Topic 7: Haemoglobin

Efficient movement of substances to and from exchange surfaces is provided by mass transport, over large distances. 

Haemoglobin 

The haemoglobins are a group of protein molecules with a quarternary structure that has evolved to make it efficient at its role of transporting oxygen.  The quaternary structure consists of four polypeptides linked together, each of which is associated with a ferrous (Fe 2+) ion. Each Fe 2+ ion can combine with a single oxygen molecule, so in humans, four oxygen molecules can be carried by a haemoglobin molecule. Haemoglobin: 

• Readily associates with/loads oxygen at the surface where gas exchange occurs 
• Readily dissociates from/ unloads oxygen at those tissues requiring it. 

It changes affinity for oxygen under different conditions because its shape changes in the presence of certain substances. Haemoglobins with high affinity load oxygen more easily but release it less easily. Those with a low oxygen affinity load oxygen less easily but unload it more easily. 

Differences in species: 

Each species produces a haemoglobin with a slightly different amino acid sequence therefore has a slightly different tertiary and quaternary structure, hence different oxygen binding properties. These have evolved within species as adaptations to different environments and conditions. For example, organisms living at lower partial pressures of oxygen have evolved to have haemoglobin that has higher affinity for oxygen than the haemoglobin of animals that live where partial pressure is higher. 

Oxygen dissociation curves:…