Organic Structure Question

[zoner]

Between oleate and sterate, which is more likely to be found in the cell memebrane of a bone cell?

a) Oleate, because it has greater flexibility than stearate due to its carbon-carbon pi-bond.

b) Stearte, becaues it has greater flexibiliyt than oleate due to its lack of a carbon-carbon pi-bond

my answer was a since pi bonds makes it more flexible, but this freaking TBR passage answer is saying that its b because not having pi bond makes it have greater ability to rotate into different conformations.

This is the main problem with TBR, they provide lengthy explanation for easy answers, and when it comes to questions like this, one sentence explanation.

which one is the right answer?

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[DougTPR]

a) Oleate, because it has greater flexibility than stearate due to its carbon-carbon pi-bond.

b) Stearte, becaues it has greater flexibiliyt than oleate due to its lack of a carbon-carbon pi-bond

In this case, a one sentence answer is enough.

A pi bond (made from side to side overlapping p orbitals) cannot rotate and is thus NOT flexible.

 

[zoner]

But I learned in my previous bio class that saturated fatty acid is INflexible due to london dispersion forces that occur between the chains where as if there is unsaturation, it can help break such interaction.

Isn't that why saturated fatty acid block artery whereas unsaturated fatty acid such as olive oil is better?

This is completely opposite of the answer given.

Another thing that is wrong is that why would the mp for Stearic acid be higher than Oleic acid if it was more flexible? It is higher due to L/D/F thus less flexzible to move around.

 

[cheesier]

The question is not asking about intermolecular forces, so you don't have to worry about London Forces and the interactions of different hydrocarbon chains with each other. If you have a molecular modeling kit, you can see that a pi bond prevents free rotation and is less flexible. The question is pretty narrow, so just ask yourself: Are molecules without pi bonds more flexible than those with them?

As for why the melting point is higher for stearic acid, intermolecular forces DO come into play here. Because the hydrocarbon tails aren't kinked by pi bonds, they pack together better which raises the melting point.

 

[zoner]

I understand why the pi bond would make it inflexible. However, the intermolecular forces will make the saturated fatty acid even more inflexible.

Isn't this the reason why the leg part of raindeers have cells memebranes more saturated with unsaturated hydrocarbons so that it would be more flexible in the cold?

 

[gmcguitar4]

from what I understand..double bonds always mean less flexible. that question is clearly asking about the flexibility of the molecule itself. as for clogging arteries, the reason why saturated fats cause problems is because their chains cannot be broken. Because of unsaturated fatty chains have double bonds, the chains are subjected to breaking at these double bonds thus reducing the size of the molecule and reducing the probability of them clotting in blood.

For the raindeer, I would think maybe they have more unsaturated fats in their legs to have the energy more readily available. Since the unsaturated chains can be broken, this just gives more surface area for enzymes to act on them.

 

[cheesier]

I understand why the pi bond would make it inflexible. However, the intermolecular forces will make the saturated fatty acid even more inflexible.

Isn't this the reason why the leg part of raindeers have cells memebranes more saturated with unsaturated hydrocarbons so that it would be more flexible in the cold?

You are still thinking about this in terms of the interaction between different molecules. Imagine a single fatty acid chain. The one with the double (pi) bond does not have free rotation. Whatever intermolecular forces exist are irrelevant because the question specifically asks about what a pi bond does to a single molecule.

 

[zoner]

You are still thinking about this in terms of the interaction between different molecules. Imagine a single fatty acid chain. The one with the double (pi) bond does not have free rotation. Whatever intermolecular forces exist are irrelevant because the question specifically asks about what a pi bond does to a single molecule.

so you are saying that the right theoretical answer to the question and the best answer choices are different?

The question was specifically asking which what kind of fatty acid chain will be more prevalent in cell membrane.

 

[Rabolisk]

I'm going to have to agree with zoner here, actually. This is a mistake from TBR. Membrane flexibility decreases with increased saturation.

 

[cheesier]

I'm going to have to agree with zoner here, actually. This is a mistake from TBR. Membrane flexibility decreases with increased saturation.

Except this question has nothing to do with membrane flexibility.

Zoner: you are reading this question all wrong. Let's start fresh and reread it.

Between oleate and sterate, which is more likely to be found in the cell memebrane of a bone cell?

a) Oleate, because it has greater flexibility than stearate due to its carbon-carbon pi-bond.

b) Stearte, becaues it has greater flexibiliyt than oleate due to its lack of a carbon-carbon pi-bond

For starters, the names of the lipids don't matter. Even the fact that they are lipids doesn't matter. If it gave you the names of two carbohydrates, you would still go about this question the same way. The fact that they're talking about lipids in a membrane is also irrelevant. Deconstruct the question and this is what you should say to yourself:

Do pi bonds increase or decrease flexibility?

That is all that this question is asking.

The theoretical answer is that pi bonds decrease flexibility. The answer that you choose is B because it says:

Molecule X is more flexible because it lacks a pi bond.

This statement is true, regardless of the molecule, regardless of where it is, etc.

 

[zoner]

Ok, so if the answers were phrased to say

a) Oleate would form the cell membrane because it can form greater cell membrane flexibility than stearate due to its carbon-carbon pi-bond.

b) Stearte would form greater flexibile cell membrane than oleate due to its lack of a carbon-carbon pi-bond

Then the choice (A) would be correct?

 

[cheesier]

Ok, so if the answers were phrased to say

a) Oleate would form the cell membrane because it can form greater cell membrane flexibility than stearate due to its carbon-carbon pi-bond.

b) Stearte would form greater flexibile cell membrane than oleate due to its lack of a carbon-carbon pi-bond

Then the choice (A) would be correct?

Correct

 

[zoner]

thats a mind torturer

 

[Rabolisk]

Except this question has nothing to do with membrane flexibility.

Zoner: you are reading this question all wrong. Let's start fresh and reread it.

Between oleate and sterate, which is more likely to be found in the cell memebrane of a bone cell?

a) Oleate, because it has greater flexibility than stearate due to its carbon-carbon pi-bond.

b) Stearte, becaues it has greater flexibiliyt than oleate due to its lack of a carbon-carbon pi-bond

For starters, the names of the lipids don't matter. Even the fact that they are lipids doesn't matter. If it gave you the names of two carbohydrates, you would still go about this question the same way. The fact that they're talking about lipids in a membrane is also irrelevant. Deconstruct the question and this is what you should say to yourself:

Do pi bonds increase or decrease flexibility?

That is all that this question is asking.

The theoretical answer is that pi bonds decrease flexibility. The answer that you choose is B because it says:

Molecule X is more flexible because it lacks a pi bond.

This statement is true, regardless of the molecule, regardless of where it is, etc.

They intend for this question to have to do nothing with lipids and cell membranes. What they want to test is whether or not you understand that there is no free rotation across pi bonds. However, the fact is, the way they asked the question did involve cell membranes and flexibility. Pi bonds decrease flexibility, but the established theory, backed by experimental data, is that unsaturated lipids increase the flexibility of the membrane. If you read just the answer choices, A makes perfect sense. However, an answer choice that does not answer the question is simply not correct. If I had written this question for an exam and a student pointed this out to me, I would throw the question out. It is an error from TBR.

 

[BerkReviewTeach]

This is the main problem with TBR, they provide lengthy explanation for easy answers, and when it comes to questions like this, one sentence explanation.

Here is the one sentence answer on page 12 of the class handout for the lecture (which you must be using).

• 24. Choice B is correct. 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 conformations. This is to say that stearate is more flexible than oleate, making choice B the best answer.

If you can elaborate as to what is unclear here, perhaps I can help to clear things up. The answer is basically saying that a flexible alkyl chain will form stronger interactions with its neighbor and thus act more like a solid (which explains the higher melting points of long chain alkanes compared to equal length alkenes and the rigidity of cell membranes rich in saturated fats). Stearate has no pi-bonds, so it is more saturated than oleate (with one pi-bond).

 

[BerkReviewTeach]

It is an error from TBR.

Actually, it is not an error.

Here is the exact question:

• 24. 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.

There is a table that shows that stearate is 18-carbons without any pi-bonds and oleate is 18-carbons with one cis pi-bond.

Let's start by accepting that a bone cell requires rigidity in its cell membrane. In order to increase rigidity (or decrease fluidity), the fatty acid chains of the phospholipid should pack tightly (like they do in any hydrocarbon solid). In order to pack tightly and thus maximize the intermolecular forces, the molecules must be flexible. When considering the two fatty acid carboxylates given, the aliphatic chain of stearate will offer more flexibility than the chain with the cis pi-bond found in oleate. Oleate molecules do not pack as tightly as stearate, which would lower the melting point (typical organic chemistry example) and increase the fluidity (typical cell bio question).

This means that the best answer of the four choices is stearate (eliminating A and C) because of its flexibility (eliminating D).

Rabolisk, if you can find an error here, please tell me what is incorrect about this question.

 

[Rabolisk]

Nothing's wrong with the question, although it is a tricky way of asking it. When the OP posted the question, he didn't provide the entire context nor did he have the "extended" explanation. I overlooked the fact that the question was really asking for more rigidity (bone). My apologies for that. Nevertheless, the question did ask for an understanding of lipid membrane fluidity, rather than just carbon double bonds as some had suggested.

 

[BerkReviewTeach]

Nothing's wrong with the question, although it is a tricky way of asking it. When the OP posted the question, he didn't provide the entire context nor did he have the "extended" explanation. I overlooked the fact that the question was really asking for more rigidity (bone). My apologies for that. Nevertheless, the question did ask for an understanding of lipid membrane fluidity, rather than just carbon double bonds as some had suggested.

I just reread my post and I want to apologize if my tone wasn't the professional and friendly one I was hoping to convey. I asked you, because you have posted many brilliant answers to question after question in the Q and A forum and have earned a great deal of trust as an expert in not only the material but more so a logical and clean way of answering questions. I figured that if you found something wrong that it was a point to consider strongly.

I actually love this question because it combines organic chemistry and cell bio.

 

[BerkReviewTeach]

thats a mind torturer

I've read a few posts from you lately and I hope I can offer some perspective that helped me. The first time I sat down to prepare for an MCAT, I was overwhelmed by the volume of material and freaked out that it was so different than the way I had been asked questions in my college classes. It's nerve-wracking and I have to admit being pretty tense. But after a while all that anxiety became detrimental, as I was constantly fighting with questions I was getting wrong. It took a while, but I learned to instead focus on what I was doing to get things right and then trying to bottle it up to use over and over.

You are clearly a smart person with a good volume of material under your belt. But you're fighting the material too much right now. I wanted to post in your "hate studying" thread, but too many people were venting there, so it wasn't going to fit the tone. People need to vent from time to time. But the truth of the matter is that studying for the MCAT at some level is enjoyable. It's like working out really hard. You hate the reps and the dead feeling you get during some workouts, but once the results start to take affect it becomes addicting. Let that moment come, and you'll find you don't really hate it. You're going to kick the MCAt's a(you know the next two constenants). Don't stress too much on the path up to that moment.

 

[zoner]

In order to pack tightly and thus maximize the intermolecular forces, the molecules must be flexible.

To Berkeley Reivew Teach

Thanks for taking your time to clear this up. I was mostly venting and I apologize if that offended you in some way. The reason why I got most frustrated with this question was because in my Biology class, there was a test question where it had asked me what fatty acid make up cell membrane of a bone cell. I had gotten that question wrong because I had picked saturated fatty acid instead of unsaturated. Maybe the professor was wrong or maybe my memory has faded. In any case, I understand this now and I see that the way the answers were phrased were bit tricky for someone reading them really fast. When it says "it has greater flexibility" I guess someone being careless could interpret it as the membrane that such fatty acid will create will be more flexible instead of the interpreting it as the structure itself is more flexible. I guess not really knowing how the real MCAT is, I misinterpreted my reading as there being something wrong with the BR. However, my frustration with some of the TBR answer explanations I think is still valid since there should have been some explanation on how someone could have read the answer choices wrong or where they could have gotten into a trouble. Some answers explanations does this beautifully and I am very grateful for that. It is just not consistent.

Well,,, I am stressing no matter what I do or what others say and in my previous experience, stresses always helped to achieve the very best.

 

[BerkReviewTeach]

Well,,, I am stressing no matter what I do or what others say and in my previous experience, stresses always helped to achieve the very best.

There is definitely truth to that. Without stress, we'd sit around and get nothing done. I guess for me it's always been about channeling that stress into productivity, which I must admit I don't always do very well.

Good luck and hopefully you take advantage of the Q and A forum. It's quite helpful.