(i) the role of haemoglobin in transporting oxygen and carbon dioxide

The idea

Haemoglobin is a quaternary protein with four haem groups, each containing an iron ion (Fe²⁺) that binds one oxygen molecule, so a fully loaded molecule carries four oxygen molecules. In the lungs, a high partial pressure of oxygen (pO₂) causes oxygen to associate with haemoglobin, forming oxyhaemoglobin; in respiring tissues, a lower pO₂ causes oxygen to dissociate and be released. The affinity changes with pO₂, giving the S-shaped oxygen dissociation curve, and a high partial pressure of carbon dioxide lowers affinity further (the Bohr shift), increasing oxygen release where respiration is greatest. For carbon dioxide, most is transported as hydrogencarbonate ions (HCO₃⁻) formed in red blood cells by carbonic anhydrase, with some carried bound to haemoglobin as carbaminohaemoglobin and a small fraction dissolved in plasma.

What earns the marks

• States that haemoglobin has four haem groups, each with an iron ion (Fe²⁺) that binds one oxygen molecule, so four oxygen molecules are carried per molecule.
• Explains that oxygen associates with haemoglobin at high pO₂ in the lungs to form oxyhaemoglobin.
• Explains that oxygen dissociates from haemoglobin at lower pO₂ in respiring tissues and is released.
• Describes the S-shaped oxygen dissociation curve and the effect of a high pCO₂ lowering affinity (the Bohr shift).
• States that most carbon dioxide is transported as hydrogencarbonate ions (HCO₃⁻), formed with the enzyme carbonic anhydrase.
• States that some carbon dioxide is carried bound to haemoglobin as carbaminohaemoglobin and a small amount dissolves in plasma.

Common mistakes and how to put them right

• Saying haemoglobin carries one oxygen molecule.

  Put it right: Each of the four haem groups binds one oxygen molecule, so four are carried.

• Claiming all carbon dioxide is carried as hydrogencarbonate.

  Put it right: Some is bound to haemoglobin and a small fraction dissolves in plasma.

• Reversing the Bohr shift.

  Put it right: A high pCO₂ lowers haemoglobin's affinity for oxygen, increasing release in respiring tissues.

Exam tips

• Link each transport statement to the partial pressure conditions in lungs versus respiring tissues.
• Use the correct terms: association and dissociation for oxygen, and name the enzyme carbonic anhydrase for hydrogencarbonate formation.
• When describing the dissociation curve, refer to the S shape and explain the plateau and steep region in terms of affinity.