Question 2

[shaun.w.mc6504]

Oxygen does not dissolve well in the blood plasma and there for it needs to bind to hemoglobin in order to be transmitted through the blood? What is wrong with this statement and therefore B as being a correct answer?

[admin]

When you consider that 97% of oxygen is associated with Hb then, you might consider the point of view that something is wrong with plasma and that is the reason for the imbalance. However, once you recognize that ALL of that 97% must first be in plasma, then you must conclude that oxygen certainly dissolves in plasma sufficiently for life to exist.

So then the question becomes, what is so special about Hb that it acts like a sponge, pulling oxygen out of plasma? In other words, the reason that Hb is so good at what it does, is NOT because of oxygen's ability to dissolve in plasma, it's because of Hb's unique ability to siphon off the oxygen and store it.

[rocketbooster]

[addiaz2714087]

[quote="rocketbooster"]

[dnpgr16513]

Totally agree with you guys.

[jsfkt78927]

hahaha you guys are some whinny students...

got this one right btw.

You know, you guys are missing a huge point in bio in general. I would say for most everything you learn in your undergrad bio, they always address structure and function.

The question did not ask about the structure: any idiot can read the last sentence of Paragraph 1 and figure the structure out. The mcat is a reasoning test, not a memorization test. And its likely that you will not get to argue questions with the proctors. And getting hot headed about a tough question (which you think is unfair) will only work against you. Let them roll off you like water on a duck's back, and use your positive momentum for the next question.

The question asked about why it binds so much oxygen, as if there is some quality extrinsic of the structure that allows Hb to bind more than usual.

Four subunits as answer is a kick in the face to much of the Biological community, because that's like saying water falls, to a question asking about the causes of rain.

additionally, if youre still whining about this is too specific and it couldn't possible be inferred from the passage. Well, if you do the POE like the answer says youre, left with C and D. Hmmm which to choose. Well its a reasoning test, and its not suppose to be easy. You dont have to know what cooperative binding is, but you should know what allosteric means.

Again, you must realize that simply stating a fact about the structure is not the best explanation here because C has more insight into the reasons why the four subunits bind so much O2 as they do.

How could you surmise that allosteric or cooperative binding is occurring if you were unsure about C or D?

It would behoove you to review the table more closely. Tables and graphs don't contain useless information. Scientific papers (like the ones on the MCAT) are proof read many times, and superfluous data and charts are excised lest any reader confusion.

The table gives some sort of clues to point out that there's something happening to the protein and its four subunits. Otherwise you couldn't expect, or explain such a large jump in % saturation of Hb between 20-40 pO2 (likewise 20 & 60, it's definitely not linear!) just by saying "four subunits".

If you choose to complain rather than actively in gauge yourself, you've already lost.