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The rate of transmembrane movement of cholesterol in the human erythrocyte. Selectively permeable membranes allow cells to keep the chemistry of the cytoplasm different from that of the external environment. It also allows them to maintain chemically unique conditions inside their organelles.
The cell membrane is not a static structure. It is a dynamic structure that allows the movement of phospholipids and proteins. Fluidity is a term used to describe the ease of movement of molecules in the membrane and is an important characteristic for cell function.
Fluidity is dependent on the temperature increased temperatures it more fluid and decreased temperatures make it more solid , saturated fatty acids and unsaturated fatty acids.
Saturated fatty acids make the membrane less fluid while unsaturated fatty acids make it more fluid. The correct ratio of saturated to unsaturated fatty acids keeps the membrane fluid at any temperature conducive to life. For example, winter wheat responds to decreasing temperatures by increasing the amount of unsaturated fatty acids in cell membranes to prevent the cell membrane from becoming too solid in the cold.
In animal cells, cholesterol helps to prevent the packing of fatty acid tails and thus lowers the requirement of unsaturated fatty acids.
A set of probes in which a 9-anthroyloxy group is attached to different positions of a long chain fatty acid provides a means of measuring a fluidity gradient into membrane bilayer.
Thus, these fluorescence methods which are simple and rapid represent a semi-quantitative approach of the so called "membrane fluidity". Modifications in membrane fluidity can control the expression of proteins, receptors exposed on cell surface and alter functional properties of cells. 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?
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