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Biology revision – Mass transport (Unit 7)
Haemoglobin
- These are a group of molecules adapted to transport O 2
Haemoglobin molecules
- Are a group of chemically similar molecules found in a wide variety of organisms
- Protein molecules with a quaternary structure that has evolved to make it efficient at
carrying O2 under one set of conditions but releasing it under different conditions
- It is made up as follows:
o Primary structure – sequence of aminos in the 4 polypeptide chains
o Secondary structure – each of these polypeptide chains are coiled into a helix
o Tertiary structure – each polypeptide chain is folded into a precise shape – this
shape is an important factor in its ability to carry O 2
o Quaternary structure – all four polypeptides are linked together to form an almost
spherical molecule
Each polypeptide chain is associated with a haem group – contains a Fe 2+
Each Fe2+ ion can combine with a single O2 molecule making a total of
four O2 molecules that can be carried by a single haemoglobin in a human
Loading and unloading oxygen
- Binding haemoglobin and O2 is known as loading – takes place in lungs in human
- Releasing O2 from haemoglobin is known as unloading – takes place in tissues in
human
- Haemoglobins with a high affinity for O2 take up O2 more easily but release it less easily
- Haemoglobins with a low affinity for O2 take up O2 less easily but release it more easily
o Affinity – Its chemical attraction
How tightly it binds – High affinity = very tight binding
Role of haemoglobin
- Role is to transport O2 and in order to be efficient at this haemoglobin must:
o Readily associate with O2 at surface where exchange readily takes place
o Readily disassociate from O2 at those tissues requiring it
- These functions are achieved by haemoglobins ability to change its affinity to O 2 under
different conditions
- Its achieves this as its shape changes in the presence of certain substances, e.g. CO 2
- In the presence of CO2 the new shape of the haemoglobin molecule binds more loosely
to O2 – as a result O2 is released
Region of the body Oxygen CO2 Affinity of haemoglobin Result
concentration concentration for oxygen
Gas exchange surface High Low High Oxygen is
associated
Respiring tissues Low High Low Oxygen is
disassociated
Why are there different haemoglobins?
- Scientists found that there are different types of haemoglobins showing different
properties of how they released and took up O2
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