Hind Leys Biology
F211
Transport in animals 5.6
Transport of oxygen
Objectives
 Describe the role of haemoglobin in the transport of oxygen.
 Explain the significance of the different affinities for oxygen of foetal
haemoglobin and adult haemoglobin.
Haemoglobin
Oxygen is transported in the red blood cells, or erythrocytes, which contain the protein
haemoglobin. When oxygen combines with haemoglobin it becomes oxyhaemoglobin.
Haemoglogin + Oxygen → Oxyhaemoglobin
Hb
+ 4O2 → HbO8
Simplified to Hb + O2 → HbO2
Haemoglobin is a complex protein with four subunits.
Each subunit consists of a polypeptide (protein) chain
and a haem (non-protein) group. The haem group
contains a single iron ion, Fe2+. This group has an
affinity for oxygen, and each haem group can bind one
oxygen molecule. In total, a haemoglobin molecule can
carry four oxygen molecules.
Figure 1 Haemoglobin structure
1. Describe the meaning of the term ‘affinity’.
2. Explain how red blood cells are adapted for the uptake and transport of oxygen.
Just small enough to pass thru capillaries-short diffusion path, flattened shape
increases surface area and increases exposure of Hb to O2, donut shape allows
flexibility, no organelles so more space for Hb, filled with Hb- O2 not v water sol
so only small amount transported in plasma
3. Red blood cells contain some enzymes which are involved in carbon dioxide
transport. Explain why these enzymes cannot be replaced.
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Hind Leys Biology
F211
Transport in animals 5.6
Taking up and releasing oxygen
Oxygen diffuses into the blood from the lungs. From the plasma, they diffuse into red
blood cells and are bound to haemoglobin. This takes the oxygen molecules out of
solution and so helps to maintain the steep concentration gradient, allowing more oxygen
to enter the blood.
In the tissues oxygen is needed for aerobic respiration. Oxyhaemoglobin must be able to
release its oxygen where it is needed in the tissues. This is called dissociation.
Haemoglobin and oxygen transport
The ability of haemoglobin to take up and release oxygen depends on the amount of
oxygen in the surrounding tissues. This is measured by the relative pressure that oxygen
contributes to a mixture of gases, known as the partial pressure, pO2. It is also known
as the oxygen tension and is measured in kilopascals, kPa.
Haemoglobin can take up oxygen in a way that produces as S-shaped curve called an
oxyhaemoglobin dissociation curve.It is S-shaped because of the behaviour of the
Haemoglobin is different at different pO2.
It shows that at the higher and lower
partial pressures, there isn’t much change
in the saturation of the Hb.
But in the middle range, a small change in
the pO2 can result in a large change in the
percentage saturation of the blood.
Venous
pO2
Arterial
pO2
Figure 2 Oxyhaemoglobin dissociation curve
Explaining the shape of the curve
 At low pO2, haemoglobin does not readily take up oxygen. This is because
the haem group is in the middle of the haemoglobin molecule. This makes it
difficult for the oxygen molecules to diffuse in and associate with the haem
group. This difficulty is responsible for the flat part of the curve at low
oxygen tension.
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Hind Leys Biology
F211
Transport in animals 5.6
 As the oxygen tension rises, the diffusion gradient into the haemoglobin
molecule
increases.
An
oxygen
molecule
eventually
diffuses
into
the
haemoglobin and associates with one of the haem groups. This causes a
slight change in shape of the haemoglobin, a conformational change. This
allows further oxygen molecules to diffuse in and bind more easily,
accounting for the steep part of the curve.
 Once 3 O2 molecules are bound, it becomes more difficult for the final
molecule to diffuse in and bind. It is therefore difficult to achieve 100%
saturation of all haemoglobin molecules, even at very high pO2, shown by
flat part of curve.
Mammalian haemoglobin is well adapted to transporting oxygen to the tissues. The
oxygen tension in the lungs is sufficient to produce almost 100% saturation. In the
respiring tissues it is sufficiently low to cause oxygen to dissociate readily from the
oxyhaemoglobin.
4. When the body is at rest, only one of the four oxygen molecules carried by
haemoglobin is normally released into the tissues. Suggest why this could be an
advantage when the organism becomes more active.
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Hind Leys Biology
F211
Transport in animals 5.6
The Bohr effect
In the presence of carbon dioxide, haemoglobin changes shape allowing it to bind more
loosely to oxygen.
The amount of O2 carried and released by
Hb depends on pH.

Low pH (due to high CO2) causes HbO2 to
dissociate from O2. A small decrease in
the pH results in a large decrease in the
% saturation of Hb with O2, even if pO2 is
high. The more CO2, the more the curve
shifts to the right
Figure 3 The effect of pCO2 on oxyhaemoglobin dissociation
.
ANY SHIFT TO THE RIGHT = DECREASED AFFINITY FOR O2
The graph can be used to explain the following points:
o
There is high [O2] in the lungs and low [CO2]
o
The haemoglobin will therefore be virtually saturated with O 2.
o
In tissues (actively respiring) there will be low [O2] and high [CO2]
The haemoglobin will therefore give up a lot of the O2 that it is transporting.
5. Explain the difference in the percentage saturation of haemoglobin at high pCO2.
In summary,
 At the gas exchange surface carbon dioxide is constantly being removed.
 The pH is raised due to the low level of carbon dioxide.
 The higher pH changes the shape of haemoglobin into one that enables it to load
oxygen rapidly.
 This 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.
 Carbon dioxide is acidic in solution, so the pH of the blood within the tissue is
decreased.
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Hind Leys Biology


F211
Transport in animals 5.6
The lower pH changes the shape of haemoglobin into one with a lower affinity
for oxygen.
Haemoglobin releases its oxygen into the respiring tissues.
Foetal haemoglobin and myoglobin

A foetus needs to be able to
extract O2 from an environment
which makes adult Hb release it.

Foetal Hb has a higher affinity for
O2 than adult Hb so picks up O2
from the blood, reducing pO2 and
making adult Hb release it.

Figure 4 Dissociation curves for foetal Hb and myoglobin
The curve for foetal Hb lies to the
left of the normal curve.
ANY SHIFT TO THE LEFT = INCREASED AFFINITY FOR O2
6. The pO2 in the placenta may be 4kPa. What percentage of the mother’s
haemoglobin will dissociate?
7. At this oxygen tension, what percentage of foetal haemoglobin will become
saturated?
8. Explain the advantage of the foetal haemoglobin having a different curve to the
mother’s.
A foetus starts to make more adult haemoglobin towards the time of birth. After birth,
the proportion of foetal haemoglobin in the blood declines quite rapidly.
9. Explain why it is important for the baby to have adult haemoglobin after birth.
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Hind Leys Biology
F211
Transport in animals 5.6
Myoglobin is a pigment found in muscles that is very similar to haemoglobin. Like
haemoglobin it contains haem but each molecule contains only one polypeptide chain and
one haem group.
10. How many oxygen molecules can a molecule of myoglobin combine with?
11. Use the graph to explain how the myoglobin in a muscle acts as an oxygen store
that is used when the muscle is particularly active.
12. How can the oxygen store in myoglobin be replaced after use?
During exercise, the body temperature rises due to the heat produced by increased
respiration in the muscles.
13. Which way has the curve shifted and
what is the significance of this?
Shift to the right. Therefore increasing
the release of O2 to respiring muscles
during exercise
Figure 5 The effect of temperature on oxyhaemoglobin dissociation
14. Complete the following table showing the affinity of haemoglobin for oxygen
under different conditions.
Region of body
O2
CO2
Affinity of Hb
Effects
concentration
concentration
for oxygen
Lungs
High
Low
High
Association
Tissues
Low
High
Low
Dissociation
This work can be reinforced using pages 79-86 of your textbook.
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