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Role of Cholesterol in Lipid Bilayer
The lipid bilayer is a thin biological membrane that is made of two lipid layers. Each layer is built with phospholipids that contain a hydrophilic head and a hydrophobic tail. This structure is fundamental for the functioning of a cellular membrane.
Cholesterol is one of the lipid components that are present in the lipid bilayer. This waxy substance belongs to the steroid family and it is crucial not only for the functionality of the cellular membrane, but also for human existence as it serves as a precursor to synthesize many of the body’s compounds, such as steroids hormones, vitamin D, and bile acids.
Keep on reading to learn about the cell membrane lipid bilayer structure, formation, components, the role of cholesterol in the lipid bilayer, and how this waxy substance affects membrane fluidity. Everything easily explained to facilitate full comprehension.
Lipid Bilayer Structure
To start off, let’s take a look at the lipid bilayer structure. Within the animal’s cell membrane, lipids constitute nearly 50% of its total mass. All of the lipids present in the cell membrane are amphipathic, which means that they possess a hydrophilic (water-loving) and a hydrophobic (water-fearing) end.
The most predominant lipids are phospholipids, they have a hydrophilic head group and a hydrophobic tail, which are usually fatty acids.
The fatty acids tails can differ from each other in different aspects. One potential difference is their length, usually, they contain between 14 and 24 carbon atoms.
Another difference can be found in their bonds, for example, they might have one or more carbon-carbon double bonds, in this case, they are called unsaturated fatty acids or they might not have any double bonds in which case they are called saturated fatty acids. This is especially relevant for membrane fluidity, as this influences the ability that the fatty acids within the cell membrane will have to pack together.
Cholesterol is also predominant in the lipid bilayer, it represents around 25-30% of it. Cholesterol acts by inserting itself into a phospholipid bilayer with its polar hydroxyl group.
This function is extremely important and guarantees cellular membrane fluidity and permeability, we will look into that in-depth later in this article.
Lipid Bilayer Formation
The lipid bilayer formation is spontaneous, this is explained by the chemical structures of the substances that form the bilayer.
As mentioned above, the lipid bilayer contains phospholipids that have a head and a tail, the tails are hydrophobic meaning that they are insoluble in water, whereas the heads are hydrophilic, which means soluble in water.
When multiple phospholipids come together in a watery solution, the tails interact with each other creating a hydrophobic center.
On the other hand, the heads get together creating a hydrophilic coat on each side of the bilayer.
These reactions are spontaneous as they are energy favorable, they follow the principles of thermodynamic self-assembly, where molecules organize themselves in a function and/or ordered structure.
Fluid Mosaic Model
In 1972, scientists proposed the mosaic fluid of the cell membrane. This model describes the cell membrane as a fluid mosaic due to its ability to move fluidly and freely, where phospholipids, cholesterol, and proteins move continually sliding past one another.
This fluid mosaic model explains a variety of observations about the functional structure of the cell membrane, its capacity to maintain fluidity, self-sealing, and permeability. For instance, if you insert a needle into a cell membrane, it will penetrate without causing it to burst and once the needle is removed, the membrane will seamlessly self-seal.
Cholesterol in Cell Membrane
Due to cholesterol’s chemical structure, it has the capacity to fit in spaces in the middle of the phospholipids and prevent the diffusion across the membrane of water-soluble molecules, thus reducing the permeability of the membrane.
In addition, cholesterol has the capacity to affect membrane fluidity by increasing the temperature range in which the plasma membrane can continue to function, keep on reading to understand more about this phenomenon.
Cholesterol in Lipid Bilayer Fluidity
In animal cells, cholesterol acts by inserting itself into a phospholipid bilayer with its polar hydroxyl group, having the capacity to either increase or decrease membrane fluidity. For instance, when the temperature rises cholesterol diminishes membrane fluidity by pulling phospholipids together and increasing intermolecular forces. On the other hand, when the temperature drops, cholesterol increases fluidity by keeping phospholipids from packing together.
In this manner cholesterol has the capacity to act as a buffer for the cell membrane, helping it keep fluidity even when the temperature rises or drops. In other words, cholesterol helps to expand the range of temperature in which the cell membrane is fluid and consequently functional.
Role of Cholesterol in Lipid Bilayer Conclusion
The lipid bilayer is a critical component in cellular membranes. This structure resembles a fluid mosaic and due to this characteristic, it is capable of controlling which substances get in and out of the cell, in addition to providing the cell with fluidity, another remarkable characteristic for the cell’s functionality.
Cholesterol is present in the lipid bilayer along with phospholipids and it plays a huge role in cell membrane capability.
Cholesterol inserts itself between phospholipids in the lipid bilayer and affects membrane fluidity not only by increasing the temperature range in which the cell membrane can continue to function, but it also acting as a barrier, as due to its chemical structure, it can fit in gap spaces, preventing water soluble substances from diffusing across the membrane.
