A Level Biology help! How does low partial pressure lead to high affinity?

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frostfly
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For example, how does low partial pressure of O2 cause high affinity of haemoglobin for O2? I mean, is there even a causal relationship, or does it link to carbon dioxide?

Explanations very much appreciated!
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AortaStudyMore
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(Original post by honour)
For example, how does low partial pressure of O2 cause high affinity of haemoglobin for O2? I mean, is there even a causal relationship, or does it link to carbon dioxide?

Explanations very much appreciated!
So affinity depends on a number of things, the concentration of 2,3-DPG (a product of metabolism), the pH (and therefore CO2 by association), and the amount of oxygen bound. The type of haemoglobin also has an affect but I won't go into that.

So the partial pressure of O2 doesn't change the affinity of haemoglobin. In the tissues, where the pO2 is low, the pCO2 and 2,3 DPG is high, which lowers the affinity of haemoglobin so that it releases its oxygen. In the lungs where pCO2 and 2,3 DPG are low, the affinity is increased to allow the haemoglobin to get as much oxygen bound as possible. Also, if you look at it in terms of correlation instead of causation, the affinity for oxygen actually increases as pO2 increases, this is because as oxygen binds, it loosens up the haemoglobin structure, which allows more oxygen to bind more easily.

So actually, affinity increases as you increase pO2, hence why a oxyhaemogobin dissociation curve is sigmoid shaped.

Does that all make sense?
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en1
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organisms that live in an environment where there is a low po2 (low concentration of oxygen) have haemoglobin that has a higher affinity to oxygen, so oxygen loads more readily on to their haemoglobin.
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frostfly
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(Original post by AortaStudyMore)
So affinity depends on a number of things, the concentration of 2,3-DPG (a product of metabolism), the pH (and therefore CO2 by association), and the amount of oxygen bound. The type of haemoglobin also has an affect but I won't go into that.

So the partial pressure of O2 doesn't change the affinity of haemoglobin. In the tissues, where the pO2 is low, the pCO2 and 2,3 DPG is high, which lowers the affinity of haemoglobin so that it releases its oxygen. In the lungs where pCO2 and 2,3 DPG are low, the affinity is increased to allow the haemoglobin to get as much oxygen bound as possible. Also, if you look at it in terms of correlation instead of causation, the affinity for oxygen actually increases as pO2 increases, this is because as oxygen binds, it loosens up the haemoglobin structure, which allows more oxygen to bind more easily.

So actually, affinity increases as you increase pO2, hence why a oxyhaemogobin dissociation curve is sigmoid shaped.

Does that all make sense?
Perfect, you've explained very well. After reading this over a few times, it made a lot of sense. Thanks dude, you're fantastic!

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I love your username too, very original! :thumbsup:
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