Section Bank Chem/Phys

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Section Bank Chem/Phys

Postby Lionroar » Thu Jan 04, 2018 1:54 pm

Hi, just had a few questions in the Chem/Phys Section Bank:

#18: What is a phosphatide, and is it any different than a phospholipid?

#19: Does the passage indicate anywhere that the liposomes fluoresced because there was dye trapped inside? Isn't light usually emitted when electrons fall in energy states (as in C)?

#20: How does one tell whether a reactant is under kinetic control simply because it is not reacting? Maybe it is the most favorable thermodynamic product?

Thank you!
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Re: Section Bank Chem/Phys

Postby NS_Tutor_Andrew » Thu Jan 04, 2018 8:07 pm

Hi Lionroar,

Going one by one...

#18: This is an interesting terminological question. 98% of the time, "phosphatide" is an older-fashioned synonym for "phospholipid". I was able to find some sources suggesting that phosphatides would refer specifically to phospholipids that contain a nitrogen in the non-acyl substituent (which is the case here), but that usage doesn't seem very common. Interestingly, the term "phosphatide" occurs nowhere in Lehninger's Biochemistry, which is a pretty authoritative textbook.

#19: As usual, you can approach this in terms of why B is right or why C is wrong. In terms of why B is right, the idea is that this answer choice best reflects the logic of the experiment -- having liposomes form in the presence of fluorescent dye allowed the dye to be trapped within the membranes, and then they measured fluorescence during elution as shown in Fig. 1. Choice B captures this without requiring extra reasoning. It may sound a little silly/obvious/circular to say that they fluoresced because of fluorescent dye, but that's not wrong. Turning to why C is wrong, the basic reason is that we have no clear support for this happening, and it would be sort of unexpected -- intramolecular factors like conjugation can affect the energy of the excited state, but that's a different situation. Plus, even if this were to be the case, we'd expect this to lead to differences in the wavelengths emitted/absorbed, not the appearance of fluorescence when none existed.

#20: That's an excellent question, and this question has received a lot of feedback online. The basic idea has to do with reversibility, and the idea that thermodynamic control is associated w/ reversibility, such that by putting energy into the system mixing you can "reverse" the random initial reactions and push towards the more stable final results (the average-sized liposomes they mention), whereas kinetic control is associated w/ irreversibility at low temperatures, so that adding a modest amount of energy by mixing won't change anything about what happened.

Hope this is helpful!!
Andrew D.
Content Manager, Next Step Test Prep.

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