TBR Organic...

[andrewsmack05]

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The question states:

Cell membranes are composed of many molecules, including phospholipids. A phospholipid is a molecule with a glycerol backbone plus two fatty acids, and a phosphate attached to the oxygen atoms of glycerol. A cell membrane would be most rigid if both of its fatty acids were:

A. Completely saturated and short molecules
B Completely saturated and long molecules
C unsaturated and short molecules
D unsaturated and long molecules

The answer they gave is B because for the membrane to be rigid, there must be many interactions between the lipid chains. The maximum degree of interaction occurs with long, saturated fatty acids.

In the informative paragraph before that, it states that unsaturated chains are more rigid because they have double bonds within the structure. The question seems sort of contradictory, but when I think about the interactions, the long, saturated fatty acids would be able to twist around each other giving a maximum degree of interaction when compared to the unsaturated fatty acids. If possible, I would just like a little clarification on this. Thanks!

 

[dougkaye]

Good question. This is a pretty high-yield topic so I'm sure there will be other responses. B is definitely correct. This question is asking "what will lead to the greatest amount of intermolecular interaction (attraction/packing) between fatty acid chains". The idea is that greater packing will result in a more rigid membrane, versus loose packing that would be more "fluid" and would be characterized by a smaller amount of intermolecular forces. For the MCAT you have to understand that SATURATED fatty acids pack together significantly tighter than unsaturated because the lack of double bonds makes them more linear and less kinky. The kinks in the unsaturated FA's reduces their ability to pack together. Think butter versus oil. Butter is saturated FA's and they pack together tighter, thus solid at room temp. Oil is unsaturated and is liquid at RT. That comparison applies nicely and directly to the question of membrane fluidity. Length of FA chain follows directly from the above analysis. Longer the chain, more surface area there is for IM forces to exist b/t the FA chains.

I think you're getting confused between rigidity in a single FA chain versus interaction between chains, the latter being what this question is really testing. It's true that a double bond will increase rigidity WITHIN the molecule but that's a red herring. it's all about how strong a friendship can form between the FA's based on length and degree of saturation.

 

[Catburr]

I think you're getting confused between rigidity in a single FA chain versus interaction between chains, the latter being what this question is really testing. It's true that a double bond will increase rigidity WITHIN the molecule but that's a red herring. it's all about how strong a friendship can form between the FA's based on length and degree of saturation.

I remember this same question confusing me. This is a really great explanation, thanks.

The question seems sort of contradictory, but when I think about the interactions, the long, saturated fatty acids would be able to twist around each other giving a maximum degree of interaction when compared to the unsaturated fatty acids.

It sounds like you already knew the answer anyway. 🙂

 

[ThirdEye]

In the informative paragraph before that, it states that unsaturated chains are more rigid

No sweat... this info is wrong.

 

[BerkReviewTeach]

No sweat... this info is wrong.

Actually, it's not.

You should go back and look at the text of the passage more carefully and you'll see the difference between molecular inflexibility (seen with pi-bonds more than alkyl groups) and packing rigidity (seen with alkyl chains more than chains with pi-bonds). Also, Dougkaye laid out a great explanation of the information in that passage.

 

[BenZq]

For the MCAT you have to understand that SATURATED fatty acids pack together significantly tighter than unsaturated because the lack of double bonds makes them more linear and less kinky. The kinks in the unsaturated FA's reduces their ability to pack together.

^^^ Exactly. I remember from marine biochem that deep sea microbes synthesize more unsaturated FA to increase membrane fluidity to counter changes in pressure. The added kinks decrease the overall Van Der Waals Forces in the membrane.

Hey BRTeach, Is O. Chem Ch. 7 ex. 7.11 a typo for answer A?