3.2.12- Adult and Fetal Haemoglobin Flashcards
What does the oxygen dissociation curve show?
The oxygen dissociation curve shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO2)
What does partial pressures of oxygen mean?
Partial pressure of oxygen refers to the pressure exerted by oxygen within a mixture of gases; it is a measure of oxygen concentration
When is haemoglobin saturated?
Haemoglobin is referred to as being saturated when all of its oxygen binding sites are taken up with oxygen; so when it contains four oxygen molecules
What is affinity and describe the curve?
The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen
When haemoglobin has a high affinity it binds easily and dissociates slowly
When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily
With haemoglobin oxygen binds at different rates as the pO2 changes; hence the resulting curve
It can be said that haemoglobin’s affinity for oxygen changes at different partial pressures of oxygen
Explain the shape of the curve.
The curved shape of the oxygen dissociation curve for haemoglobin can be explained as follows
Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to haemoglobin; this means that binding of the first oxygen occurs slowly, explaining the relatively shallow curve at the bottom left corner of the graph
After the first oxygen molecule binds to haemoglobin, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind; this speeds up binding of the remaining oxygen molecules and explains the steeper part of the curve in the middle of the graph
The shape change of haemoglobin leading to easier oxygen binding is known as cooperative binding
As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites, explaining the levelling off of the curve in the top right corner of the graph
When the curve is read from left to right what can you interpret?
When the curve is read from left to right, it provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen
At low pO2, in the bottom left corner of the graph, oxygen binds slowly to haemoglobin; this means that haemoglobin cannot pick up oxygen and become saturated as blood passes through the body’s oxygen-depleted tissues
Haemoglobin has a low affinity for oxygen at low pO2, so saturation percentage is low
At medium pO2, in the central region of the graph, oxygen binds more easily to haemoglobin and saturation increases quickly; at this point on the graph a small increase in pO2 causes a large increase in haemoglobin saturation
At high pO2, in the top right corner of the graph, oxygen binds easily to haemoglobin; this means that haemoglobin can pick up oxygen and become saturated as blood passes through the lungs
Haemoglobin has a high affinity for oxygen at high pO2, so saturation percentage is high
Note that at this point on the graph increasing the pO2 by a large amount only has a small effect on the percentage saturation of haemoglobin; this is because most oxygen binding sites on haemoglobin are already occupied
When the curve is read from right to left what can you interpret?
When read from right to left, the curve provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen
In the lungs, where pO2 is high, there is very little dissociation of oxygen from haemoglobin
At medium pO2, oxygen dissociates readily from haemoglobin, as shown by the steep region of the curve; this region corresponds with the partial pressures of oxygen present in the respiring tissues of the body, so ready release of oxygen is important for cellular respiration
At this point on the graph a small decrease in pO2 causes a large decrease in percentage saturation of haemoglobin, leading to easy release of plenty of oxygen to the cells
At low pO2 dissociation slows again; there are few oxygen molecules left on the binding sites, and the release of the final oxygen molecule becomes more difficult, in a similar way to the slow binding of the first oxygen molecule
Describe the importance of foetal haemoglobin.
The haemoglobin of a developing foetus has a higher affinity for oxygen than adult haemoglobin
This is vital as it allows a foetus to obtain oxygen from its mother’s blood at the placenta
Fetal haemoglobin can bind to oxygen at low pO2
At this low pO2 the mother’s haemoglobin is dissociating with oxygen
Describe the presence of foetal haemoglobin on a dissociation curve.
On a dissociation curve graph, the curve for foetal haemoglobin shifts to the left of that for adult haemoglobin
This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin
What happens to foetal haemoglobin after birth?
After birth, a baby begins to produce adult haemoglobin which gradually replaces foetal haemoglobin
This is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual
Describe the importance of different types of haemoglobin.
Haemoglobin is a quaternary protein, made up of four globin polypeptides and four haem groups
The structure of haem is identical in all types of haemoglobin
The globin chains however can differ substantially between species
The globin polypeptides determine the precise properties of haemoglobin
There are a wide range of haemoglobin types that exist
They vary in their oxygen-binding properties
They bind to and release oxygen in different conditions
Environmental factors can have a major impact on the evolution of haemoglobin within a species
Describe the effects of altitude on partial pressures of oxygen.
The partial pressure of oxygen is lower at higher altitudes
Species living at high altitudes have haemoglobin that is adapted to these conditions
For example, llamas have haemoglobin that binds very readily to oxygen
This is beneficial as it allows them to obtain a sufficient level of oxygen saturation in their blood when the partial pressure of oxygen in the air is low
What is the bohr effect?
The bohr effect describes haemoglobins lower affinity for oxygen secondary to increases in the partial pressure of carbon dioxide.
