AQA AS Biology Unit 2: Haemoglobin

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  • Created on: 22-08-13 17:12
Preview of AQA AS Biology Unit 2: Haemoglobin

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Red blood cells contain haemoglobin.
The structure of a haemoglobin molecule is made up as follows:
Primary structure Consists of four polypeptide chains
Secondary structure Each of the polypeptides chains is coiled into
a helix
Tertiary structure Each polypeptide chain is folded into a
precise shape
Quaternary structure All four polypeptides are linked together to
form an almost spherical molecule.
Haemoglobin is made up of four amino acid chains.
In each of these chains you have a heme group.
An iron atom in the ferrous state (Fe 2+ ) sits in the middle of the heme.
This is where the oxygen binds.

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The Role of Haemoglobin
The role of haemoglobin is to transport oxygen.
To be efficient at transporting oxygen, haemoglobin must:
Readily associate with oxygen at the surface where gas exchange takes place
Readily dissociate with oxygen at those tissues requiring it
Haemoglobin changes its affinity for oxygen under different conditions.
It achieves this because its shape changes in the presence of certain substances, such
as carbon dioxide.…read more

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In the presence of carbon dioxide, the new shape of the haemoglobin molecule binds more
loosely to oxygen.
As a result haemoglobin releases its oxygen.…read more

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An organism living in an environment with little oxygen requires a haemoglobin
that readily combines with oxygen if it is to absorb enough of it.
Provided that the organisms metabolic rate is not very high, the fact that this form
of haemoglobin does not release its oxygen as readily into the tissues will not be a
An organism with a high metabolic rate needs to release oxygen readily into its
tissues.…read more

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The higher pH changes the shape of the haemoglobin into one that enables it to load
oxygen readily
The shape also increases the affinity of haemoglobin for oxygen, so it is not released
while being transported in the blood to the tissues
In the tissues, carbon dioxide is produced by respiring cells
Actively respiring tissues produce carbon dioxide which dissolves into tissue fluid to
make carbonic acid and so lowers the pH
The lower pH changes the shape of haemoglobin into one with a lower…read more

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The graph of this relationship is known as the oxygen dissociation curve.
Where pO2 is high, haemoglobin
has a high affinity for oxygen so it has a high saturati has a high affinity for oxygen,
it has a high saturation of
Where pO2 is low, haemoglobin
has a low affinity for oxygen,
so it has a low saturation of
This graph shows a very small decrease in the partial pressure of oxygen leads to a lot of
oxygen becoming dissociated from haemoglobin.…read more

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There are a number of different types of haemoglobin molecules, each with a different shape
and hence a different affinity for oxygen.
In addition, the shape of any one type of haemoglobin molecule can change under different
These facts both mean that there are a large number of different oxygen dissociation curves.
They all have a roughly similar shape but differ in their position on the axes.
Curve further to the left The greater the affinity of haemoglobin for
oxygen.…read more

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Rapidly Muscles Level of carbon Affinity of Oxygen
respiring dioxide is high. haemoglobin for dissociation
tissues oxygen is curve is
reduced, which, shifted to
coupled with the the right.
concentration of
oxygen in the
muscles means
that oxygen is
readily unloaded
from the
into the muscle
The greater the concentration of carbon dioxide, the more readily haemoglobin
releases its oxygen.
This is because dissolved carbon dioxide is acidic and the low pH causes haemoglobin to
change shape.…read more

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A human foetus makes a different kind of
haemoglobin from an adult. Foetal haemoglobin
has a higher affinity for oxygen at low partial
pressures, so its oxygen dissociation curve is
shifted up. A developing foetus obtains its oxygen,
not through its lungs, but from its mother's blood
in the placenta. So this different haemoglobin
allows oxygen to diffuse from the mother's blood to the foetus, and to be unloaded in the
foetal tissues.…read more

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Mice lose heat very quickly due to their large surface
area: volume ratio, so they have a high metabolic rate to generate more heat. Their tissues
therefore have a constant demand for oxygen for respiration. The oxygen dissociation curve
for mouse haemoglobin is shifted down compared to humans, so plenty of oxygen is
unloaded to all tissues all the time.…read more


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