TBR O Chem chapter 1 question 10 practice exam

[deleted492904]

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Hey guys, I have a question about one of the questions from TBR's Ochem book.

The question is : Between oleate and stearate, which is more likely to be found in the cell membrane of a bone cell?

Answer choices:
A-oleate, because it has greater flexibility than stearate due to its carbon carbon pi bond
B-stearate, because it has greater flexibility than oleate due to its lack of pi bond
C-oleate, because it has greater rigidity than stearate due to its pi bond
D-stearate, because it has greater rigidity than oleate due to its lack of pi bond

The answer in the back is B.

The passage states that the fluidity of the cell membrane increases as the units of unsaturation increases. In other words as the saturation increases the cell membrane becomes more rigid. For bone cells we would want them to be more rigid or at least that's how bones are. Based on this information the answer would be stearate because it is more saturated. So, my question is why wouldn't the answer be D. I understand that having pi bond also makes the bond unable to rotate more freely so B is technically correct in terms of describing the rotation around the bonds, but it doesn't answer the question. What makes B a better answer choice than D? To me D sounds more correct. Thanks in advance.

 

[sps27]

This is what I have in my notes; taken from TBR.

1) Saturated fat molecules are flexible and solid. (like strings)
2) UnSaturated fat molecules are rigid and liquid. (like straws)
3) Rigid cell membranes have more saturated fats because they pack very well. (strings can be tied and twisted. more chance for intermolecular interactions)
4) Flexible cell membranes have more of un-saturated (pi bonds) fats. (straws can easily be pulled apart, not very rigid)
5) More pi bonds increase fluidity in a membrane.
6) Greater pi bonds means lower melting point.

Using this information above, Stearate molecule is not more rigid than Oleate molecule. In fact, Stearate is more flexible than Oleate. It packs well because it lacks a pi bond and hence gives more rigidity to cell membrane.

 

[deleted492904]

This is what I have in my notes; taken from TBR.

1) Saturated fat molecules are flexible and solid. (like strings)
2) UnSaturated fat molecules are rigid and liquid. (like straws)
3) Rigid cell membranes have more saturated fats because they pack very well. (strings can be tied and twisted. more chance for intermolecular interactions)
4) Flexible cell membranes have more of un-saturated (pi bonds) fats. (straws can easily be pulled apart, not very rigid)
5) More pi bonds increase fluidity in a membrane.
6) Greater pi bonds means lower melting point.

Using this information above, Stearate molecule is not more rigid than Oleate molecule. In fact, Stearate is more flexible than Oleate. It packs well because it lacks a pi bond and hence gives more rigidity to cell membrane.

Thanks for the reply.

I think I was processing the answer choices wrong. I thought choice D was kind of right because I interpreted the word rigidity in terms of the whole molecule. Whereas the answer choice is just talking about stearate itself being more rigid (flexible in the right answer choice). I guess that stems from me being confused about something that can be rigid and fluid at the same time. Based on what you said and I actually thought about this when I was typing the question, being more fluid does not mean less rigid? The way I remember fluidity is that it is the ability for molecules to move across the membrane but the technical definition is the viscosity of the membrane (Please feel free to correct me on this). Therefore, the molecule itself can be flexible but the membrane can be rigid because the individual fat molecules (fatty acids) pack well due to it being so flexible?