Every human and animal cell is enveloped by a semipermeable membrane. It protects the cell interior against harmful influences from the outside, and it is responsible for the necessary exchange of substances from the outside to the inside as well as from the inside to the outside. In a third function, the membrane takes over the communication between the cells, provided that the cell is located within a cell association.
What is a cell membrane?
The cell membrane surrounds every human and animal cell and separates it from other cells or from the extracellular space. It must be selectively permeable in both directions to allow needed substances into the cell or to carry degradation products out of the cell interior. If the cell is within a cell association, the membrane must be able to form some sort of mechanical bond with the membrane of the adjacent cell to provide the necessary strength to the cell association. In addition, the membrane must be able to communicate with the attached neighboring cells. It must be able to pass on “messages” from its cell to its neighboring cells as promptly as possible in a kind of intercellular communication, or to receive a message from the neighboring cells and pass it on to its own cell. To prevent the cell from being attacked by the body’s own defenses via an autoimmune reaction, the membrane must have features on the side facing the extracellular space that, as it were, identify it to the immune system as an endogenous cell.
Anatomy and structure
The cell membrane is composed of a double layer of lipids and reaches a thickness of only 6 to 10 nanometers. The lipophilic groups of the two lipid layers face each other, forming an insurmountable hydrophobic barrier for aqueous fluids. The lipids of the outer layer are partially glycolized, and saccharides may have attached and combined with the lipids to form glycolipids. The cell membranes are interspersed with so-called membrane proteins, which perform a variety of tasks. Glycoproteins are attached to the outward-facing surface of the membrane and serve, among other things, to identify the cell as endogenous to the immune system. Other proteins (integral proteins) penetrate the cell membrane and communicate with the extracellular and intracellular space. Another important structure is formed by the so-called ion channels, which are formed by channel proteins and enable certain substance exchanges. Specifically for the exchange with water to overcome the hydrophobic barrier between the two lipid layers of the cell membrane, so-called water channels (aquaporins) are present, which function roughly analogous to ion channels.
Function and Tasks
The cell membrane demarcates the interior of the cell from the exterior or from other cells and protects the nucleus, organelles, cytoplasm, and other parts located within the cell. Despite its semi-permeability, the membrane can separate the aqueous fluid inside the cell from the aqueous fluid outside the cell – even at different osmotic pressures. Another function and task is the selective exchange of substances between the cell interior and the extracellular space. The cell membrane has three different options available to it for this purpose:
- The first option is to use osmotic gradient.
- The second possibility is to use the ion and water channels that have formed in the cell membrane. Through various types of channels, ions can be transported along an electrical voltage gradient.
- However, there is also the possibility of so-called transport proteins ions under energy expenditure against the electrical voltage gradient or electrically neutral molecules to pass through.
Mass transport via ion channels works in both directions. For exchange with macromolecules that cannot be transported by either osmosis or ion channels, the cell membrane can form protrusions that can envelop the macromolecules and then transport them through the cell membrane to the interior of the cell. For cells that are not directly connected to nerves, communication with each other is important.Special proteins are responsible for this, which are anchored in the cell membrane and are connected to both the intracellular and the extracellular space (transmembrane proteins), so that information can be exchanged in both directions. Information exchange in the broader sense also includes the fact that the cell membrane signals to the immune system by means of docked peripheral proteins that it is an endogenous cell that must not be attacked.
Diseases and disorders
The regular functioning of the two basic functions of substance exchange and signal conduction of a cell membrane formed the prerequisite for the emergence of higher life. The effects can be correspondingly serious if only one basic function of the cell membrane is disturbed. Autoimmune diseases, which are triggered by a misguided immune system, can be causally related to a malfunction of the cell membranes of the affected tissue. In the case of a defect in the docked membrane proteins, the immune system may classify the cells not as the patient’s own tissue but as foreign tissue and initiate corresponding attacks. The autoimmune disease antiphospholipid syndrome (APS) leads to an altered composition of the cell membranes of red blood cells (erythrocytes) because the immune system leads to a destruction of membrane proteins associated with phopholipids. This strongly promotes coagulation, leading to an increased incidence of thrombosis, stroke, myocardial infarction and pulmonary embolism. Impaired intercellular communication can also lead to serious consequences. For example, if transmembrane proteins, transmit a “death command” to neighboring cancer cells, triggering their spontaneous cell death (apoptosis), are not taken up by the cancer cell due to a disruption in the communication mechanism, this means that tumor cells can develop unhindered. Amyloid deposits in the brains of Alzheimer’s patients are most likely caused by a certain membrane protein being broken down by the enzyme beta-secretase and thus rendered physiologically ineffective. This means that the disease is caused by a malfunction in the cell membrane.
