The haemoglobins are a group of chemically similar molecules found in many different organisms. Haemoglobin is a protein with a quaternary structure. The role of haemoglobin in the transport of oxygen. The loading, transport and unloading of oxygen in relation to the oxygen dissociation curve. The effects of carbon dioxide concentration.Candidates should be aware that different organisms possess different types of haemoglobin with different oxygen transporting properties. They should be able to relate these to the environment and way of life of the organism concerned.

Haemoglobins are molecules consisting of four polypeptide chains and four haem groups. Different ones are found in different organisms.

There is a primary level of four polypeptide chains, two alpha and two beta, which turn into helices at the secondary level and fold up at the tertiary level much like any other protein. The difference is an additional quaternary structure in which the four chains are bonded to each other and four haem groups (iron).

Because the haem groups can form temporary bonds with oxygen, this means that the haemoglobin is able to carry four molecules of oxygen.

The dissociation curve shows the concentrations of oxygen at which the haem groups bind to oxygen.
On this graph we see the four points at which oxygen binds to the haemoglobin molecule.

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The curve can be moved left or right depending on the type of haemoglobin and conditions.

Moving right means it has a lower affinity to oxygen and means it will give up oxygen more easily. High concentrations of CO2 will move the curve to the right. This means that near the muscles where there is a high concentration of CO2 (because it is a waste product of respiration that happens there) the molecule will easily give oxygen to the muscles that need it.

Moving to the left will mean the molecule has a greater affinity for oxygen and give it up less easily. In the lungs there are low amounts of CO2 because it is being removed by the lungs, this means the curve is moved to the left and haemoglobin easily picks up the oxygen it needs to.

This is called the Bohr effect. The movement happens because the shape of the molecule effects how it binds to oxygen, PH effects the shape and so CO2 which has a low PH will cause these changes.

Different organisms need different affinities of their haemoglobin because of the environment they live in and their metabolic rates.

By moving the curve to the left, an organism in an environment with a low partial pressure of oxygen can still fully load its haemoglobin.

An organism with a high metabolic rate needs oxygen frequently to respire and provide ATP (energy) to cells, this is achieved by moving the curve to the right so that oxygen dissociates easily even at higher partial pressures.