TBR Orgo Ch 1 Phase II q. 10 - error?

[Jepstein30]

Was doing orgo passages in TBR today and got this question wrong.. don't understand the answer key since it seems to contradict itself.

10. Between oleate and stearate, which is more likely to be found in the cell membrane of a bone cell?
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 a carbon-carbon pi-bond
C) Oleate, because it has greater rigidity than stearate due to its carbon-carbon pi-bond
D) Stearate, because it has greater rigidity than oleate due to its lack of a carbon-carbon pi-bond

My reasoning: D should be the answer. Bone cells want to have rigid membranes for structural support. Passage states that as unsaturation increases, fluidity increases. Therefore, as saturation increases, rigidity increases. Of the two, oleate has a carbon-carbon pi bond and stearate has no carbon carbon pi bond. Therefore, stearate is more rigid than oleate due its lack of unsaturation.

Book's answer: Choice B is the best answer. Bone cells are used for structural support, so their cell membranes should be fairly rigid. Fluidity decreases with saturation, so bone cells are apt to have phospholipids containing saturated fats in their cell membranes. Stearate is more saturated than oleate, so choices A and C are eliminated. Stearate lacks the pi-bond in its backbone that oleate has, so it has a greater ability to rotate into different combinations. This is to say that stearate is more flexible than oleate, making Choice B the best answer.

I'm fairly confused because the first part of the answer established we want more rigidity but the second part of the answer chose stearate because of its flexibility?

Also, wouldn't it be the opposite? Aren't molecules with many double binds "stuck" in certain conformations = more rigidity? Given that you can rotate around any single bond, a saturated fatty acid is way more flexible..

Maybe i'm associating fluidity with flexibility and shouldn't be? honestly not sure which way is up on this one because the answer just seems inconsistent to me..

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[BestDoctorEver]

Was doing orgo passages in TBR today and got this question wrong.. don't understand the answer key since it seems to contradict itself.

10. Between oleate and stearate, which is more likely to be found in the cell membrane of a bone cell?
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 a carbon-carbon pi-bond
C) Oleate, because it has greater rigidity than stearate due to its carbon-carbon pi-bond
D) Stearate, because it has greater rigidity than oleate due to its lack of a carbon-carbon pi-bond

My reasoning: D should be the answer. Bone cells want to have rigid membranes for structural support. Passage states that as unsaturation increases, fluidity increases. Therefore, as saturation increases, rigidity increases. Of the two, oleate has a carbon-carbon pi bond and stearate has no carbon carbon pi bond. Therefore, stearate is more rigid than oleate due its lack of unsaturation.

Book's answer: Choice B is the best answer. Bone cells are used for structural support, so their cell membranes should be fairly rigid. Fluidity decreases with saturation, so bone cells are apt to have phospholipids containing saturated fats in their cell membranes. Stearate is more saturated than oleate, so choices A and C are eliminated. Stearate lacks the pi-bond in its backbone that oleate has, so it has a greater ability to rotate into different combinations. This is to say that stearate is more flexible than oleate, making Choice B the best answer.

I'm fairly confused because the first part of the answer established we want more rigidity but the second part of the answer chose stearate because of its flexibility?

Also, wouldn't it be the opposite? Aren't molecules with many double binds "stuck" in certain conformations = more rigidity? Given that you can rotate around any single bond, a saturated fatty acid is way more flexible..

Maybe i'm associating fluidity with flexibility and shouldn't be? honestly not sure which way is up on this one because the answer just seems inconsistent to me..

Pi bond decrease flexibily and increase fluidity...When fatty acid does have only sigma bonds, they are more flexible because the bond are easier to bend and that increases the dipole due to Van der Wals interaction of the neighboring atoms. Dont confuse fluidity with flexibility in the case of fatty acids... When fatty acid are more flexible, they become "less" fluid due to the reason I just stated.

 

[Jepstein30]

Pi bond decrease flexibily and increase fluidity...When fatty acid does have only sigma bonds, they are more flexible because the bond are easier to bend and that increases the dipole due to Van der Wals interaction of the neighboring atoms. Dont confuse fluidity with flexibility in the case of fatty acids... When fatty acid are more flexible, they become "less" fluid due to the reason I just stated.

Okay, but the answer key says the membrane of bone cells should be rigid.. thus, we should want MORE pi bonds, no?

Shouldn't the answer be C then?

 

[BerkReviewTeach]

Okay, but the answer key says the membrane of bone cells should be rigid.. thus, we should want MORE pi bonds, no?

This question combines two concepts that unfortunately have wording that sounds contradictory, but actually isn't.

From an organic chemistry point of view, we consider molecular flexibility. When there is a pi-bond present, then the molecule cannot rotate as easily and is said to be less flexible. As a result, it doesn't pack with its neighbors as readily, and thus has a higher melting point. Saturated fats have a much higher melting point than unstaurated fats of the same carbon chain-length, because they interact with neighboring molecules to a greater extent.

From a cell biology point of view, we only care about the packing ability. If the molecules can pack well in the membrane, then the membrane is less fluid. If they don't pack well, then the membrane is more fluid. So fatty acids with molecular flexibility form rigid cell membranes.

Combining the two thoughts we find that if there is a pi-bond present, then there is less molecular flexibility and thus a more fluid membrane forms, because the molecules don't interact (intertwine) as well.

This subject makes a great passage topic, because it combines two concepts from different disciplines. This could easily be presented as a percent omega fats per food item passage where they ask biology and organic chemistry questions. A big part of what the BR organic chemistry book is trying to do, given that O chem isn't test as much, is to mix in biology concepts everywhere they can fit. If you break down the BR O Chem passages, you'll notice that about a third of the passages are borderline (or full blown) biology passages answered using organic chem explanations.

 

[BestDoctorEver]

Okay, but the answer key says the membrane of bone cells should be rigid.. thus, we should want MORE pi bonds, no?

Shouldn't the answer be C then?

C is not the answer... Rigidity (more pi bond) in that case makes the molecule more fluid. You are still equating l rigidity due to pi bonds to being less flexible in real life. For instance, cooking oil molecular structure ( chemical formula) is rigid due to many pi bonds in its molecule; however, oil itself is fluid as oppose to butter which molecule is more flexible (less pi bonds). This seems really confusing or counterintuitive. BerkReviewTech provides a perfect explanation of the concepts involved. Answer C is false because more carbon--carbon pi bonds make the molecule more fluid (less rigid).