Practice MCAT test: Is one of the questions wrong?

[Wiscobadger13]

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Here is a question from a practice MCAT from mcat-prep.com. It accompanies a table showing Hb % saturation and pO2.

[FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif]2) Which of the following best explains why 97% of oxygen in blood is in the HbO2 form?

1. Oxygen binds irreversibly to the iron atoms in hemoglobin.
2. Oxygen does not dissolve well in blood plasma.
3. There are allosteric interactions between hemoglobin subunits.
4. Hemoglobin consists of four proteinacious subunits.

...
[FONT=Verdana, Arial, Helvetica, sans-serif]The answer given is C. I strongly disagree and argue it B.

I realize that Hb has allosteric interactions (I'm not stupid), but that has no affect on the O2 in blood plasma. If Hb bound the same amount of O2 it does with allosteric interactions, but the O2 in the blood increased, the %O2 in the blood in the HbO2 form would decrease. This coupled with the known fact O2 isn't super soluble in blood plasma makes B seem right.

I believe they meant to word the question as why is 97% of the Hb in the HbO2 form. This would C make more sense because allosteric interactions would decrease Hb while increaseing HbO2.

Comments?

.

 

[aSagacious]

Moving to the MCAT discussions forum.

 

[sliceofbread136]

this will be move to the mcat forum.

I think B would be the better answer to be honest.

 

[dmf2682]

Really belongs in the gold standard forum, it's their test the op is referring to.

Could go either way on this one. I tend to go towards c myself. I'd need the table to confirm.

 

[sliceofbread136]

Really belongs in the gold standard forum, it's their test the op is referring to.

Could go either way on this one. I tend to go towards c myself. I'd need the table to confirm.

Why C? Hemoglobin could hold 97% of the oxygen in blood even if it didnt have the positive allosteric reactions.

I see the rationale for both b and c, but b seems like a better answer. This is just a bad question OP, I wouldn't sweat it.

 

[dmf2682]

Why C? Hemoglobin could hold 97% of the oxygen in blood even if it didnt have the positive allosteric reactions.

I see the rationale for both b and c, but b seems like a better answer. This is just a bad question OP, I wouldn't sweat it.

Without seeing the sat table you can't really say if it would, for a given amount of saturation. You also couldn't quantify how much oxygen would bind to hb if plasma was better at absorbing oxygen. These are enzymatic effects we're talking about here, and everything I know about enzymes (it isn't much, tbh but it's what I got) says that they lower the activation energy of physical reactions by a whole lot, increasing the rate by orders of magnitude. So to me, increasing oxygen solubility in plasma is first order, and the enzymatic effect would win. But I could be wrong, that's just my put.

 

[chiddler]

I think it is because B does not answer the question very well. B tells us that oxygen must be bound to hemoglobin but not which configuration (HbO, HbO2, HbO3, etc). C tells us why HbO2 is the dominant form of HbOX.

However, i'm curious why it's not HbO4? Cooperative effect, right?

 

[syoung]

there are "other" forms of HbOX? Isn't the HbO2 the only form? (but with up to 4 binding spots for O2?)

 

[chiddler]

there are "other" forms of HbOX? Isn't the HbO2 the only form? (but with up to 4 binding spots for O2?)

Up to 4 oxygens can bind Hb, right? Isn't it HbO4? Or...is it HbO8 since 4 O2's bind?

What is notation like for hemoglobin-oxygen?

 

[dmf2682]

I was under the impression that hemoglobin had four heme groups each made of HbO2. Not sure though.

 

[chiddler]

no idea, would love input if anybody knows, please!

 

[hellocubed]

I think it is because B does not answer the question very well. B tells us that oxygen must be bound to hemoglobin but not which configuration (HbO, HbO2, HbO3, etc). C tells us why HbO2 is the dominant form of HbOX.

However, i'm curious why it's not HbO4? Cooperative effect, right?

The question states:

Which of the following best explains why 97% of oxygen in blood is in the HbO2 form?

B answers the question effectively. the vast majority of Oxygen is mostly dissolved in Hb because it is more soluble as HbO2 than as free oxygen.


C answers the question very poorly. yes there are allosteric effects, but why does that mean 97% of Oxygen is found as HbO2? This answer choice explains why Oxygen that first binds to Hb would alleviate the binding of the second subunit. But this does not do as good of a job as choice B in explaining why such a vast majority of O2 is found in Hb.


unless there is more information that the OP is leaving out, the answer should be B.

Edit: fixed question, added more

 

[chiddler]

The question states: why 97% of Oxygen is in HbO2 instead of in blood?

No the question certainly does not state that.

"why 97% of oxygen in blood is in the HbO2 form?"

 

[hellocubed]

Both statements are valid obviously.

But B answers the question better.

 

[syoung]

All the articles I'm finding JUST say
because of the low solubility of O2 in plasma, very little is there.
because the O2 has .4g/mL solubility in plasma, etc etc...blah blah doesn't really say WHY it's insoluble, but CO2 is quite soluble!

 

[chiddler]

Both statements are valid obviously.

But B answers the question better.

that's where i explained my disagreement a few posts up.

 

[chiddler]

Ok I got some information that cleared a lot up. For me anyway.

Q: "how do you notate it when 4 oxygen bind hemoglobin, then? Hb(O2)4?"

A: (med student friend) "its generally implied if its completely saturated we don't wright partial saturation".

yeah!

so HbO2 = completely saturated hemoglobin

 

[ipodtouch]

So there is no notation for a single O2 on a Hemoglobin?

Should we imply a completely saturated hemoglobin everytime we see HbO2?
It would make more sense to see an MCAT source for this other than word of mouth.

 

[bajoneswadup]

I'm pretty sure this question is talking about the T and R state. The reason why O2 bonds to Hb so well in the lungs vs not well in the tissues is because of allosteric binding of H+ and CO2. H+ and CO2 bind to different parts of the HB molecule in the tissues (where pH is very low- lots of H+ and CO2) and HB changes its configuation from relaxed to tense, which makes it have low O2 affinity. In the lungs where pH is high (low H+ and CO2 b/c it diffuses out) the Hb changes configuations to relaxed state which has high affinity for O2 binding. This is also why 2,3BPG lowers O2 affinity, because it too binds on a separate site of HB changing it to T state.

Sorry I duno if somebody alrdy said this I just skimmed and went to the bottom