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Cell membrane fluidity

Cell membrane fluidity is the property of the cell membrane that allows it to adapt its shape and movement to different conditions. Three key factors influence cell membrane fluidity: temperature, cholesterol, and the kind of fatty acids in the phospholipids that form the cell membrane. These factors alter the space and interaction between the phospholipids, and how this affects the fluidity of the cell membrane. Created by William Tsai.

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  • blobby green style avatar for user amy
    Since cholesterol acts as a "buffer", does it only affect fluidity when temperature is also in play (at low/high temperatures)? So in normal temperature conditions, would cholesterol have no affect on the fluidity?
    (13 votes)
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    • piceratops ultimate style avatar for user Cassandra LaMarche
      Cholesterol still effects membrane fluidity at normal temperature. Think of it sort of like a gradient. At low T's, cholesterol increases fluidity by preventing overly tight packing of the phospholipids . At higher T's, cholesterol decreases fluidity by pulling the phospholipids closer together so at medium T's, cholesterol is keeping the bilayer together but also preventing the phospholipids from overpacking. It is essentially playing both roles at medium temperatures.
      (2 votes)
  • hopper jumping style avatar for user Lucas De Oliveira
    Can somebody give a example of a medical situation that the doctor could intervene in membrane fluidity?
    (12 votes)
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    • starky tree style avatar for user Jacob Andrews
      Spur cell anemia (acanthocyte formation) would be the overproduction of cholesterol in RBCs. This high concentration of cholesterol would cause increased rigidity leading to a rupture of the hemeglobin. An overactive spleen would be the pathology of this anemia. So the treatment would involve regulation of the splenic activity and not the fluidity of the membrane directly.
      (6 votes)
  • leafers ultimate style avatar for user James Greenhalgh
    What affects how much saturated fat and how much unsaturated fat the phospholipid bilayer is composed of?
    (11 votes)
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    • blobby green style avatar for user Raphael.Urbin
      Maybe you are not right. Think about the case in which you are genetically programed efficiently but you eat only saturated fats and lots of trans fats. Well, your body will become very rigid, you can't get enough nutrients through the cells, and you will die pretty soon. So, I think you should be able to modify and training the cells to work efficiently and in balance. I think that unsaturated fats can't be more than 10% in the total membrane, for a lot of reasons and naturally divine occurred limitations: temperature, blood density, oxygenation, etc. For example, high activity levels, diets and environment influences a lot.
      (2 votes)
  • purple pi purple style avatar for user Felicia Wright
    How does cholesterol decrease the fluidity in lipid rafts? I thought that the glycosphingolipids in the rafts are packed tightly together so shouldn't cholesterol increase the distance between these lipids thus increasing the fluidity?
    (5 votes)
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  • aqualine ultimate style avatar for user milind
    what exactly is fluidity ?
    (3 votes)
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    • mr pants orange style avatar for user woolichooks
      What I got from the video was that the fluid nature of the lipid bilayer means that the molecules within the layer such as the phospholipids can move around as freely as they want. If the temperature is low, then the phospholipids will not be able to move around as much and will be in its crystallized state. Therefore, fluidity just means movement.
      (4 votes)
  • blobby green style avatar for user mmk145
    So cis-unsaturated fats increase fluidity because they cause these kinks or bends. Do trans-unsaturated fats also increase fluidity? To my knowledge, they do not create the same kinks/bends as cis configuration does.
    (4 votes)
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    • piceratops ultimate style avatar for user Darmon
      Trans-unsaturated fats will not have kinks in them, and will therefore not significantly affect membrane fluidity. However, it is interesting to note that nearly all unsaturated biological fatty acids have cis double bonds in their tails; trans-unsaturated fats are extremely rare in living organisms. :)
      (2 votes)
  • blobby green style avatar for user kiwinoelaa
    Why do the phospholipids move towards the cholesterol when its too warm?
    (4 votes)
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    • boggle yellow style avatar for user Anuja
      Cholesterol has an OH group on one of its ends. Since the oxygen in the OH group is highly electronegative, it hogs all of the electrons and a positive charge is created on the hydrogen side of the group. That side then forms a hydrogen bond with the negatively charged oxygen on the phosphate group.
      The rest of the cholesterol molecule is made up of carbon and hydrogen, so it is non-polar. It aligns itself with the non-polar, hydrophobic lipid tails of the phospholipids. It attracts the lipid tails through Van der Waal forces (London dispersion forces).
      Because of these forces of attraction, phospholipids tend to stay around cholesterol molecules when it is warm (and in other temperatures). So, they stop nearby phospholipids from moving about, thereby reducing the fluidity of the membrane.
      (1 vote)
  • primosaur ultimate style avatar for user Asaad Khattab
    Is it the double bond in the unsaturated fatty acid that gives it the kink or bend.?
    (2 votes)
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  • blobby green style avatar for user Kalulong
    I understand how and why temperature changes effect fluidity of phospholipids. I also understand that cholesterol counteracts the change in fluidity. However, the cholesterol, as you mention, acts as a buffer, which implies that as temperature increases, membrane fluidity still has a net increase in fluidity, correct? In other words, do cholesterol interactions ever actually have such an effect that an increase in temperature decreases membrane fluidity overall? Thanks!
    (3 votes)
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    • male robot hal style avatar for user Abid Ali
      Don't overthink this one. In a hypothetical cell membrane where cholesterol doesn't exist, temperature increase and decrease will increase and decrease fluidity, respectively. Now, if you add cholesterol to a cell membrane, the reverse is true. Also, cis-fatty acids will increase membrane fluidity as well.
      (0 votes)
  • aqualine seed style avatar for user michelle.92003
    How do steroids maintain the fluidity of a membrane?
    (2 votes)
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Video transcript

Why doesn't our cell membrane fall apart when it's too hot, or why doesn't our cell membrane freeze when it gets too cold? Well, our cell membrane actually has a very unique property called membrane fluidity. Now, a lot of different factors can affect membrane fluidity. But the three most important ones that we're going to focus on today are, number one, temperature, number two, cholesterol, and number three, which is whether we have unsaturated or saturated fatty acids. Now, just to quickly to remind us, the building blocks of a cell membrane are what we call phospholipids. And it looks like this. There's a phosphate head group that's represented by a circle and two fatty acid chains, kind of like strings hanging below. And in this video, we're actually really going to focus in on the impact of phospholipids in our cell membrane. So the first thing we're going to start off with is temperature. We have low temperature, and obviously we have high temperature. So let's pretend that our cell membrane is only made up of phospholipids. What do you think our cell membrane's going to look like at low temperatures? Since the temperature is low, our phospholipids are actually going to start clustering together really closely, kind of like that. And the reason why is because these phospholipids are at low temperature, which means they don't have a lot of energy to move around a lot. So they're going to huddle really close together. At extremely low temperatures, we actually call this a crystallized state. And since they're huddled so close together and they don't have a lot of energy to move around, the fluidity is actually pretty low. So as the temperature decreases, the fluidity of the cell membrane also decreases. What happens at high temperatures? Well, at high temperatures, our phospholipids have a little more energy. So they're going to move around a little bit more and cause themselves to have more of a distance between each other, kind of like that. So you'll notice that the distance between phospholipids is now much greater than what it was over here, at low temperatures, which is very, very small. So this increased distance allows our fluidity to increase, because there's much more room for the cell membrane to move around. So as the temperature increases, our membrane fluidity also increases. What happens when we add cholesterol? Well, at low temperatures, our phospholipids still tend to cluster pretty closely together. But occasionally, something really interesting happens, which is when cholesterol actually inserts itself between the phospholipids, like this. And it doesn't do this for every single phospholipid, but it'll occasionally insert itself into the membrane. The same goes for the phospholipids that are underneath. And you'll notice that the membrane doesn't always have to line up in the sense that the phospholipids can actually be in the same place as the ones above or in a slightly different place. In some membranes there's more cholesterol, and in others there is less. But the presence of cholesterol itself does something really unique. And what that is, is it actually increases the distance between some of the phospholipids. And like we've talked about for the high temperatures, as the distance between the phospholipids increases, the fluidity can also increase. What happens at high temperatures with cholesterol? At high temperatures, our phospholipids are already pretty far apart, just like the above picture. But just like before, the cholesterol will insert itself into the membrane at random places. And what this will actually do is it will cause the phospholipids to pull themselves closer together, because they kind of want to attach to that cholesterol. So now there's more stuff inserted throughout the membrane, and so the molecules in the membrane are now closer together. So the fluidity actually decreases. So cholesterol is actually really interesting, because at low temperatures, the fluidity will increase. And at high temperatures, the fluidity will decrease. You can kind of think about cholesterol like a buffer, kind of like in chemistry. It allows our cell membrane to remain at a fairly stable and normal level of fluidity. When the temperature gets too low, the fluidity will increase a little. And when the temperature gets too high, the fluidity will decrease. So moving on to our third one, which is the presence of saturated versus unsaturated fats, we're going to go ahead and make a new canvas to give ourselves a little bit of room. So in number three, we're comparing the presence of saturated versus unsaturated fats. And when we're talking about saturated versus unsaturated, we're talking about the fatty acid chains that are hanging below our phosphate head group. So just to remind us from chemistry, a saturated fatty acid can be represented like this, where every angle or pointy end is a carbon. In the case of an unsaturated fatty acid, it can look pretty different, because an unsaturated fatty acid means that we have some double bonds. So let's say we have two double bonds like that. By themselves, it doesn't seem to be anything special. Granted, they look different. But how will these interact with multiple fatty acids next to them? So in the case of a saturated one and in the case of an unsaturated one, our molecule will still have some double bonds. And what's really unique is you'll notice that in the saturated fatty acid, these two fatty acid chains stack together really neatly, kind of like Legos. But in our unsaturated fatty acid, these two don't really stack together that neatly. How will this affect our membrane fluidity? Well, for the sake of this particular explanation, we're going to draw the saturated fatty acid chains as straight lines, like this and just because we're trying to represent the fact that these straight lines stack together really well. So what's going to happen is they'll stack pretty closely together, and so will the ones underneath. And since the distances between the molecules is pretty small, our fluidity is actually pretty low. So what do you think will happen with our unsaturated fatty acids? Well, you'll notice that there's a little bit of a bend now in these fatty acid chains. So I'm actually going to represent the phospholipid with a little bend in it. And these might occur at different places, or they might have both of them being bent. But you'll notice that I'm unable to draw these phospholipids as closely together. There becomes more distance between these phospholipids because of this unsaturated bend in our phospholipids. So since there's more distance between our phospholipids, the fluidity increases. Just to quickly sum up, today we learned the three factors that can affect membrane fluidity, the first being temperature. As temperature increases, fluidity also increases. The second is cholesterol. And cholesterol acts as a buffer, increasing fluidity at low temperatures and decreasing fluidity at high temperatures. And the last are unsaturated fatty acids in our phospholipid. When we increase the amount of unsaturated fatty acids in our cell membrane, the fluidity also increases.