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Oxygen Dissociation Curves:
When haemoglobin is exposed to different partial pressures
of oxygen, it doesn't bind to oxygen evenly
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•

The shape of the haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of its
four polypeptides subunits because they are closely
united
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•

The binding of this first oxygen molecule changes the
quaternary structure of the haemoglobin molecule,
causing it to change shape
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•

It therefore takes a smaller increase in the partial
pressure of oxygen to bind the second oxygen molecule than it did to bind the first one
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The gradient of the curve steepens
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This is all to do with
probability, with the majority of the binding sites occupied, its less likely that a single oxygen molecule
will find an empty site to bind to
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The further to the left of the curve, the greater is the affinity
of haemoglobin for oxygen
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Effects of CO2 Concentration:
Transport of oxygen
by haemoglobin

Haemoglobin has a reduced affinity for oxygen in
the presence of carbon dioxide
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At the gas exchange surface (lungs) the concentration of CO2 is low because it diffuses
across the exchange surface and is excreted
by the organism
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The reduced CO2 concentration has shifted the oxygen dissociation
curve to the left
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The affinity of
haemoglobin for oxygen is reduced, which
coupled with the low concentration of oxygen
in the muscles, means that oxygen is readily
unloaded from the haemoglobin into the
muscle cells
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The greater the concentration of CO2, the more
readily haemoglobin releases its oxygen
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Loading, Transport and Unloading of Oxygen:
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At the gas-exchange surface CO2 is constantly
being removed
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Oxygen diffuses into the lugworms blood
from the water
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In pracculate a fresh supply of oxygenated water through its
tise not all haemoglobin molecules are loaded with
burrow
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progressively less oxygen as the lugworm uses it up
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As a consequence, the overall saturation of haemohas to extract as much oxygen as possible from the
•
This shape also increases the affinity of haemo- globin at atmosphere pressure is normally around
water in the burrow if it is to survive until the tide coglobin for oxygen, so it isn't released while being 97%
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transported in the blood to the tissues
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of a human
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The blood returning to the
lugworm is fully loaded with oxygen even where there
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CO2 is acidic in solution, so the pH of the blood
lungs will therefore contain haemoglobin that is still
is little available in its environment
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75% saturated with oxygen
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At these altitudes the atmospheric pressure is
globin into one with a lower affinity for oxygen
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These different types have evolved within spelower
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cies as adaptations to different environments and
with oxygen
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This process is a flexible way of ensuring that there is
left of a human as they have a type of haemoglobin that
always sufficient oxygen for respiring tissues, the more For example, species of animals that live in an environ- has a greater affinity for oxygen than humans
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This
ment with a lower partial pressure of oxygen have
works as follows:
evolved haemoglobin that has a higher affinity for oxygen than the haemoglobin of animals that live where
Higher the rate of respiration —> the more CO2 the
the partial pressure of oxygen is higher
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the haemoglobin shape change —> the more readily
oxygen is unloaded —> the more oxygen is available
for respiration

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