Cholesterol and phospholipid membrane fluidity

[dnovikov]

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So in TBR Bio chapter 6, it states "Addition of cholesterol to a membrane acts to decrease fluidity. The planar steroid ring inserts between neighboring fatty acid side chains and interferes with movement of those chains.." [can be found on page 34 of the 2nd book]

However when doing passage X on page 76, question #65 asks: "In eukaryotic cells, the plasma membrane contains relatively large amounts of cholesterol. At such high concentrations, cholesterol has the effect to:
A: decreasing the fluidity of membrane by inhibiting hydrocarbon-chain crystallization
B: decreasing the fluidity of membrane by promoting hydrocarbon-chain crystallization
C: increasing the fluidity of membrane by inhibiting hydrocarbon-chain crystallization
D: increasing the fluidity of the membrane by promoting hydrocarbon-chain crystallization

I chose A and in the explanation to why answer C is correct, TBR states: "at the high concentrations that cholesterol is found in eukaryotic cells, it acts to increase the fluidity of the membrane"

While it seems as if the explanation contradicts what is said earlier in the chapter, is it because the concentration of cholesterol is HIGH that fluidity is increased? Like if there is just a normal amount of cholesterol, would fluidity decrease as previously stated in the chapter?

 

[YouNeverKnow22]

As far as I know Cholesterol contributes to fluidity in Eukaryotic cells and Hopanoids in Prokaryotic cells., I use EK, and I'm pretty sure that's right.

 

[cloak25]

Inhibiting hydrocarbon chains from crystallizing = increasing fluidity. Choice A says it decreases fluidity which would be true if cholesterol is promoting hydrocarbon-chain crystallization. That being said, wouldn't choice B be true?😕

 

[Nontrad85]

Inhibiting hydrocarbon chains from crystallizing = increasing fluidity. Choice A says it decreases fluidity which would be true if cholesterol is promoting hydrocarbon-chain crystallization. That being said, wouldn't choice B be true?😕

While choice B is a true statement, it is not the best answer for the question.
At high concentration of cholesterol, it help separate phospholipids at the plasma membrane so that fatty acid chains (ie. long hydrocarbon chains) do not come close to each other and crystallize; in turn, this increases cell membrance fluidity.

 

[dnovikov]

While choice B is a true statement, it is not the best answer for the question.
At high concentration of cholesterol, it help separate phospholipids at the plasma membrane so that fatty acid chains (ie. long hydrocarbon chains) do not come close to each other and crystallize; in turn, this increases cell membrance fluidity.

So does a low concentration decrease fluidity as stated in the text?

 

[SLC]

Just know that in a membrane, cholesterol prevents extremes. It keeps the membrane from becoming too fluid, but also keeps it from becoming too rigid. It's a goldilocks molecule with respect to the membrane.

 

[Temperature101]

what is crystallizing?

 

[Danlee07]

what is crystallizing?

Normal membrane fluidity refers to the state where the lipid bilayer is at a liquid crystallizing state, which means hydrocarbon tails are disordered with cholesterol stuck between them. When in the absence of cholesterol, the lipid bilayer would be in a crystalline state, the hydrocarbon tails are fully extended with just van de Waal attraction and other weak interactions.

Cholesterol at a "normal level" is already concentrated but evenly spaced throughout the lipid bilayer.

Not A. bc the fluidity would need to be increased to maintain inhibition of CH tails from interaction.
Not B. bc promotion would mean CH interact in a crystalline state.
Not D. bc promotion would mean CH interact in a crystalline state.

 

[gettheleadout]

Just know that in a membrane, cholesterol prevents extremes. It keeps the membrane from becoming too fluid, but also keeps it from becoming too rigid. It's a goldilocks molecule with respect to the membrane.

Yeah that's how I learned it too; cholesterol has a "moderating" effect.

 

[Nontrad85]

Just know that in a membrane, cholesterol prevents extremes. It keeps the membrane from becoming too fluid, but also keeps it from becoming too rigid. It's a goldilocks molecule with respect to the membrane.

Yes, I completely agree.

Also, I think it's important to note that too much cholesterol on the cell membrane can be necrotic/apoptotic to the cell, which has been shown in pancreatic tissues/cells.

 

[411309]

For the record TPR specifically says cholesterol increases fluidity 😵

 

[gettheleadout]

For the record TPR specifically says cholesterol increases fluidity 😵

This is what I have in my notes from cell bio

Intercalation of the cholesterol molecules lowers fluidity at high T, increases fluidity at low T; moderating effect, helps maintain Tm-level fluidity

 

[Godric]

So take away from this would be,

> Pertaining to cholesterol concentration
-at high concentration cholesterol would increase fluidity

-at lower concentration cholesterol would decrease fluidity

>Pertaining to temperature

- At higher temp cholesterol increases fluidity

-At lower temp cholesterol decreases fluidity, and would impart rigidity

does anyone want to check/correct/agree with the statements above?
btw this question drove insane!

 

[typicalindian]

So take away from this would be,

> Pertaining to cholesterol concentration
-at high concentration cholesterol would increase fluidity

-at lower concentration cholesterol would decrease fluidity

>Pertaining to temperature

- At higher temp cholesterol increases fluidity

-At lower temp cholesterol decreases fluidity, and would impart rigidity

does anyone want to check/correct/agree with the statements above?
btw this question drove insane!

You have it backwards my friend, at high temperatures cholesterol decreases fluidity and at low temperatures it increases it. I just had an exam on this last week in my cell bio class and we learned cholesterol as being a "fluidity buffer". As for the concentration, I don't know. The answer choice in OP contradicts what we learned in class >_>

 

[Godric]

You have it backwards my friend, at high temperatures cholesterol decreases fluidity and at low temperatures it increases it. I just had an exam on this last week in my cell bio class and we learned cholesterol as being a "fluidity buffer". As for the concentration, I don't know. The answer choice in OP contradicts what we learned in class >_>

lol just went back to tbr and the part about temp, they wrote pertaining to fats, but I assumed that because fats are lipids, and so is cholesterol it must apply to them also. So now I am just confused, I googled and I found this.

source- http://en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Membrane_Fluidity#Melting_Temperature

So is what typicalindian said correct or what I said ?

 

[gettheleadout]

Godric, the picture you've posted is accurate, but the function of cholesterol is to maintain the fluidity of the membrane around the level it has at Tm temperature. Temperature makes it change fluidity according to your graph, but cholesterol acts counter that change.

 

[typicalindian]

lol just went back to tbr and the part about temp, they wrote pertaining to fats, but I assumed that because fats are lipids, and so is cholesterol it must apply to them also. So now I am just confused, I googled and I found this.

source- http://en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Membrane_Fluidity#Melting_Temperature

So is what typicalindian said correct or what I said ?

I'm right haha. The picture you posted shows membranes becoming more fluid like as temperature increases so cholesterol will act to lower the fluidity to maintain the integrity of the membrane. Likewise, when temperature decreases, the fluidity also decreases. In order to fix this the cholesterol will function to bring fluidity back to normal.
Just remember "cholesterol is a fluidity buffer" and you'll be golden. What's a buffer do? It acts to resist a change. So with cholesterol it functions to oppose changes to fluidity that are caused by other factors.

One other thing you should know is that prokaryotes don't have cholesterol in their membranes so they must use other methods to regulate fluidity. They use an enzyme called "desaturase" which desaturates the C-C bonds which INCREASES fluidity because they are less able to pack together. And I believe, but am not sure, that they use an enzyme to do the opposite.
Hope that helps!

 

[jpfoh]

Passage from "Molecular Biology of the Cell" - Fifth Edition by Alberts

[cholesterol] enhances the permeability-barrier properties of the lipid bilayer. It inserts into the bilayer with its hydroxyl group close to the polar head groups of the phospholipids, so that its rigid, platelike steroid rings interact with- and partially immobilize- those regions of the hydrocarbon chains closest to the polar head groups. By decreasing mobility of the first few CH2 groups of the hydrocarbon chains of the phospholipid molecules, cholesterol makes the lipid bilayer less deformable in this region and thereby decreases the permeability of the bilayer to small water-soluble molecules. Although cholesterol tightens the packing of the lipids in a bilayer, it does not make membranes any less fluid. At high concentrations found in most eucaryotic plasma membranes, cholesterol also prevents the hydrocarbon chains from coming together and crystallizing.

Now to be honest, I am having trouble making sense of all this. Can somebody help simplify please.

 

[NonTraditional3]

Has to be answer C. Here's why.

FACT: cholesterol imparts fluidity to biological membranes.

We can consider fluidity to be analogous to "liquidity" that is to say, exhibiting fluid-like properties, such as flow and assuming the shape of whatever it is contained in. Now, if hydrocarbon chains were crystallizing, that can be considered analogous to solidifying. As solidification (crystallization) takes place, the constituents become more rigid, and thus less fluid-like. So, promotion of crystallization cannot lead to enhanced membrane fluidity.

With this, we can eliminate A and B, as they both say decrease fluidity, and this brings you to 50/50 chance if you guessed on C vs D, but D cannot be correct as the second half of the answer contradicts the first half.

 

[Lunasly]

I know this is an old thread, but I also wanted to get some clarification on this. Cholesterol does provide a moderating effect, but it is to my understanding that its effect will depend on temperature. If the membrane is always in its fluid phase, then adding cholesterol will make it more rigid (less fluid). However, if the membrane is in a relatively solid-like phase, then adding cholesterol will increase fluidity (disrupts the electrostatic forces between the fatty acid tails. If a passage does not indicate the phase of the membrane, how can we accurately determine whether or not fluidity has increased or decreased?