saturation of haemoglobin against the partial pressure of oxygen (pO₂, in kPa). It is S-shaped (sigmoid):

• At low pO₂ the first oxygen molecule binds slowly, because the haem groups are hard to reach.
• Binding changes the shape of the molecule, making it easier for further oxygen to bind (cooperative binding), so the curve becomes steep.
• At high pO₂ the curve flattens as almost all the haem groups are occupied.

In the lungs (high pO₂) haemoglobin is almost fully saturated, so it loads oxygen. In respiring tissues (low pO₂) it unloads oxygen, because the curve is steep there and a small fall in pO₂ gives a large release.

The horizontal axis is partial pressure, not concentration: a curve further left means a higher affinity; further right, a lower affinity.

Respiring cells produce carbon dioxide. It dissolves in plasma and in red blood cells, where the enzyme carbonic anhydrase converts it to carbonic acid (H₂CO₃), which dissociates into H⁺ and hydrogencarbonate ions. The H⁺ ions lower the pH.

A high pCO₂ and low pH change the shape of haemoglobin and reduce its affinity for oxygen. The dissociation curve shifts to the right: this is the Bohr effect. More oxygen is released at a given pO₂, and the shift is greatest in the most active tissues, which have the highest CO₂ production. In the lungs the pCO₂ is low, the pH higher, the curve moves left and loading is easier.

Most carbon dioxide is carried as hydrogencarbonate ions in the plasma; the haemoglobin binds H⁺ ions, acting as a buffer.

Myoglobin is a protein in muscle with one polypeptide chain and one haem group, so it has no quaternary structure and binds a single O₂ molecule. Its dissociation curve lies to the left of that of haemoglobin and is hyperbolic, not S-shaped, because there is no cooperative binding.

Myoglobin has a higher affinity for oxygen, so it takes oxygen from haemoglobin and stores it in the muscle, releasing it only when the pO₂ falls very low, for example in vigorous exercise. Haemoglobin, in contrast, transports oxygen, loading it in the lungs and unloading in tissues.

Both are globular proteins with haem groups that bind oxygen reversibly.

The fetus gets its oxygen from the mother's blood in the placenta, where fetal blood has a lower pO₂ than maternal blood. Fetal haemoglobin has a different polypeptide composition and a higher affinity for oxygen; its dissociation curve lies to the left of the adult curve.

At the pO₂ in the placenta, fetal haemoglobin is more saturated than maternal haemoglobin, so it takes oxygen from the mother's haemoglobin and a gradient is maintained. After birth, fetal haemoglobin is gradually replaced by adult haemoglobin, which releases oxygen more readily to the tissues of a more active, independent baby.