Berkley Review Chapter 1 Organic Chemistry (Fats, Saturation vs. Unsaturation)

[theyellowking]

Question: "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 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 carbon-carbon pi bond

Information given:
Stearic acid has 18 carbons and 0 pi bond
Oleic acid has 18 carbons and 1 pi bond (cis)

Immediately I think to myself that the fatty acid should be rigid. Then I recall that as saturation increases, fluidity decreases, which makes me think that Stearic acid is the choice, eliminating A and C. Because of the absence of the double bond, I then think that this makes the cell membrane of Stearate more rigid, picking D.

However, looking back, this was the answer:

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 greater ability to rotate in different conformations. This is to say that stearate is more flexible than oleate, making choice B the best answer.

I just found it odd that the answer talks about Stearate having more flexibility when we're trying to find something that has greater rigidity. While I understand that looking at one fatty acid that is saturated compared to one that is unsaturated shows that the saturated one is more "flexible" because of the lack of pi bonds, isn't a membrane composed of saturated fatty acids more rigid? In other words, wasn't this question asking which fatty acid is more rigid and why?

(Maybe I'm overthinking this question, but I'm extremely confused with their choice of words for the answer)

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[BerkReviewTeach]

The cell membrane fluidity depends on the packing of the alkyl groups on the molecules. Flexible molecules pack better leading to more rigid solids, such as cell membranes.

 

[theyellowking]

Ah, I didn't make the connection between the flexibility to rigidness. Thank you