The phospholipid bilayer

[Lunasly]

I was just reading BR and a small section came up on the phospholipid bilayer. It starts off by stating that saturated fats are flexible and unsaturated fats are rigid. It then goes on to ask: A cell membrane would be most rigid if both of its fatty acids were:

A) Completely saturated short molecules
B) Completely saturated long molecules
C) Unsaturated short molecules
D) Unsaturated long molecules

The answer says that its B because there needs to be many interactions between the lipid chains. Interactions are greatest with long, saturated fatty acids.

If this is the case, then the fatty acids needs to be flexible (hence why the answer says to pick long, saturated fatty acids). The issue I have is, why does the question ask for the answer that would make the membrane the most rigid. On top of that, I thought that saturated fatty acids were more rigid then unsaturated fatty acids.

Thanks,
Lunasly.

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[pm1]

I was just reading BR and a small section came up on the phospholipid bilayer. It starts off by stating that saturated fats are flexible and unsaturated fats are rigid. It then goes on to ask: A cell membrane would be most rigid if both of its fatty acids were:

A) Completely saturated short molecules
B) Completely saturated long molecules
C) Unsaturated short molecules
D) Unsaturated long molecules

The answer says that its B because there needs to be many interactions between the lipid chains. Interactions are greatest with long, saturated fatty acids.

If this is the case, then the fatty acids needs to be flexible (hence why the answer says to pick long, saturated fatty acids). The issue I have is, why does the question ask for the answer that would make the membrane the most rigid. On top of that, I thought that saturated fatty acids were more rigid then unsaturated fatty acids.

Thanks,
Lunasly.

It seems like there is contradicting information. If the answer is B, it is stating that completely saturated is more rigid - which disagrees with the previous statement that said unsaturated fats are more rigid. 😕

I also thought that saturated fats were more rigid.

Sorry I didn't answer your question 😛 I would agree with the right answer being B, but I just don't know about the statement on the question stem.

I guess I would also like to hear other opinions on this.

 

[Lunasly]

Yes, exactly. That is the explanation that they provide right above the example. I also thought saturated fats were more rigid (think of them stacking up on top of one another like pancakes) while unsaturated fats were more flexible (hence why the kinks within one of the two fatty acids is important to maintain fluidity of the membrane).

 

[MedPR]

As individual units:

Unsaturated fats = double bonds. Double bonds = no rotation.

Saturated fats = single bonds. Single bonds = free to rotate.

Rotation = more flexible = less rigid.

Since individual saturated fats are flexible, they can interact more closely with other saturated fats. Therefore, if you stick a bunch of them together they will interact with each other way more than if you stuck a bunch of rigid unsaturated fats together.

 

[gettheleadout]

MedPR is right. They're trying to see if you're paying close attention with this question because the answer isn't as simple as going from "rigid" fatty acid to "rigid" membrane. You have to think as MedPR said in the context of what flexibility of the lipids does to the membrane overall.

 

[Lunasly]

Sorry, but what do you mean by "free to rotate" and "not free to rotate"?

If I am understanding this correctly: The question asks what would each individual fatty acid have to be like in order to form a rigid structure.

The unsaturated fatty acids are rigid by nature due to the kink formed by the double bond. In reality, that kink allows for the membrane to be a little more "fluid," does it not? However, since the saturated fatty acid which does not have any double bonds is flexible, we can form a nice, neat stack of them. This stacking of flexible structures allows it to be more rigid while stacking a bunch of unsaturated fatty acids makes the overall structure more flexible.

Do I got that right?

 

[MedPR]

Sigma bonds can rotate, pi bonds cannot rotate. That's what I mean by "free to rotate" and "not free to rotate".

And yes, you got it.

 

[pm1]

Since individual saturated fats are flexible, they can interact more closely with other saturated fats. Therefore, if you stick a bunch of them together they will interact with each other way more than if you stuck a bunch of rigid unsaturated fats together.

so by saturated (less rigid) interacting make them more rigid overall/not as individual units?

 

[MedPR]

so by saturated (less rigid) interacting make them more rigid overall/not as individual units?

Yes. Tie a knot with two pieces of string (saturated fat) and a knot with two straws (unsaturated fat). Which one is harder to untie (more rigid)?

 

[Lunasly]

Nice analogy!

Thanks 🙂

 

[MedPR]

I stole it from TBR.

 

[ThirdEye]

Sorry, but what do you mean by "free to rotate" and "not free to rotate"?

If I am understanding this correctly: The question asks what would each individual fatty acid have to be like in order to form a rigid structure.

The unsaturated fatty acids are rigid by nature due to the kink formed by the double bond. In reality, that kink allows for the membrane to be a little more "fluid," does it not? However, since the saturated fatty acid which does not have any double bonds is flexible, we can form a nice, neat stack of them. This stacking of flexible structures allows it to be more rigid while stacking a bunch of unsaturated fatty acids makes the overall structure more flexible.

Do I got that right?

Yes, the rigidity of the cis double bonds in unsat. F.A.s allows the membrane to be more fluid by creating more space and having less attraction to adjacent molecules.

Saturated fatty acids, though not rigid by themselves, become very rigid in the membrane because they pack tightly and have attraction to eachother. The longer the chain, the more attraction and more stiff the membrane becomes.