#3 - Hemoglobin, a substance in human blood, transports

[moshei24]

I got this one right, but I didn't realize until now was (E) was wrong. It's because the stimulus doesn't tell us anything about the minimum amount of oxygen molecules that gets picked up. It tells us the max of 4 molecules. When it says that is it telling us that it has to be pick up 4?

Thanks!

[Nikki Siclunov]

We cannot infer that each hemoglobin molecule in human blood picks up at least one oxygen molecule. Solely based on the information contained in the stimulus, it is possible that some hemoglobin molecules pick up no oxygen at all, or that they serve an entirely different purpose.

[moshei24]

Aha, so the key word was "each"? Right?

[kappe]

Answer choice E would this answer be a Shell answer choice?

[Nicholas Bruno]

Yes it would seem so. Answer Choice E is incorrect because it assumes that hemoglobin takes at least one oxygen molecule. The stimulus does not mention a minimum number of oxygen molecules.

I hope that helps!

[kcho10]

Hi,
Can someone explain how (C) is a better answer than (E)?

Also, if we see the statement like the one we saw 'hemogoblin, a substance in the human blood, transports oxygen..." Is it safe to assume that the author is only speaking in general terms, and he is not trying to indicate that every hemogoblin acts this way? So it cannot be converted to a sufficient condition, it would have to be converted to 'most hemogoblin'? Someone explained it to me this way...but I'm not sure if that's right

Then, is it also safe to say that 'a hemogoblin' can be said to be 'any hemogoblin'? What's troubling to me is that it seems like a similar line of reasoning used to eliminate (E) can also be used to eliminate (C). If (E) is wrong because we have no way of knowing that each molecule picks up 1 to 4 molecules, can't we also say that we have no way of knowing about the starting points of each individual hemogoblin in terms of likelihood of catching another molecule?

I hope these questions make sense...

thank you

[Jon Denning]

Hey kcho,

Thanks for the question! This is a Must Be True question, where we'll need to use the information in the stimulus to reasonably support an answer choice, so let's start with the stimulus and see what we can make of it.

The stimulus here is notably scientific in nature, but fortunately not terribly complex: hemoglobin carries oxygen from the lungs to the rest of the body, and gets better at doing so (picking up more oxygen) until it's carrying four oxygen molecules (it opens itself more and more to new oxygen molecules). So there's not a lot we can take away from that, aside from the basic idea that there's a progressive improvement in carry-ability as hemoglobin goes from one to four oxygen molecules.

So which answer can we then reliably support with this information? Well answer choice (C) reflects exactly what we're told: hemoglobin with three O molecules will be better at picking up new oxygen than hemoglobin with only two oxygen molecules, which would be better at getting more O than hemoglobin with just one oxygen molecule, etc.

But what about (E)? I see two issues with it: first, just because we know hemoglobin's general role doesn't mean we know that *every single hemoglobin molecule* ("each") is always doing that role (I'll expand on this in a moment). Second, we can't know numerically what hemoglobin does when it carries oxygen: we know it can collect up to four, but that doesn't mean it will transport exactly one, or two, or three, or even four O molecules. Perhaps it only transports once it's reached four, for instance, meaning (E) could be wrong in that regard.

As to your question about what we can assume from an author's general, but somewhat vague, statements like we see here, I think it's safe to say that this author is describing a purpose/behavior of hemoglobin, but not necessarily saying that every hemoglobin molecule is performing that function at all times. But this is a contextual reading. That is, a statement like, "people desire happiness" pertains to all people, but not necessarily their sole/exclusive motivation at all times. Similarly, something like "eyes detect light" pertains to all eyes but doesn't imply that light is always being detected (eyes can be closed, after all). So does all hemoglobin perform the task ascribed here? Yes. Is every hemoglobin molecule performing that task at all times without exception? That's probably going too far.

I hope that helps!

[kcho10]

Hi Jon,
Thank you for your thorough response. I'm still unsure about (E) though. Doesn't the part of the stimulus that says that its maximum capacity is four oxygen molecules (line 5) imply that it can't have more than 4 molecules? Thank you!

[Jon Denning]

Ahhh, I see your point. That actually goes back to the notion of what hemoglobin can do (its purpose) versus what every hemoglobin molecule must always be doing. "Each hemoglobin" is simply too strong for what we're told here in the stimulus; we know only what hemoglobin's role in general is, not necessarily what every single hemoglobin molecule is always doing, or, interestingly enough, exactly how many oxygen molecules will be transported.

That is, the stimulus says hemoglobin can carry up to four O molecules and gets better at picking them up until it reaches four...but based on that it's possible that hemoglobin only ever carries four for transport! In other words, it could be the case that hemoglobin collects until it has four and then carries them to the rest of the body, in which case (E)'s statement that each hemoglobin carries between one and four is possibly mistaken (again, we can't know that hemoglobin ever carries one, or two, etc.). So I can find a few faults with (E), and that's certainly reason enough to dismiss it, especially when (C) is directly provable from what we've been told.

I've edited my initial response above to better emphasize that idea so that when/if others read this exchange it will be more clear for them, too

Thanks for following up!

[ChicaRosa]

I see why C is correct but I'm trying to understand why B is wrong?

I thought it was B because of the last sentence in the stimulus which states that an oxygen molecule changes the shape of a hemoglobin, causing it to open itself to receive more. But is it wrong because the more oxygen molecules it picks up the more it becomes effective which will change its shape to catch more oxygen molecules?

Thank you!