Cells are the smallest, coherent units that make up organs and tissues. Each cell is surrounded by a cell membrane, a barrier that consists of a special double layer of fat particles, the so-called lipid double layer. Lipid bilayers can be imagined as two fat films stacked on top of each other, which due to their chemical properties cannot separate from each other and thus form a very stable unit. Cell membranes fulfill many different functions: They are used for communication, protection and as a control station for cells.
Which different cell membranes are there?
Not only is the cell itself surrounded by a membrane, but also the cell organelles. Cell organelles are small, membrane-delimited areas within the cell, each of which has its own task. They differ in their proteins, which are embedded in the membranes and act as transporters for substances that are to be transported across the membrane.
The inner mitochondrial membrane is a special form of the cell membrane. Mitochondria are organelles that are important for the cell to generate energy. They were only absorbed into the human cell later in the course of evolution. Therefore they have two lipid bilayer membranes. The outer one is the classical human one, the inner one the membrane specific for the mitochondrion. It contains cardiolipin, a fatty acid that is built into the fatty film and can only be found in the inner membrane and no other.
The human body only contains cells that are surrounded by a cell membrane. However, there are also cells, such as bacteria, that are also surrounded by a cell wall. The terms cell wall and cell membrane cannot therefore be used synonymously. Cell walls are significantly thicker and also stabilize the cell membrane. Cell walls are not necessary in the human body as many individual cells can join together to form strong associations. Bacteria, on the other hand, are single-celled cells, i.e. they only consist of a single cell, which would be significantly weaker without the cell wall.
Read more on the subject at: bacteria
Structure of the cell membrane
Cell membranes separate different areas from one another. 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 from a water-insoluble, hydrophobic Tail. The heads attach to each other in one plane, so that the mass of tails all point in one direction. On the other hand, another series of fatty acids accumulate in the same pattern. This creates the double layer, which is bounded on the outside by the heads and in this way one inside hydrophobic Area, i.e. an area into which no water can penetrate, creates.
Depending on which molecules the head of a fatty acid is made up of, they have different names and different properties, but these only play a subordinate role. Fatty acids can be unsaturated or saturated, depending on the tail and its chemical structure. Unsaturated fatty acids are significantly more rigid and cause a decrease in the fluidity of the membrane, whereas saturated fatty acids increase the fluidity. The 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 rigidity are required, which can be achieved through the additional incorporation of one or the other type of fatty acid.
In addition, cholesterol can be built into the membrane, which massively lowers the fluidity and thus stabilizes the membrane. Because of 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 in order to be transported into or out of the cell, transport proteins and channels are necessary. These are stored in the membrane between the fatty acids. Since these channels are passable for some molecules and not for others, one speaks of one Semi-permeability the cell membrane, i.e. a partial permeability.
The last building block of the cell membrane are receptors. Receptors are also large proteins that are mostly produced in the cell itself and then built into the membrane. You can either span them 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 and cannot easily be detached from it. However, they can be moved laterally to different locations within the membrane, depending on where they are needed.
Finally, there may still be sugar chains on the outside of the cell membrane, in technical terms Glycocalyx called. For example, they are the basis of the blood type system. Since the cell membrane consists of so many different building blocks that can also vary their exact location, it is also known as the liquid mosaic model.
Read more on the subject at: Blood types
Cell membrane thickness
Cell membranes are around 7 nm thick, i.e. extremely thin, but still robust and insurmountable for most substances. The head areas are each about 2 nm thick, during the hydrophobic Tail area measures 3 nm wide. This value hardly varies between the different types of cells in the human body.
What are the components of the cell membrane?
Basically, the cell membrane is made up of a phospholipid double layer. Phospholipids are building blocks that consist of a water-loving, i.e. hydrophilic, head and a tail, which is formed by two fatty acids. The part that is made up of fatty acids is hydrophobic, which means that it repels water.
In the double layer of phospholipids, the hydrophobic components point towards each other. The hydrophilic parts point to the outside and inside of the cell. This structure of the membrane allows two aqueous environments to be separated from each other.
The cell membrane also contains sphingolipids and cholesterol. These substances regulate the structure and fluidity of the cell membrane. Fluidity is a measure of how well proteins can move in the cell membrane. The higher the fluidity of a cell membrane, the easier it is for proteins to move in it.
In addition, there are many different proteins in the cell membrane. These proteins are used to transport substances through the membrane or to interact with the environment. This interaction can be achieved through a direct bond between neighboring cells or through messenger substances that bind to the membrane proteins.
The following topic could also be of interest to you: Cell plasma in the human body
Phospholipids in the cell membrane
Phospholipids are the main component of the cell membrane. Phospholipids are amphiphilic. This means that they consist of a hydrophilic and a hydrophobic part. This property of the phospholipids enables the interior of the cell to be separated from the environment.
There are different forms of phospholipids. The hydrophilic backbone of the phospholipids consists either of glycerine or sphingosine. Both forms have in common that two hydrophobic hydrocarbon chains are attached to the basic structure.
Cholesterol in the cell membrane
Cholesterol is contained in the cell membrane to help regulate fluidity. A constant fluidity is very important to maintain the transport processes of the cell membrane. At high temperatures, the cell membrane tends to become too fluid. The bonds between the phospholipids, which are already weak under normal circumstances, are even weaker at high temperatures. Due to its rigid structure, cholesterol helps to maintain a certain strength.
It looks different at low temperatures. Here the membrane can become too tight. Phospholipids, which have saturated fatty acids as a hydrophobic component, become particularly solid. This means that the phospholipids can lie very close to each other. In this case, cholesterol stored in the cell membrane causes increased fluidity, since cholesterol contains a rigid ring structure and thus acts as a spacer.
You can find detailed information on the subject of "cholesterol" at:
- LDL - "low density lipoprotein"
- HDL - "high density lipoprotein"
- Cholesterol Esterase - That's what it's important for
Functions of the cell membrane
As the complex structure of cell membranes suggests, they have to fulfill many different functions, which can vary greatly depending on the type and location of the cell. On the one hand, membranes generally represent a barrier. A function that should not be underestimated. Innumerable reactions take place in parallel in our body at any given time. If they all took place in the same room, they would strongly influence and even cancel each other out. A regulated process of metabolism would not be possible and humans as they exist and function as a whole would be inconceivable.
They also serve as a transport medium for a wide variety of substances that are transported across the membrane by means of transporters. In order to be able to work together as an organ, the individual cells must be in contact via their membranes. This is achieved through various connecting proteins and receptors. Cells can use the receptors to identify one another, communicate with one another and exchange information. E.g. the glycocalyx as one of the many distinguishing features between the body's own and foreign cells. Receptors are proteins that pick up signals from outside the cell and pass them on to the cell nucleus and thus the “brain” of the cell. Depending on the chemical properties of the chemical particle that has docked on the receptor, it is located either on the outside of the cell, in the cell or in the cell membrane.
But cells themselves can also convey information. The most famous of our bodies are the nerve cells. In order for them to perform their function, their membranes must be able to conduct electrical signals. Electrical signals arise due to different charges inside and outside the cells. This difference in charge, also known as the gradient, must be maintained. In this context, one speaks of a membrane potential. Cell membranes separate the differently charged areas from one another, but at the same time contain channels that allow a brief reversal of the charge ratios so that the actual current and thus the information that is to be transmitted can flow. This phenomenon is also called action potential.
Read more on the subject at: Nerve cell
Transport processes in the cell membrane
The cell membrane as such is impermeable to larger molecules and ions. So that an exchange between the cell interior and the environment can take place, there are proteins in the cell membrane that transport various molecules into and out of the cell.
With these proteins a distinction is made between channels through which a substance passes passively into or out of the cell along the difference in concentration. Other proteins have to generate energy to actively transport substances across the cell membrane.
Another important form of transport are the vesicles. Vesicles are small bubbles that are pinched off from the cell membrane. Substances that are produced in the cell can be released into the environment through these vesicles. In addition, substances can also be removed from the cell environment.
Differences to the cell membrane of bacteria - penicillin
The cell membrane of bacteria hardly differs from that of the human body. The big difference between cells lies in the additional cell wall of the bacteria. The cell wall attaches itself to the outside of the cell membrane and in this way stabilizes and protects the bacterium, which without it would be vulnerable. she is off Murein, a special sugar particle, into which other proteins can be incorporated, such as the Locomotion and reproduction serve. penicillin can disrupt the synthesis of the cell wall and thus works bactericidal, that is, it kills the bacterium. In this way, a targeted action against disease-causing bacteria is possible without destroying the body's own cells at the same time.