AAMC Official Guide to the MCAT 2015 CHEM/Physics Q 19.

[RSK25]

Hi,

Can someone explain q19 from AAMC Official Guide to the MCAT, the one about bacterial cells synthesizing PE?

---

Members don't see this ad.

 

[bee17]

Can you explain what part confuses you? The question is asking how to tell if the labeled phosphate is from solution rather than bacterial PE synthesis. You can determine this by looking at how fast acellular PE incorporates labeled phosphate from the environment. As the explanation says, this rate should be different than rate of cellular phosphate incorporation.

 

[lostintranslation]

This makes sense to me, but I want to learn more! I have been trying to find research papers that investigate this very question, whether 32P can be incorporated by direct exchange (which certainly would seem unlikely, but I would love to know if anyone is aware of such a paper.)

 

[deleted647690]

Can you explain what part confuses you? The question is asking how to tell if the labeled phosphate is from solution rather than bacterial PE synthesis. You can determine this by looking at how fast acellular PE incorporates labeled phosphate from the environment. As the explanation says, this rate should be different than rate of cellular phosphate incorporation.

In the first part of the experiment, when they immediately reacted TNBS with the bacteria, why were the labeled phosphates only found on the inside of the membrane? I understand that once TNBS is incorporated, the exchange of phosphates cannot occur because TNBS renders the phosphate heads lipophilic. However, what I'm trying to ask is, why is it that all of the phosphates are starting out on the inside of the membrane? In the second part of the experiment, where there was a delay before TNBS treatment, the phosphates had time to move from the inside to the outside. I just don't understand why the labeled phosphates start out on the inside in the first place.