Functions of the plasma membrane in the cell

The plasma membrane is a lipid bilayer with proteins, ion channels and receptor molecules built into it. This is a mechanical barrier that separates the cytoplasm of the cell from the surrounding space, while being the only link to the external environment. Therefore, the plasmolemma is one of the most important structures of the cell, and its functions allow it to exist and interact with other cellular groups.

A general idea of the functions of the cytolemma

Plasma membrane in the form in which it is present in the animal cell, is characteristic of a variety of organisms from different kingdoms. Bacteria and protozoa, whose organisms are represented by a single cell, have a cytoplasmic membrane. And animals, mushrooms and plants as multicellular organisms have not lost it in the process of evolution. However, in different realms of living organisms the cytolemma differs somewhat, although its functions are still the same. They can be divided into three groups: delimiting, transport and communication.

The group of delimiting functions includes mechanical protection of the cell, maintaining its shape, protecting from the extracellular environment. The transport group functions as a membrane due to the presence of specific proteins, ion channels and carriers of certain substances. To the communicative functions of the cytolemma should be attributed receptor. On the surface of the membrane there is a set of receptor complexes through which the cell participates in the mechanisms of humoral information transfer. However, it is also important that the plasmolemma surrounds not only the cell, but also some of its membrane organelles. In them, it plays the same role as in the case of the whole cell.

Barrier function

Barrier functions of the plasma membrane are multiple. It protects the internal environment of the cell with the prevailing concentration of chemicals from its change. In solutions, a process of diffusion occurs, that is, an independent equalization of the concentration between media with different contents of certain substances in them. Plasmolemma just blocks diffusion by preventing the flow of liquid and ions in any directions. Thus, the membrane limits the cytoplasm with a certain concentration of electrolytes from the surrounding cell environment.

The second manifestation of the barrier function of the plasma membrane is protection from strong acidic and strong alkaline media. The plasmolemma is constructed in such a way that the hydrophobic ends of the lipid molecules face outward. Therefore, it often delimits intracellular and extracellular media with different pH values. This is necessary for cellular life.

Barrier function of membranes of organelles

Barrier functions of the plasma membrane are different, and because they depend on the location of the plasma membrane. In particular, the karyolemma, that is, the lipid bilayer of the nucleus, protects it from mechanical damage and separates the nuclear environment from the cytoplasmic one. And it is believed that the karyoolma is inextricably linked with the membrane of the endoplasmic reticulum. Therefore, the whole system is considered one as a storehouse of hereditary information, a protein synthesizing system and a cluster of post-translational modification of protein molecules. The membrane of endoplasmic nets is necessary to maintain the shape of the transport intracellular channels through which the protein, lipid and carbohydrate molecules move.

The mitochondrial membrane protects the mitochondria, and the plastid membrane protects the chloroplasts. The lysosomal membrane also acts as a barrier: inside the lysosome is an aggressive pH medium and reactive oxygen species that can damage the structures inside the cell if they get there. The membrane is a universal barrier, simultaneously allowing lysosomes to "digest" solid particles and limiting the site of action of enzymes.

The mechanical function of the plasmolemma

The mechanical functions of the plasma membrane are also heterogeneous. First, the plasmolemma supports the cellular form. Secondly, it limits the deformability of the cell, but does not interfere with the change in shape and fluidity. At the same time, the strengthening of the membrane is also possible. This is due to the formation of the cell wall protista, bacteria, plants and fungi. In animals, including the human species, the cell wall is the simplest and is represented only by the glycocalysis.

In bacteria it is glycoprotein, in plants it is cellulose, in mushrooms it is chitinous. Diatom algae completely incorporate silica (silicon oxide) into their cell walls, which significantly increases the strength and mechanical stability of the cell. And every body needs a cell wall for this. And the plasmolemma itself has much less strength than a layer of proteoglycans, cellulose or chitin. The fact that the cytolemma plays a mechanical role can not be doubted.

Also, the mechanical functions of the plasma membrane allow the mitochondria, chloroplasts, lysosomes, nucleus and endoplasmic reticulum to function within the cell and be protected from subthreshold lesions. This is typical for any cell that has these membrane organelles. Moreover, the plasma membrane has cytoplasmic outgrowths, through which intercellular contacts are created. This is an example of the realization of the mechanical function of the plasma membrane. The protective role of the membrane is also ensured by the natural resistance and fluidity of the lipid bilayer.

Communicative function of the cytoplasmic membrane

Among the communicative functions is transport and reception. These two qualities are typical for the plasma membrane and karyolemma. The membrane of the organelles does not always have receptors or is permeated with transport channels, while in karyoloma and cytolemma these formations are present. It is through their implementation of these communicative functions.

Transport is realized by two possible mechanisms: with energy expenditure, that is, in an active way, and without costs, by simple diffusion. However, the cell can transport substances and by phagocytosis or pinocytosis. This is realized by capturing a cloud of liquid or solid particles with protrusions of the cytoplasm. Then the cell as if by hands grasps a particle or a drop of liquid, pulling it inside and forming a cytoplasmic layer around it.

Active transport, diffusion

Active transport is an example of selective absorption of electrolytes or nutrients. By means of specific channels represented by protein molecules consisting of several subunits, the substance or hydrated ion penetrates into the cytoplasm. Ions change potentials, and nutrients are built into metabolic chains. And all these functions of the plasma membrane in the cell actively contribute to its growth and development.

Lipidosolubility

Highly differentiated cells, for example, nervous, endocrine or muscle, use these ion channels to generate rest and action potentials. It is formed due to the osmotic and electrochemical difference, and the tissues are able to contract, generate or impulse, respond to signals or transmit them. This is an important mechanism for the exchange of information between cells, which underlies the nervous regulation of the functions of the whole organism. These functions of the plasma membrane of the animal cell provide the regulation of vital activity, protection and movement of the whole organism.

Some substances can penetrate the membrane at all, but this is characteristic only of molecules of lipophilic fat-soluble molecules. They simply dissolve into the bilayer of the membrane, easily falling into the cytoplasm. This mechanism of transport is characteristic of steroid hormones. And the hormones of the peptide structure are unable to penetrate the membrane, although they also transmit information to the cell. This is achieved due to the presence on the surface of the plasmolemma of receptor (integral) molecules. The associated biochemical mechanisms of signal transmission to the nucleus together with the mechanism of direct penetration of lipid substances through the membrane constitute a simpler system of humoral regulation. And all these functions of the integral proteins of the plasma membrane are needed not only by one cell, but by the whole organism.

Table of cytoplasmic membrane functions

The most obvious way to isolate the functions of the plasma membrane is the table, which indicates its biological role for the cell as a whole.

This table clearly shows what functions the plasma membrane performs. However, these roles are played only by the cell membrane, that is, the lipid bilayer that surrounds the whole cell. Inside it there are also organelles, which also have membranes. Their roles should be expressed in the form of a scheme.

Functions of the plasma membrane: scheme

In the cell, the presence of membranes is characterized by the following organelles: nucleus, rough and smooth endoplasmic reticulum, Golgi complex, mitochondria, chloroplasts, lysosomes. In each of these organelles, the membrane plays a crucial role. You can consider it using the example of a table scheme.

Membranes of an animal cell

Such are the functions of the plasma membrane in the cell, where it plays an important role for each organelle. And a number of functions should be combined into one - into a protective one. In particular, barrier and mechanical functions are combined into a protective one. Moreover, the functions of the plasma membrane in the plant cell are almost identical to those in the animal and bacterial.

The animal cell is the most complex and highly differentiated. There are much more integral, semi-integrated and surface proteins here. In general, in multicellular organisms, the membrane structure is always more complex than in unicellular organisms. And what functions the plasma membrane of a particular cell performs, determines whether it is related to epithelial, connective or excitable tissue.