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Haemoglobin molecule

There is a famous graph of no.of oxygen moles held by Haemoglobin vs the partial pressure of oxygen. Could someone explain the shape of the curve?

Reply 1

tomtheone

If you are talking about the S shaped dissociation curve then here goes,
At a high partial pressure there is more Oxygen to associate with the haemoglobin so the haemoglobin has a higher saturation of Oxygen. This part of the curve occurs in the lungs where there will be high ammounts of Oxygen and so willl associate with the haemoglobin.

At alow partial pressure the haemoglobin will have a low saturation. This means that less Oxygen will associate with the haemoglobin but more will dissociate. So Oxygen will be given up by the haemoglobin. This occurs at the tissues so they can receive a lot of Oxygen. I think that is it, hope it helps.

PS if you didn't know associate = joins
dissociate = unjoin

Reply 2

corkskrew

The reason it's an odd shape and not linear (straight line) is that each Hb molecule can hold 4 oxygen molecules. When one oxygen binds to Hb an allosteric change occurs which makes the other binding sites on Hb more susceptible to bind oxygen as well (increases their affinity). So the presence of oxygen on Hb will make the ability of Hb to bind oxygen better.

Edit...
Note: Allosteric just means that one interaction by the enzyme changes another part of the molecule, either making it more or less able to do something.

Reply 3

aiman

A large change in pp of oxygen is needed to increase the no.of moles from 3 to 4. Is this due to a steric factor?

(I am interested in the Biochemistry of the disassociation curve)

Reply 4

corkskrew

I think you're getting at the fact that you need to almost double the pp of O2 in order to move from 70%-ish saturation to 90-100%.
This represents the activity in the lungs... as most of the Hb is saturated already a small change in pO2 is not going to add significantly to the amount of bound O2.
The curve is showing the percentage of Hb molecules which are carrying 4 oxygens... it's not that every Hb molecule must pick up one oxygen molecule before they all start getting a second... that would give a 4 phase graph.

Reply 5

Stranded

I have a question regarding this topic too. I've come across some terms as 'loading & unloading tension'. What does that mean?

Reply 6

corkskrew

Neophyte~

I have a question regarding this topic too. I've come across some terms as 'loading & unloading tension'. What does that mean?

Just the partial pressures of oxygen at which oxygen is released by the Hb molecule. At low partial pressures oxygen is released (this low pp is the unloading tension). At higher pps Oxygen binds Hb... the loading tension.

OK thank you :smile:

Neophyte~

I have a question regarding this topic too. I've come across some terms as 'loading & unloading tension'. What does that mean?

I *THINK* the unloading tension is the pressure at which oxygen is 50% saturated and the loading tension is 95% saturated.

I am not sure about the numbers the this is the general idea.