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Test Question I think I got Right but was Marked WRONG!!! ??

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Test Question I think I got Right but was Marked WRONG!!! ??

Postby Wafflez » Thu Jul 01, 2010 12:25 am

Okay,

Well today I took a test for Biology 101. I thought I did fairly well. But there was one question (and I don't have the exact question right now but will post it when I get the results back) which I was discussing with my classmates about because we all had different answers. It went something to this affect:

Of the listed molecules, which one will find it hardest "diffusing" through the plasma membrane [of a cell]?

A. H2O (water)
B. O2 (Oxygen)
C. CO2 (carbon dioxide)
D. Na+ (a sodium ion)

I picked C Carbon dioxide.

Apparently the answer was D the sodium ion. I figured this out after speaking to the professor.

But that's not right!

Because the question specifically asks which molecule will find it hardest diffusing across the plasma membrane. An sodium ions DO NOT DIFFUSE ACROSS PLASMA MEMBRANES.

Diffusion is the act of moving from higher concentration to lower concentration and DOES NOT USE UP ANY ENERGY.

Sodium ions must use an active transport protein in order to cross the membrane. Thus that can't even be the answer because sodium cannot diffuse across the plasma membrane PERIOD!.

That leaves the other three possible answers which all are capable of diffusing across, so it's just a matter of which one is largest, which is the carbon dioxide, which will have the most trouble.

I explained this to the professor and yet she did not budge, am I missing something????

It's like asking which of four organisms has the hardest time flying and listing three birds and a human.... but humans do not have wings, so why would that even be the answer???? (this is just an analogy)

Any help, kthx!
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Postby mith » Thu Jul 01, 2010 2:06 am

sodium does not need active transport to cross. You can blame her for not teaching about facilitated diffusion or sodium ion channels.

even if sodium does not cross, you could argue that it is hardest to diffuse across, because it has infinite non-diffusivity compared to everything else that does diffuse.
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Re: Test Question I think I got Right but was Marked WRONG!!! ??

Postby Wafflez » Thu Jul 01, 2010 6:13 am

Nice!

Okay I figured out where I went wrong... The bloody textbook is misleading as hell and the professor didn't spot [or know] which part I was mislead by. She wasn't even a Bio major so it makes sense that she'll follow the book, but miss some obvious details.

The whole time I was thinking that diffusion was impossible where energy is involved. That's what I was getting wrong. Diffusion CAN involve energy and as you said Na+ can pass through using a method related to ''facilitated diffusion'' which does in fact use energy [ATP] to move them through.

The damn textbook made out that diffusion was soley a non-kinetic act. damn this!

But if it WERE true as I had thought I was, then my arguement would still have been valid. I undestand what you mean, but by definition, ''hardest'' implies increased difficulties in reaching a common goal. But that doesn't include imposibilities. Something that is 'hard' signifies complications but in the end, said goal is reachable. If diffusion was limited to non-kinetic acts of crossing the membrane like I thought, then it would have been impossible for Na+ to diffuse in that manner; thus, it would have been an invalid answer altogether.


But sadly, the misleading material of my textbook has resulted in anxiety and confusion over one dumb question, which given the knowledge I now hold over this topic, I would have answered that question correctly.

I do not settle for anything less than a perfect score =]

Thanks Mith for your supportive and informative feedback. At least you didn't make me feel even more of a cluts than I already do over trusting the misleading nature of that text.

I knew there was something wrong when I spoted a couple typos lmao

fyi text = Mader, Biology 10th edition [only half the text, it's customized for this college]
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Postby mith » Sun Jul 04, 2010 7:18 pm

Let's not be too harsh on your teach, the problem with our school system is teachers aren't that highly respected or paid. What happens is the best and brightest go into industry or research jobs while people who don't do so well and can't get in are the ones left with the jobs like teaching high school bio(albeit there are some dedicated people who really want to teach such as my old calc teacher who was actually good at his job and a math major).

Let me put it this way, high school is just a stopover if you're going to college and if you understand what qualifications you need to get in, you really wouldn't be stressing about getting perfect scores.
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Postby JackBean » Thu Jun 23, 2011 8:32 am

just because Na+ uses channels or transporters, doesn't mean, it's 100% incapable of diffusion through the membrane.
But even if the sodium was wrong, your answer would be wrong also, since H2O is polar and will diffuse harder than nonpolar CO2 ;)
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
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