Question on Cell Membrane

[Hexon]

i understand that according to the fluid mosaic model the cell membrane is comprised of a hydrophilic head and a hydrophobic tail;

so would this mean that the hydrophilic head (that faces the outside) is polar? and if so, how is it possible that lipid soluble or nonpolar molecules such as cholesterol can easily pass through the membrane when non lipid soluble molecules such as peptide hormones (glucagon, prolactin, etc) require cell membrane receptors ?

i think i'm confusing myself here, but i'd appreciate any help and insight🙂 what am i missing over here?

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[Erythropoietin]

i understand that according to the fluid mosaic model the cell membrane is comprised of a hydrophilic head and a hydrophobic tail;

so would this mean that the hydrophilic head (that faces the outside) is polar? and if so, how is it possible that lipid soluble or nonpolar molecules such as cholesterol can easily pass through the membrane when non lipid soluble molecules such as peptide hormones (glucagon, prolactin, etc) require cell membrane receptors ?

i think i'm confusing myself here, but i'd appreciate any help and insight🙂 what am i missing over here?

Cholesterol has both a polar and a nonpolar part, so it is amphipathic. The OH group (see figure: http://www.paleovillage.com/wp-content/uploads/2011/07/440px-Cholesterol.svg_.png) aligns with the polar phosphate group of the phospholipids. The remaining part of the molecule (the 4 rings and the carbon chain) aligns with the fatty acids (the hydrophobic portion of the membrane). The non-lipid molecules require receptors or some sort of channels to go through because it would require energy for them to pass through the long carbon chains in between the polar heads of the membrane.

 

[Erythropoietin]

Here's another cool picture http://www.uic.edu/classes/bios/bios100/lectf03am/cholesterol.jpg

 

[bidiboom]

And the phospholipid bilayer is a "general picture" of the membrane.. actually they all the time turning here and there upside down.. and hop! rearrange themselves again.. I mean membrane actually is not static, the phospholipid components are all the time dynamically moving in their places..

I am not sure but this may have an effect as well.

 

[Hexon]

thanks for the responses guys🙂

Cholesterol has both a polar and a nonpolar part, so it is amphipathic. The OH group (see figure: http://www.paleovillage.com/wp-content/uploads/2011/07/440px-Cholesterol.svg_.png) aligns with the polar phosphate group of the phospholipids. The remaining part of the molecule (the 4 rings and the carbon chain) aligns with the fatty acids (the hydrophobic portion of the membrane). The non-lipid molecules require receptors or some sort of channels to go through because it would require energy for them to pass through the long carbon chains in between the polar heads of the membrane.

i see, so whilst the OH functional group of the cholesterol bonds to the polar head(s) of the cell membrane, the non polar parts of cholesterol would bind to the 'freely moving' fatty acids that reside in and around the cell membrane?

Here's another cool picture http://www.uic.edu/classes/bios/bios100/lectf03am/cholesterol.jpg

nice pic🙂

And the phospholipid bilayer is a "general picture" of the membrane.. actually they all the time turning here and there upside down.. and hop! rearrange themselves again.. I mean membrane actually is not static, the phospholipid components are all the time dynamically moving in their places..

I am not sure but this may have an effect as well.

good points, thank for that🙂

 

[Erythropoietin]

thanks for the responses guys🙂

i see, so whilst the OH functional group of the cholesterol bonds to the polar head(s) of the cell membrane, the non polar parts of cholesterol would bind to the 'freely moving' fatty acids that reside in and around the cell membrane?

nice pic🙂


good points, thank for that🙂

I'm not exactly sure how exactly it moves inside the membrane but I'm guessing the polar OH binds to the polar heads and "makes room?" for the nonpolar part of the cholesterol to enter inside the membrane.

 

[Hexon]

I'm not exactly sure how exactly it moves inside the membrane but I'm guessing the polar OH binds to the polar heads and "makes room?" for the nonpolar part of the cholesterol to enter inside the membrane.

ah ok, so from your perspective and to use the analogy of the OH being a spearhead, the rest of the nonpolar cholesterol body follows the polar OH as it 'clears the way' through the cell membrane?

 

[Erythropoietin]

ah ok, so from your perspective and to use the analogy of the OH being a spearhead, the rest of the nonpolar cholesterol body follows the polar OH as it 'clears the way' through the cell membrane?

Yes but I'm not sure; but it makes sense that OH binds to the polar heads first, otherwise there's no way for the hydrocarbon chain of cholesterol to go inside the membrane. This bit of info is beyond the scope of MCAT though.

 

[Hexon]

Yes but I'm not sure; but it makes sense that OH binds to the polar heads first, otherwise there's no way for the hydrocarbon chain of cholesterol to go inside the membrane. This bit of info is beyond the scope of MCAT though.

yeah i was thinking it through in my head, trying to visualise how cholesterol and other lipid soluble molecules easily move thru the cell membra,e interesting regardless though.

THanks for the help, and to everyone else🙂

 

[helicase8]

I'm not exactly sure how exactly it moves inside the membrane but I'm guessing the polar OH binds to the polar heads and "makes room?" for the nonpolar part of the cholesterol to enter inside the membrane.

yes. the oh group of the cholestrol interacts with the polar head group and the nonpolar fa interacts with the non polar region of the membrane.
remember that cholestrol influences membrane fluidity, so the more cholestrol you have in the membrane the more rigid it becomes.
but you are right about the "making room" in a sense, because membranes are not symmetric. one leaflet is different from the other. the hard part about science when you really try to analyze it to that level, is that scientists now-a-days still don't get it 100%. you also have to take into consideration that there are alot of types of phospholipids, and the different ways that things enter a cell.

i hope it also helped a little. if you still don't get it i will dig up my notes. i guess its something i should know 🙂

 

[Erythropoietin]

^ Yes true. and since cholesterol is something that's needed by the membrane itself (just like water is needed for other things), I don't think there would be a reason for a phospholipid membrane to be impermeable to cholesterol...meaning that it should be very easy for it to get inside

 

[Ssina]

yes. the oh group of the cholestrol interacts with the polar head group and the nonpolar fa interacts with the non polar region of the membrane.
remember that cholestrol influences membrane fluidity, so the more cholestrol you have in the membrane the more rigid it becomes.
but you are right about the "making room" in a sense, because membranes are not symmetric. one leaflet is different from the other. the hard part about science when you really try to analyze it to that level, is that scientists now-a-days still don't get it 100%. you also have to take into consideration that there are alot of types of phospholipids, and the different ways that things enter a cell.

i hope it also helped a little. if you still don't get it i will dig up my notes. i guess its something i should know 🙂

I just wanted to check with you that statement again, I thought you can't really say if we have more cholesterol the fluidity is decreases.... I thought it can regulate the fluidity of a cell membrane either way, depending on the temperature... now I am a bit rusty on this, so I could be wrong!

 

[Erythropoietin]

I just wanted to check with you that statement again, I thought you can't really say if we have more cholesterol the fluidity is decreases.... I thought it can regulate the fluidity of a cell membrane either way, depending on the temperature... now I am a bit rusty on this, so I could be wrong!

Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules.

 

[Ssina]

Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules.

I realize that, but I was under the impression that it is temperature dependant- so basically at hight temp it will reduce fluidity and at low temp actually increase fluidity of the membrane.... under normal conditions and at normal body temperature I guess we can assume it will just decrease it

 

[askamsky51]

And the phospholipid bilayer is a "general picture" of the membrane.. actually they all the time turning here and there upside down.. and hop! rearrange themselves again.. I mean membrane actually is not static, the phospholipid components are all the time dynamically moving in their places..

I am not sure but this may have an effect as well.

Just a simple correction to this, but generally speaking molecules in the phospholipid bilayer do not flip. They certainly move across in lateral diffusion (at amazing rates), however flipping is a "relatively" rare occurrence which is facilitated by other enzymatic-type activity.

I realize that, but I was under the impression that it is temperature dependant- so basically at hight temp it will reduce fluidity and at low temp actually increase fluidity of the membrane.... under normal conditions and at normal body temperature I guess we can assume it will just decrease it

cholesterol is independent from temperature in membrane fluidity. Remember, as temperature decreases the membrane becomes more ordered (therefore less fluid). When considering temperature I always think about absolute zero, the closer molecules are to absolute zero--the less they move.

 

[Ssina]

Just a simple correction to this, but generally speaking molecules in the phospholipid bilayer do not flip. They certainly move across in lateral diffusion (at amazing rates), however flipping is a "relatively" rare occurrence which is facilitated by other enzymatic-type activity.

flipaaseee... good pt!

 

[Erythropoietin]

Yes membranes do not flip; they just move laterally.
About temperature, I don't think cholesterol is too much temperature dependent because there are still the same functional groups (OH of cholesterol) being involved. Too high temperature will introduce more kinetic energy and melt the fatty acids down. As temperature increase, both the cell membrane and the proteins can be affected.
On the other hand, a decrease in temperature will cause the phospholipids to be more rigid and thus less fluid. When this happens, permeability decreases.

 

[blazinfury]

I have a question about temperature and cholesterol. When a membrane freezes, it would be ideal to add more cholesterol to maintain fluidity so that it does not freeze. So would this mean that a membrane with more cholesterol has a lower freezing pt (more neg) than a membrane that is more saturated? On that same note, a membrane with more cholesterol would have a lower melting point than a membrane with less cholesterol due to the weaker bond interaction between the surrounding molecules as they are not packed together well?

 

[dmf2682]

I have a question about temperature and cholesterol. When a membrane freezes, it would be ideal to add more cholesterol to maintain fluidity so that it does not freeze. So would this mean that a membrane with more cholesterol has a lower freezing pt (more neg) than a membrane that is more saturated? On that same note, a membrane with more cholesterol would have a lower melting point than a membrane with less cholesterol due to the weaker bond interaction between the surrounding molecules as they are not packed together well?

All this talk of melting and freezing makes me wonder how relevant this conversation is... Isn't that well outside the range of human body temps?