Aamc #10 - bs # 126

[SephirothXR]

Most bacterial and human cells are alike in:

a)the ability to produce ATP through ATP synthase
b)The chemical composition of their ribosomes
c)their enclosure within cell walls
d)the shape of the self-replicating structures that carry their DNA

I got it down to A and B, and I KNOW that ribosomes are smaller in prokaryotes than in eukaryotes. They are however, all made up of rRNA, correct? That is, they're all made of nitrogen, carbon, hydrogen, and whatever amino acids are made up of. Apparently not, and the explanation was more or less that the bacterial ribosome is smaller (made of two different sub units, which I knew..). Is this really the chemical composition though? If the question had stated same type of ribosome, it would have been a red flag. But the answer is A. And now of course I agree with it b/c it's true and if I knew that to be true, I would have chosen it, but is my reasoning of the definition of "chemical composition" reasonable?

---

Members don't see this ad.

 

[MT Headed]

MCAT strategy tip: when you have (yes,maybe,no,no) you go with yes and you move on.

Of course the phrase "chemical composition" is ambiguous. But you do know that prokaryote ribosomes are 30S and 50S, while eukaryote ribosomes are 40S and 60S. They are made of chemicals but they aren't the same. So answer B is now demoted from a yes to a maybe, and you hunt extra hard within the other 3 answers looking for a better answer.

This AAMC question gets discussed a lot in the official AAMC10 thread: http://forums.studentdoctor.net/showthread.php?t=516259

 

[Axon hillock]

Most bacterial and human cells are alike in:

a)the ability to produce ATP through ATP synthase
b)The chemical composition of their ribosomes
c)their enclosure within cell walls
d)the shape of the self-replicating structures that carry their DNA

I got it down to A and B, and I KNOW that ribosomes are smaller in prokaryotes than in eukaryotes. They are however, all made up of rRNA, correct? That is, they're all made of nitrogen, carbon, hydrogen, and whatever amino acids are made up of. Apparently not, and the explanation was more or less that the bacterial ribosome is smaller (made of two different sub units, which I knew..). Is this really the chemical composition though? If the question had stated same type of ribosome, it would have been a red flag. But the answer is A. And now of course I agree with it b/c it's true and if I knew that to be true, I would have chosen it, but is my reasoning of the definition of "chemical composition" reasonable?

This is a pretty bad/difficult question IMO. However, prokaryotic ribosomes are actually made up of different type or rRNA's and proteins compared to Eukaryotic ribosomes. They are different, not only in terms of size, but also Eukaryotic have a different ratio of rRNA to protein. Its really very complex. It's a case of choosing the "best" answer, which in this case happens to be A.

 

[paul411]

Prokaryotes have 70S (50S, 30S subunits) ribosomes and eukaryotes have 80S (60S, 40S subunits). This is something you need to know. If you look at the process behind the nomenclature, you will realize that they are of varying density/composition.

There should be no doubt about ATP production via ATP synthase in both groups.