Oxygen Dissociation Curve - AS

If you decrease the partial pressure of O₂ by a small amount (just above when it is steep), then you will notice there is a large drop in the saturation of Hb with O₂. So a small decrease in pO₂ leads to a lot of O₂ becoming dissociated with Hb and available for respiration.

Reply 2

Ellie4

2

Ahhrrrr. Bohr shift *Cries and runs away*

Reply 3

Basically, the oxygen dissociation curve means that where the oxygen is in a very high concentration, for example in the lungs, haemoglobin has a very high affinity for oxygen and so it has a very high saturation of oxygen. This allows as much oxygen as possible to be taken up into the haemoglobin in the red blood cells from the alveoli in the lungs.

Where the oxygen concentration is low, for example in the muscles, haemoglobin has a low affinity of oxygen. This allows oxygen to be relased at the muscles where it is needed for aerobic respiration.

Therefore the shape of the oxygen dissocation curve allows efficient transportation of oxygen.

Hope that explains things! :cool:

Reply 4

Revenged

Basically, the oxygen dissociation curve means that where the oxygen is in a very high concentration, for example in the lungs, haemoglobin has a very high affinity for oxygen and so it has a very high saturation of oxygen. This allows as much oxygen as possible to be taken up into the haemoglobin in the red blood cells from the alveoli in the lungs.

Where the oxygen concentration is low, for example in the muscles, haemoglobin has a low affinity of oxygen. This allows oxygen to be relased at the muscles where it is needed for aerobic respiration.

Therefore the shape of the oxygen dissocation curve allows efficient transportation of oxygen.

Hope that explains things! :cool:

That's a much better explanation, nice one :cool:

Reply 5

glance

17

I hate oxygen. I'm going to have to understand sometime this week..

Reply 6

This part of biology becomes awesome after you do buffers and pH in chemistry :cool:

Reply 7

endeavour

This part of biology becomes awesome after you do buffers and pH in chemistry :cool:

How are they connected... hameoglobin acts as a buffer?

Reply 8

onlylittleme

OP

endeavour

This part of biology becomes awesome after you do buffers and pH in chemistry :cool:

MMM...if you say so...much prefer health and disease myself...much easier...

Anyway thanks for that you are all stars of the highest variety. :smile:

.

So...let me clarify.. the advantage of the sigmoid curve is that its easy to get oxygen when the partial pressure is high (lungs) and easy to lose when the partial pressure is low (Respiring tissues) BUT inbetween its just normal?

Reply 9

glance

17

Yep, haemoglobin acts as a buffer in the blood, and reduces the concentration of hydrogen ions in the blood by forming haemoglobonic acid.

Reply 10

Also, just to add - those H+ ions are due to

H_2O \ + \ CO_2 \ == \ H_2CO_3 \ == \ H^+ \ + \ HCO_3^-

ie from CO₂, so when Hb combines with the H+ ions, it releases oxygen which can diffuse into respiratory tissue (Bohr effect)

Reply 11

onlylittleme

So...let me clarify.. the advantage of the sigmoid curve is that its easy to get oxygen when the partial pressure is high (lungs) and easy to lose when the partial pressure is low (Respiring tissues) BUT inbetween its just normal?

Yeah,

inbetween the curve is steep... therefore a small increase in the amount of oxygen around would greatly increase haemoglobin affinity of oxygen

Reply 12

Golden Maverick

4

glance

Yep, haemoglobin acts as a buffer in the blood, and reduces the concentration of hydrogen ions in the blood by forming haemoglobonic acid.

And protonated Hb more readily releases oxygen, making it all the more interesting.

Reply 13

oxymoron

7

Golden Maverick

And protonated Hb more readily releases oxygen, making it all the more interesting.

Do you know why? <Has just revised this :wink:

>

Reply 14

corkskrew

2

Is it to do with the equilibria:

H-Hb + O2(g) <--> HbO2- + H3O+(aq)
and
CO2(g) + 2H2O(l) <--> HCO3-(aq) + H3O+(aq)

...?
So protonated Hb is present only when there is a high pp of CO2 and a lower p of O2, due to the forward reaction in that second equation. High [CO2] pushes Eqb of eqn 2 to the right, so higher [H3O+] is apparant. This then pushes the Eqb of the 1st eqn to the left, so the protonated Hb is more dissociated???
I'd be chuffed if I'm anywhere near being right - this isnt on my syllabus!

Reply 15

corkskrew

2

....Le Chatelier's Principle......???????........

Reply 16

oxymoron

7

corkskrew

Is it to do with the equilibria:

H-Hb + O2(g) <--> HbO2- + H3O+(aq)
and
CO2(g) + 2H2O(l) <--> HCO3-(aq) + H3O+(aq)

...?
So protonated Hb is present only when there is a high pp of CO2 and a lower p of O2, due to the forward reaction in that second equation. High [CO2] pushes Eqb of eqn 2 to the right, so higher [H3O+] is apparant. This then pushes the Eqb of the 1st eqn to the left, so the protonated Hb is more dissociated???
I'd be chuffed if I'm anywhere near being right - this isnt on my syllabus!

No - the first eqation is actually wrong as this is not how O2 binds. In fact it just acts as a ligand on the Fe2+ ion found in the Haem group, so no H+ is lost when O2 binds.

What happens is that H+ can protonate a histidine amino acid in part of the protein. This gives it a postive charge and enables it to Hydrogen bond with an aspartate amino acid in one of the other subunits. This favours a form of the protein in which the O2 binding site is blocked (T Form) so lowers the affinity for Oxygen.

Reply 17

oxymoron

No - the first eqation is actually wrong as this is not how O2 binds. In fact it just acts as a ligand on the Fe2+ ion found in the Haem group, so no H+ is lost when O2 binds.

What happens is that H+ can protonate a histidine amino acid in part of the protein. This gives it a postive charge and enables it to Hydrogen bond with an aspartate amino acid in one of the other subunits. This favours a form of the protein in which the O2 binding site is blocked (T Form) so lowers the affinity for Oxygen.

I'm sure your right but you really are complicating things.

All you need to know for edexcel bio is that in a high concentration of CO2 causes the Bohr effect.

This confused me at AS because I wasn't really wasn't told the reason behind it - all I was told was that it causes the oxygen dissociation graph to shift to the left.

This is caused by CO2 dissolving in water in the blood plasma, forming carbonic acid (H2CO3-). This carbonic acid then dissociates into hydrocatbonate ions (HCO3-) and H+.

CO2(g) + 2H2O(l) <--> H2C03- <--> HCO3-(aq) + H3O+(aq)

That's what the reaction above shows. Don't worry about it being a revisible reaction you only need to consiser it going forwards. H30+ is just H+.

This links in nicely to what you learn since the HCO3- ions are the main form that CO2 is transported in the body. Also, the H+ ions are taken up by the haemoglobin, causing haemoglobin to realease oxygen. This is the Bohr shift.

To help clarify a few things. A buffer is something that resists changes in pH (concentration of H+ ions). Haemoglobin acts as an acidic buffer since it resists increases in the H+ concentration of the blood by taking up H+ ions.

The importance of the Bohr shift is that in areas of high carbon dioxide concentraion, for example in respiring muscles, it causes more oxygen to be released by the method above.

Reply 18