Mass transport
Haemoglobin
Haemoglobin is a water soluble globular protein which consists of two beta
polypeptide chains and two alpha helices (chains). Each molecule forms a
complex containing a haem group. It carries oxygen in the blood as oxygen can
bind to the haem (Fe2+) group. Each molecule can carry four oxygen molecules
because each polypeptide chain is associated with a haem group. Haemoglobin
has a quaternary structure.
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 tissues.
Haemoglobin with a high affinity (liking) for oxygen takes up oxygen more
easily but releases it less easily. Haemoglobin with a low affinity for oxygen
takes up oxygen less easily but releases it more easily.
- Condition: neutral pH, cool (lungs)
- High affinity for oxygen
- Gas exchange surface has a high concentration of oxygen and a low
concentration of carbon dioxide.
- Conditions: acidic, warm (tissues)
- Low affinity for oxygen
- Gas exchange surface has a high concentration of carbon dioxide and a
low concentration of oxygen.
Haemoglobin changes its affinity (chemical liking) for oxygen under different
conditions. It changes its shape in the presence of certain substances, such as
in the presence of carbon dioxide, the new shape of haemoglobin binds more
loosely to oxygen so it releases it.
,How DNA leads to different haemoglobin molecules having different affinities
for oxygen
- Each species produces a haemoglobin with a slightly different amino acid
sequence (primary structure)
- So they will have different tertiary and quaternary structures and hence
different oxygen binding properties
- The affinity will therefore depend on its structure
How the whole process works:
The affinity of oxygen for haemoglobin varies depending on the partial
pressure of oxygen which is a measure of oxygen concentration. The greater
the concentration of dissolved oxygen in cells the greater the partial pressure.
Therefore, as partial pressure increases, the affinity of haemoglobin for
oxygen increases, that is, oxygen binds to haemoglobin tightly. This occurs in
the lungs in the process known as loading. During respiration, oxygen is used
up and therefore the partial pressure decreases, thus decreasing the affinity
of oxygen for haemoglobin. As a result of that, oxygen is released in respiring
tissues where it is needed. After the unloading process, the haemoglobin
returns to the lungs where it binds to oxygen again.
, Oxygen dissociation curves
Lungs
Tissues
- Initially, the gradient of the curve is shallow because the shape of the
haemoglobin makes it difficult for the first molecule of oxygen to bind to
one of the sites on its four polypeptide subunits because they are closely
united so at low concentrations of oxygen, little oxygen binds to the
haemoglobin.
- The binding of the first oxygen molecule changes the tertiary
(quaternary) structure of the haemoglobin making it easier for the next
oxygen to bind.
- It therefore takes a smaller increase in the partial pressure of oxygen to
bind the second oxygen than it did for the first one, this is known as
positive cooperativity. The same principle makes the binding of the third
oxygen easier. The gradient now steepens.
- The binding of the last oxygen is harder again because the majority of
the binding sites are occupied, so it’s less likely that an oxygen will find
an empty site to bind to. The gradient of the curve reduces, and the
graph flattens off.
Haemoglobin
Haemoglobin is a water soluble globular protein which consists of two beta
polypeptide chains and two alpha helices (chains). Each molecule forms a
complex containing a haem group. It carries oxygen in the blood as oxygen can
bind to the haem (Fe2+) group. Each molecule can carry four oxygen molecules
because each polypeptide chain is associated with a haem group. Haemoglobin
has a quaternary structure.
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 tissues.
Haemoglobin with a high affinity (liking) for oxygen takes up oxygen more
easily but releases it less easily. Haemoglobin with a low affinity for oxygen
takes up oxygen less easily but releases it more easily.
- Condition: neutral pH, cool (lungs)
- High affinity for oxygen
- Gas exchange surface has a high concentration of oxygen and a low
concentration of carbon dioxide.
- Conditions: acidic, warm (tissues)
- Low affinity for oxygen
- Gas exchange surface has a high concentration of carbon dioxide and a
low concentration of oxygen.
Haemoglobin changes its affinity (chemical liking) for oxygen under different
conditions. It changes its shape in the presence of certain substances, such as
in the presence of carbon dioxide, the new shape of haemoglobin binds more
loosely to oxygen so it releases it.
,How DNA leads to different haemoglobin molecules having different affinities
for oxygen
- Each species produces a haemoglobin with a slightly different amino acid
sequence (primary structure)
- So they will have different tertiary and quaternary structures and hence
different oxygen binding properties
- The affinity will therefore depend on its structure
How the whole process works:
The affinity of oxygen for haemoglobin varies depending on the partial
pressure of oxygen which is a measure of oxygen concentration. The greater
the concentration of dissolved oxygen in cells the greater the partial pressure.
Therefore, as partial pressure increases, the affinity of haemoglobin for
oxygen increases, that is, oxygen binds to haemoglobin tightly. This occurs in
the lungs in the process known as loading. During respiration, oxygen is used
up and therefore the partial pressure decreases, thus decreasing the affinity
of oxygen for haemoglobin. As a result of that, oxygen is released in respiring
tissues where it is needed. After the unloading process, the haemoglobin
returns to the lungs where it binds to oxygen again.
, Oxygen dissociation curves
Lungs
Tissues
- Initially, the gradient of the curve is shallow because the shape of the
haemoglobin makes it difficult for the first molecule of oxygen to bind to
one of the sites on its four polypeptide subunits because they are closely
united so at low concentrations of oxygen, little oxygen binds to the
haemoglobin.
- The binding of the first oxygen molecule changes the tertiary
(quaternary) structure of the haemoglobin making it easier for the next
oxygen to bind.
- It therefore takes a smaller increase in the partial pressure of oxygen to
bind the second oxygen than it did for the first one, this is known as
positive cooperativity. The same principle makes the binding of the third
oxygen easier. The gradient now steepens.
- The binding of the last oxygen is harder again because the majority of
the binding sites are occupied, so it’s less likely that an oxygen will find
an empty site to bind to. The gradient of the curve reduces, and the
graph flattens off.
