Cytoplasmic membrane: functions, structure. External cytoplasmic membrane
The outer cytoplasmic membrane is a thin film. Its thickness is about 7-10 nm. The film is viewed only in an electron microscope.Next, consider what constitutes a cytoplasmic membrane. Functions of the film will also be described in the article.
What is the composition of the cytoplasmic membrane? The structure of the film is quite diverse. According to the chemical organization, it is a complex of proteins and lipids. The cytoplasmic membrane of the cell includes a bilayer. It acts as a basis. In addition, the cytoplasmic membrane contains cholesterol and glycolipids. These substances are peculiar to amphipatricity. In other words, they contain hydrophobic (“moisture-wary”) and hydrophilic (“water-loving”) ends.The latter (phosphate group) are directed outward from the membrane, the latter (residues from fatty acids) are oriented towards each other. Due to this, the lipid bipolar layer is formed. Lipid molecules have mobility. They are able to move in their own monolayer or (rarely) from one to another.The lipid layer may have a solid or liquid crystal state. Monolayers are asymmetric. This means that the composition of lipids is different. Due to this property, cytoplasmic membranes have specificity even within a single cell. The second required component of the film include proteins. Many of these compounds can move in the membrane plane or make rotations around its own axis. However, they are not able to move from one part of the bilayer to another. Protection of the internal environment - the main task that the cytoplasmic membrane performs. The structure of the film, in addition, ensures the flow of various processes. For the performance of certain tasks are proteins. Due to lipids, the structural features of the film are provided.
Cytoplasmic membrane: functions
The main tasks are:
- Barrier. Protective film provides an active, passive, selective, regulated exchange of compounds with the external environment. Due to selective permeability, the cell and its compartments are separated and supplied with the necessary substances.
- Transport. Through the film is the transition of compounds from cell to cell. Thanks to this, nutrient compounds are delivered, the end products of metabolism are removed, secretion of various substances occurs. In addition, ionic gradients are formed, the ion concentration and pH are maintained at the optimal level. They are necessary for the vigorous activity of cell enzymes.
- Matrix. This function provides a certain orientation and interposition of membrane proteins, as well as their optimal interaction.
- Mechanical. Due to it, the autonomy of the cell, internal structures is ensured. An element is also connected with other similar ones.
- Energy. Against the background of photosynthesis in chloroplasts and during cell respiration in membranes, energy transfer systems are active.Protein compounds are also involved.
- The receptor. A number of proteins that are present in the membrane, provides the perception of various signals. For example, steroids circulating in the blood affect only those target cells that have the appropriate hormone receptors. Chemical compounds that provide impulses (neurotransmitters) are also associated with specific target cell proteins.
The specific functions of the membrane include:
- Enzymatic. Often, proteins that contain the cytoplasmic membrane act as enzymes.
- Generation and implementation of biopotentials.
- Marking. The cytoplasmic membrane includes special antigens. They act as shortcuts. Thanks to them, recognition of cells is carried out. Markers are glycoproteins - proteins containing branched oligosaccharide side chains. They act as "antennas".Due to the huge number of variants of the side chains for one or another type of cells a special marker can be formed.With their help, the elements recognized by each other begin to act in concert. For example, this happens during the formation of tissues and organs. The labeling also allows the immune system to identify foreign antigens.
If some particles are not able to pass through a phospholipid bilayer for one reason or another (for example, due to hydrophilic properties, because the cytoplasmic membrane is hydrophobic inside and does not let such compounds pass, or because of the large size of the particles themselves), but they are necessary, then they can pass through special carrier proteins (transporters) and channel proteins. Or their penetration is carried out through endocytosis.In the process of passive transport, the intersection of substances by the lipid layer occurs by diffusion. At the same time energy is not spent. One of the variants of such a mechanism can be facilitated diffusion. In the course of it, any specific molecule facilitates the passage of a substance. It may have a channel capable of passing only particles of the same type. With active transport energy is expended.This is due to the fact that this process is carried out against the concentration gradient. The cytoplasmic membrane contains specific protein pumps, including ATPase, which promotes the active entry of potassium and the elimination of sodium ions.
There are several of them:
- "Sandwich model". The idea of a three-layer structure of all membranes was expressed by scientists Dawson and Daniel in 1935. In their opinion, the structure of the film was as follows: proteins-lipids-proteins. Such a view has existed for a long time.
- "Liquid mosaic structure".This model was described by Nicholson and Singer in 1972. In accordance with it, protein molecules do not form a continuous layer, but are immersed in a bipolar lipid in the form of a mosaic at a different depth. This model is considered the most universal.
- "Protein-crystal structure".In accordance with this model, membranes are formed due to the intertwining of protein and lipid molecules, which are combined on the basis of hydrophilic-hydrophobic bonds.