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| Red Blood Cells |
The unique
nature of haemoglobin, together with the advent of complex circulatory systems,
has allowed terrestrial vertebrates to optimise oxygen uptake.
Haemoglobin is
a respiratory pigment found in red blood cells. It consists of four protein, or
‘globin’ chains to which are attached four haem groups. Each haem group has an
iron atom at its centre that is capable of binding to oxygen. It is the
oxygen-carrying capacity of haemogobin that has allowed vertebrates to produce
more aerobic energy, and therefore to become larger in size, than other
organisms.
Haemoglobin’s Oxygen- Carrying Capacity
More primitive
animals such as sea anemones rely solely upon the diffusion of oxygen into
their cells from their surroundings. This process is slow, limiting the size of
organisms and restricting them to an aquatic lifestyle. Slightly more complex
organisms, such as nematodes, possess a rudimentary circulatory system that
allows them to push interstitial fluid around their bodies with muscular
movements, but the movement of oxygen and nutrients to cells still relies
principally upon diffusion.
Oxygen,
moreover, has limited solubility in water – only 1.5% of the oxygen carried in
mammalian blood is carried in a dissolved form. The remainder is instead
carried on haemoglobin, which increases the oxygen-carrying capacity of the
blood one hundred-fold. Haemoglobin’s unique ability to bind to up to four
oxygen molecules, along with the development of complex circulatory systems in
more advanced vertebrates, has accordingly produced a more efficient system of
supplying oxygen to cells.
This capacity
to rapidly supply cells with oxygen has allowed complete aerobic cellular
respiration to occur, which produces 18 times more energy (in the form of
energy storing ATP molecules) than anaerobic respiration in simpler organisms
such as yeasts. As a result, oxygen can access and power the cells of animals
as large as the blue whale.
Respiratory Pigments in Other Organisms
Most
organisms, including plants, contain some form of haemoglobin or other
oxygen-binding pigment. However, among the more advanced animals only
vertebrate haemoglobin is tetrameric; that is, it contains four haem groups
that enable it to carry four oxygen molecules. Invertebrates such as molluscs
and sea cucumbers, for instance, have dimeric haemoglobin, which can only carry
two oxygen molecules.
Other
invertebrates, such as crabs and earthworms, possess a different respiratory
pigment altogether: crabs have haemocyanin, a copper-containing pigment that
gives their blood a bluish colour, while earthworm blood can either contain chlorocruorin,
a greenish pigment, or hemerythrin, a reddish pigment.
The Adaptability of Haemoglobin to Metabolic Needs
Unlike these
more primitive respiratory pigments, the unique nature of mammalian haemoglobin
has allowed it to maximize oxygen uptake and delivery in an unprecedented
manner. The haemoglobin molecule responds, for instance, to partial pressures
of oxygen in the body. In areas of high oxygen concentration, such as the
lungs, it is stimulated to bind with oxygen molecules, whereas in oxygen-poor
environments it releases oxygen to the cells.
This is partly
facilitated by the haem group’s ability to form only loose bonds with oxygen,
which allows for the easy attachment or release of oxygen molecules. As a
result, it can rapidly bind with four oxygens to become ‘oxyhaemoglobin’
(responsible for the bright red colour of oxygenated blood), or release one
oxygen molecule at a time, as cells require them.
Furthermore,
oxygen attachment becomes easier with the addition of subsequent oxygen
molecules. This is because as each molecule of oxygen is added it slightly
changes the shape of the haemoglobin, making bonding less difficult. As a
consequence of this 'cooperative' binding, a slight increase in oxygen partial
pressure in the lungs can result in rapid blood oxygen saturation.
Haemoglobin and Carbon Dioxide
The presence
of excess carbon dioxide in the blood from respiring cells can also trigger
haemoglobin to release oxygen to these cells; in other words, precisely where
it is needed. This is because another part of the haemoglobin molecule is able
to bind to carbon dioxide, creating a conformational change in the molecule
that allows oxygen to be readily released by the haem group.
Although most
carbon dioxide is carried in the blood as dissolved bicarbonate ions,
haemoglobin’s ability to attach to carbon dioxide and to release it in the
lungs greatly assists the respiratory processes in higher vertebrates. Whether
or not haemoglobin attaches to carbon dioxide or oxygen depends on the partial
pressures of these gases in different parts of the body - a phenomenon known as
the ‘Bohr effect’.
Haemoglobin and Red Blood Cells
The flat,
disc-like nature of red blood cells also provides a greater surface area for
haemoglobin attachment, adding to the oxygen-carrying capacity of vertebrate
blood. In addition, red blood cells do not contain a nucleus, which means there
is more room for haemoglobin – one red blood cell can in fact contain up to 280
million haemoglobnin molecules. Moreover, the very fact that haemoglobin is
contained within these blood cells means that it does not interfere with the
osmotic pressure of the blood plasma, as it does in invertebrates such as
earthworms.
Another
advantage of haemoglobin is that its blood concentration can increase at high
altitudes, allowing populations such as the natives of the Andes in South
America to survive in oxygen-poor environments. The actual number of red blood
cells, and thus haemoglobin, can also increase in such conditions, allowing for
even greater oxygen uptake.
It is evident,
then, that the structure of the haemoglobin molecule itself, along with its
ability to adapt to its immediate surroundings, has given higher vertebrates
the respiratory advantage they need to thrive in terrestrial environments.
Resources
•
Fago, A., Muller, G. and Weber, e., 2003, ‘Water Regulates Oxygen
Binding in Hagfish Haemoglobin’, University of Aarhus, Denmark, Journal of
Experimental Biology
•
Hot Topics, ‘The Non-Vertebrate Haemoglobins’, Brookscole.com
•
McDowell, J., 2005, ‘Haemoglobin’, ebi.ac.uk
•
Pennsylvania State University, ‘Evolution of Myoglobin/Haemoglobin
Proteins’, pearsonhighered.com
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