Structure of the cell membrane
Cell membranes separate different areas from each other. To do this, they have to meet many different requirements: First of all, cell membranes are made up of a double layer of two fat films, which in turn are composed of individual fatty acids. The fatty acids consist of a water-soluble, hydrophilic head and a water-insoluble, hydrophobic tail.
The heads attach themselves to each other in one plane so that the mass of tails all point in one direction. Opposite to this, another row of fatty acids attach themselves to each other in the same pattern. This creates the bilayer, which is bounded on the outside by the heads and thus creates a hydrophobic area on the inside, i.e. an area into which no water can penetrate.
Depending on the molecules that make up the head of a fatty acid, they have different names and properties, but these play only a minor role. Fatty acids can be unsaturated or saturated, which depends on the respective tail and its chemical structure. Unsaturated fatty acids are much stiffer and cause a decrease in the fluidity of the membrane, whereas saturated ones increase the fluidity.
Fluidity is a measure of the mobility and deformability of the lipid bilayer. Depending on the task and condition of the cell, different degrees of mobility and stiffness are required, which can be achieved by the additional incorporation of one or the other type of fatty acid. In addition, cholesterol can be incorporated into the membrane, which massively reduces the fluidity and thus stabilizes the membrane.
Due to this structure, only very small, water-insoluble substances can easily overcome the membrane. However, since significantly larger and water-insoluble substances also have to cross the membrane to be transported in or out of the cell, transport proteins and channels are necessary. These are incorporated into the membrane between the fatty acids.
Since these channels are passable for some molecules and not for others, we speak of a semi-permeability of the cell membrane, i.e. partial permeability. The last building block of cell membranes are receptors. Receptors are also large proteins, which are usually produced in the cell itself and then incorporated into the membrane.They can either be spanned completely or only be supported on the outside.
Due to their chemical structure, the transporters, channels and receptors remain firmly in and on the membrane, so they cannot be detached from it so easily. However, they can be moved laterally to different locations within the membrane, depending on where they are needed exactly. On the outside of the cell membranes, sugar chains may still be present, known as glycocalyx.
They are for example the basis of the blood group system. Since the cell membrane consists of so many different building blocks that can also variably change their exact localization, it is also called the liquid mosaic model. Cell membranes are about 7 nm thick, i.e. extremely thin, but nevertheless robust and insurmountable for most substances. The head areas are each about 2 nm thick, while the hydrophobic tail area measures 3 nm wide. This value hardly varies between the different cell types of the human body.
