7.1 – Haemoglobin:
Haemoglobins are protein molecules with a
quaternary structure that has evolved to make it
efficient at loading oxygen under one set of
conditions but unloading it under a different set of
conditions
Primary structure: the sequence of amino acids in
the 4 polypeptide chains
Secondary structure: where each of the
polypeptide chains is coiled into a helix
Tertiary structure: in which each polypeptide chain
is folded into a precise shape – an important factor
in its ability to carry oxygen
Quaternary structure: where all 4 polypeptides are linked together to form an almost
spherical molecule. Each polypeptide is associated with a haem group which contains a
ferrous (Fe2+) ion. Each Fe2+ ion can combine with a single oxygen molecule making a
total of 4 oxygen molecules that can be carried by a single haemoglobin molecule in
humans
Loading and unloading oxygen:
The process by which haemoglobin binds with oxygen is called loading or associating.
In humans this takes place in the lungs
The process by which haemoglobin releases oxygen is called unloading or dissociating.
In humans this takes place in the tissue
Haemoglobins with a high affinity for oxygen take up oxygen more easily but release it
less readily
Haemoglobins with a low affinity for oxygen take up oxygen less easily but release it
more easily
Haemoglobin: the protein that transports oxygen around the body
Oxyhaemoglobin: is a haemoglobin with one or more oxygens loads
Myoglobin: is the molecule in muscles that is used to store oxygen for aerobic respiration
The role of haemoglobin:
To be efficient at transporting oxygen, haemoglobin must…
Readily associate with oxygen at the surface where gas exchange takes place
Readily dissociate from oxygen at those tissues requiring it
Although these properties contradict one another, haemoglobin can change its affinity
(chemical interaction) for oxygen under different conditions
It achieves this because of its shape
changes in the presence of certain
substances, such as carbon dioxide
In the presence of carbon dioxide, the
new shape of the haemoglobin molecule
binds more loosely to oxygen. As a result,
releases its oxygen
Why are there different haemoglobins?
Each species produces a haemoglobin with a slightly different amino acid sequence
, The haemoglobin of each species therefore has a slightly different tertiary and
quaternary structure and hence different oxygen binding properties
Depending on its structure, haemoglobin molecules range from those that have a high
affinity for oxygen to those that have a low affinity for oxygen
7.2 - Transport of oxygen by haemoglobin:
Oxygen dissociation curves:
When haemoglobin is exposed to different partial
pressures of oxygen, it does not bind the oxygen evenly
The graph of the relationship between the saturation of
haemoglobin with oxygen and the partial pressure of
oxygen is known as the oxygen dissociation curve
The shape of the haemoglobin makes it difficult for the
first oxygen molecule to bind to one of the sites on its 4-
polypeptide subunit because they are closely united.
Therefore, at low oxygen concentrations, little oxygen
binds to haemoglobin. The gradient of the curve is shallow initially
However, the binding of the first oxygen molecule changes the quaternary structure of
the haemoglobin molecule, causing it to change shape. This change makes it easier for
subunits to bind to an oxygen molecule. In other words, the binding of the first oxygen
molecule induces the other subunits to bind to an oxygen molecule
It therefore takes a smaller increase in the partial pressure of oxygen to bind the
second oxygen molecule that it did to bind the first one. This is known as a positive
cooperativity because binding on the first molecule makes binding of the second easier
and so on. The gradient of the curve steepens
The situation changes, however, after the binding of the third molecule. While in theory
it is easier for haemoglobin to bind the fourth oxygen molecule, in practice it is harder.
This is simply due to probability, with the majority of the binding sites occupied, it is
less likely that a single oxygen molecule will find an empty site to bind to. The gradient
of the curve reduces, and the graph flattens off
The further to the left the curve, the greater the affinity of haemoglobin for oxygen
(loads oxygen readily but unloads it less easily)
The further to the right the curve, the lower is the affinity of haemoglobin for oxygen
(loads oxygen less readily but unloads it more easily)
Effects of carbon dioxide concentration:
Haemoglobin has a reduced affinity for oxygen in the
presence of carbon dioxide
The greater the concentration of carbon dioxide, the more
readily the haemoglobin releases its oxygen (the Bohr
effect)
This explains why the behaviour of haemoglobin changes
in different regions of the body
At the gas exchange surface (e.g lungs) the concentration
of carbon dioxide is low because it diffuses across the
exchange surface and is excreted from the organism. The
affinity of haemoglobin for oxygen is increased, which
coupled with the high concentration of oxygen in the lungs
means that oxygen is readily loaded by haemoglobin. The reduced carbon dioxide
concentration has shifter the oxygen dissociation curve to the left
, In rapidly respiring tissues (e.g muscles) the concentration of carbon dioxide is high.
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
haemoglobin into the muscle cells. The increased carbon dioxide concentration has
shifted the oxygen dissociation curve to the right
Dissolved carbon dioxide is acidic, and the low pH causes haemoglobin to change
shape
Loading, transport and unloading oxygen:
At the gas exchange surface carbon dioxide is constantly being removed
The pH is slightly raised due to the low concentration of carbon dioxide
The higher pH changes the shape of 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
which being transports in the blood to the tissues
In the tissues, carbon dioxide is produced by respiring cells
Carbon dioxide is acidic in solution so the pH of the blood within the tissues is lowered
The lower PH changes the shape of the haemoglobin into one with a lower affinity for
oxygen
Haemoglobin releases its oxygen into the respiring tissues
The
lugworm:
The lugworm is not very active
Most of the time, the lugworm is covered by sea water
which it circulates through its burrow
Oxygen diffuses into the lugworm’s blood from the water,
and it uses haemoglobin to transport its oxygen to its
tissues
When the tide goes out, the lugworm can no longer
circulate a fresh supply of oxygen through its burrow
As a result, the water in the burrow contains progressively
less oxygen as the lugworm uses it up
The lugworm has to extract as much oxygen as possible from the water in the burrow if
it is to survive until the tide cover it again
This dissociation curve is shifted far to the left of that of a human meaning that the
haemoglobin of the lugworm is fully loaded with oxygen even when there is little
available in its environment
Another example is the llama. It is an animal that lives at high altitudes meaning the
partial pressure of oxygen is lower, so they have haemoglobin with a higher affinity for
oxygen that human haemoglobin
7.3 – Circulatory system of a mammal:
Diffusion is fast enough for transport over short distances. The efficient supply of materials
over larger distances requires a mass transport system
Why large organisms have a transport system:
Haemoglobins are protein molecules with a
quaternary structure that has evolved to make it
efficient at loading oxygen under one set of
conditions but unloading it under a different set of
conditions
Primary structure: the sequence of amino acids in
the 4 polypeptide chains
Secondary structure: where each of the
polypeptide chains is coiled into a helix
Tertiary structure: in which each polypeptide chain
is folded into a precise shape – an important factor
in its ability to carry oxygen
Quaternary structure: where all 4 polypeptides are linked together to form an almost
spherical molecule. Each polypeptide is associated with a haem group which contains a
ferrous (Fe2+) ion. Each Fe2+ ion can combine with a single oxygen molecule making a
total of 4 oxygen molecules that can be carried by a single haemoglobin molecule in
humans
Loading and unloading oxygen:
The process by which haemoglobin binds with oxygen is called loading or associating.
In humans this takes place in the lungs
The process by which haemoglobin releases oxygen is called unloading or dissociating.
In humans this takes place in the tissue
Haemoglobins with a high affinity for oxygen take up oxygen more easily but release it
less readily
Haemoglobins with a low affinity for oxygen take up oxygen less easily but release it
more easily
Haemoglobin: the protein that transports oxygen around the body
Oxyhaemoglobin: is a haemoglobin with one or more oxygens loads
Myoglobin: is the molecule in muscles that is used to store oxygen for aerobic respiration
The role of haemoglobin:
To be efficient at transporting oxygen, haemoglobin must…
Readily associate with oxygen at the surface where gas exchange takes place
Readily dissociate from oxygen at those tissues requiring it
Although these properties contradict one another, haemoglobin can change its affinity
(chemical interaction) for oxygen under different conditions
It achieves this because of its shape
changes in the presence of certain
substances, such as carbon dioxide
In the presence of carbon dioxide, the
new shape of the haemoglobin molecule
binds more loosely to oxygen. As a result,
releases its oxygen
Why are there different haemoglobins?
Each species produces a haemoglobin with a slightly different amino acid sequence
, The haemoglobin of each species therefore has a slightly different tertiary and
quaternary structure and hence different oxygen binding properties
Depending on its structure, haemoglobin molecules range from those that have a high
affinity for oxygen to those that have a low affinity for oxygen
7.2 - Transport of oxygen by haemoglobin:
Oxygen dissociation curves:
When haemoglobin is exposed to different partial
pressures of oxygen, it does not bind the oxygen evenly
The graph of the relationship between the saturation of
haemoglobin with oxygen and the partial pressure of
oxygen is known as the oxygen dissociation curve
The shape of the haemoglobin makes it difficult for the
first oxygen molecule to bind to one of the sites on its 4-
polypeptide subunit because they are closely united.
Therefore, at low oxygen concentrations, little oxygen
binds to haemoglobin. The gradient of the curve is shallow initially
However, the binding of the first oxygen molecule changes the quaternary structure of
the haemoglobin molecule, causing it to change shape. This change makes it easier for
subunits to bind to an oxygen molecule. In other words, the binding of the first oxygen
molecule induces the other subunits to bind to an oxygen molecule
It therefore takes a smaller increase in the partial pressure of oxygen to bind the
second oxygen molecule that it did to bind the first one. This is known as a positive
cooperativity because binding on the first molecule makes binding of the second easier
and so on. The gradient of the curve steepens
The situation changes, however, after the binding of the third molecule. While in theory
it is easier for haemoglobin to bind the fourth oxygen molecule, in practice it is harder.
This is simply due to probability, with the majority of the binding sites occupied, it is
less likely that a single oxygen molecule will find an empty site to bind to. The gradient
of the curve reduces, and the graph flattens off
The further to the left the curve, the greater the affinity of haemoglobin for oxygen
(loads oxygen readily but unloads it less easily)
The further to the right the curve, the lower is the affinity of haemoglobin for oxygen
(loads oxygen less readily but unloads it more easily)
Effects of carbon dioxide concentration:
Haemoglobin has a reduced affinity for oxygen in the
presence of carbon dioxide
The greater the concentration of carbon dioxide, the more
readily the haemoglobin releases its oxygen (the Bohr
effect)
This explains why the behaviour of haemoglobin changes
in different regions of the body
At the gas exchange surface (e.g lungs) the concentration
of carbon dioxide is low because it diffuses across the
exchange surface and is excreted from the organism. The
affinity of haemoglobin for oxygen is increased, which
coupled with the high concentration of oxygen in the lungs
means that oxygen is readily loaded by haemoglobin. The reduced carbon dioxide
concentration has shifter the oxygen dissociation curve to the left
, In rapidly respiring tissues (e.g muscles) the concentration of carbon dioxide is high.
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
haemoglobin into the muscle cells. The increased carbon dioxide concentration has
shifted the oxygen dissociation curve to the right
Dissolved carbon dioxide is acidic, and the low pH causes haemoglobin to change
shape
Loading, transport and unloading oxygen:
At the gas exchange surface carbon dioxide is constantly being removed
The pH is slightly raised due to the low concentration of carbon dioxide
The higher pH changes the shape of 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
which being transports in the blood to the tissues
In the tissues, carbon dioxide is produced by respiring cells
Carbon dioxide is acidic in solution so the pH of the blood within the tissues is lowered
The lower PH changes the shape of the haemoglobin into one with a lower affinity for
oxygen
Haemoglobin releases its oxygen into the respiring tissues
The
lugworm:
The lugworm is not very active
Most of the time, the lugworm is covered by sea water
which it circulates through its burrow
Oxygen diffuses into the lugworm’s blood from the water,
and it uses haemoglobin to transport its oxygen to its
tissues
When the tide goes out, the lugworm can no longer
circulate a fresh supply of oxygen through its burrow
As a result, the water in the burrow contains progressively
less oxygen as the lugworm uses it up
The lugworm has to extract as much oxygen as possible from the water in the burrow if
it is to survive until the tide cover it again
This dissociation curve is shifted far to the left of that of a human meaning that the
haemoglobin of the lugworm is fully loaded with oxygen even when there is little
available in its environment
Another example is the llama. It is an animal that lives at high altitudes meaning the
partial pressure of oxygen is lower, so they have haemoglobin with a higher affinity for
oxygen that human haemoglobin
7.3 – Circulatory system of a mammal:
Diffusion is fast enough for transport over short distances. The efficient supply of materials
over larger distances requires a mass transport system
Why large organisms have a transport system:
