The plasma membrane is a lipid bilayer with proteins, ion channels and receptor molecules built into its thickness. This is a mechanical barrier that separates the cytoplasm of the cell from the pericellular space, at the same time being the only connection with 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 cell groups.
Overview of the functions of the cytolemma
The plasma membrane in the form in which it is present in an animal cell is characteristic of many organisms from different kingdoms. Bacteria and protozoa, whose organisms are represented by a single cell, have a cytoplasmic membrane. And animals, fungi and plants as multicellular organisms have not lost it in the process of evolution. However, in different kingdoms of living organismsthe cytolemma is somewhat different, 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, protection from the extracellular environment. The membrane plays a transport group of functions due to the presence of specific proteins, ion channels and carriers of certain substances. The communicative functions of the cytolemma include the receptor function. 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, she 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. Diffusion occurs in solutions, that is, self-equalization of concentration between media with different contents of certain substances in them. The plasmalemma just blocks diffusion by preventing the flow of liquid and ions in any direction. Thus, the membrane limits the cytoplasm with a certain concentration of electrolytes from the pericellular environment.
The second manifestation of the barrier function of the plasma membrane is protection from strong acidic and strong alkaline environments. Plasma membrane builtso that the hydrophobic ends of the lipid molecules are facing outward. Therefore, it often distinguishes between intracellular and extracellular environments with different pH values. It is essential for cellular life.
Barrier function of organelle membranes
The barrier functions of the plasma membrane are also different because they depend on its location. 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. Moreover, it is believed that the karyolemma is inextricably linked with the membrane of the endoplasmic reticulum. Therefore, the whole system is considered as a single repository of hereditary information, a protein synthesizing system and a cluster of post-translational modification of protein molecules. The membrane of the endoplasmic reticulum is necessary to maintain the shape of intracellular transport channels through which protein, lipid and carbohydrate molecules move.
The mitochondrial membrane protects the mitochondria, while the plastid membrane protects the chloroplasts. The lysosomal membrane also plays the role of a barrier: inside the lysosome there is an aggressive pH environment and reactive oxygen species that can damage the structures inside the cell if they penetrate there. The membrane, on the other hand, is a universal barrier, both allowing lysosomes to "digest" solid particles and limiting the site of action of enzymes.
Mechanical function of the plasma membrane
Mechanical functions of the plasma membrane are also heterogeneous. First, the plasma membrane supportscellular form. Secondly, it limits the deformability of the cell, but does not prevent the change in shape and fluidity. In this case, the strengthening of the membrane is also possible. This occurs due to the formation of the cell wall by protists, bacteria, plants and fungi. In animals, including the human species, the cell wall is the simplest and is represented only by the glycocalyx.
In bacteria it is glycoprotein, in plants it is cellulose, in fungi it is chitinous. Diatoms even incorporate silica (silicon oxide) into their cell wall, which significantly increases the strength and mechanical resistance of the cell. And every organism needs a cell wall for this. And the plasmalemma itself has a much lower strength than a layer of proteoglycans, cellulose or chitin. There is no doubt that the cytolemma plays a mechanical role.
Also, the mechanical functions of the plasma membrane allow mitochondria, chloroplasts, lysosomes, the nucleus and the endoplasmic reticulum to function inside the cell and protect themselves from subthreshold damage. 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 implementation 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
Transport and reception are among the communicative functions. Theseboth qualities are characteristic of the plasma membrane and karyolemma. The membrane of organelles does not always have receptors or is permeated with transport channels, but the karyolemma and cytolemma have these formations. It is through them that these communicative functions are implemented.
Transport is implemented by two possible mechanisms: with the expenditure of energy, that is, in an active way, and without expenditure, by simple diffusion. However, the cell can also transport substances by phagocytosis or pinocytosis. This is realized by capturing a cloud of liquid or solid particles by protrusions of the cytoplasm. Then the cell, as if with its hands, captures a particle or a drop of liquid, drawing it in and forming a cytoplasmic layer around it.
Active transport, diffusion
Active transport is an example of selective uptake of electrolytes or nutrients. Through specific channels represented by protein molecules consisting of several subunits, a substance or a hydrated ion penetrates into the cytoplasm. Ions change potentials, and nutrients are built into metabolic circuits. And all these functions of the plasma membrane in the cell actively contribute to its growth and development.
Lipid solubility
Highly differentiated cells such as nerve, endocrine or muscle cells use these ion channels to generate resting and action potentials. It is formed due to the osmotic and electrochemical difference, and the tissues gain the ability to contract,generate or conduct an 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 an animal cell provide the regulation of vital activity, protection and movement of the whole organism.
Some substances can even penetrate the membrane, but this is typical only for molecules of lipophilic fat-soluble molecules. They simply dissolve in the bilayer of the membrane, easily entering the cytoplasm. This transport mechanism is typical for 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 of receptor (integral) molecules on the surface of the plasmalemma. 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 functions of the cytoplasmic membrane
The most visual way to highlight the functions of the plasma membrane is a table that indicates its biological role for the cell as a whole.
| Structure | Function | Biological role |
| Cytoplasmic membrane in the form of a lipid bilayer withhydrophobic ends located outward, equipped with receptor complexes of integral and surface proteins | Mechanical | Maintains cellular shape, protects against mechanical subthreshold effects, preserves cellular integrity |
| Transportation | Transports liquid droplets, solid particles, macromolecules and hydrated ions into the cell with or without energy expenditure | |
| Receptor | It has receptor molecules on its surface that serve to transmit information to the nucleus | |
| Adhesive | Due to protrusions of the cytoplasm, neighboring cells form contacts with each other | |
| Electrogenic | Provides conditions for generation of action potential and rest potential of excitable tissues |
This table clearly shows what functions the plasma membrane performs. However, only the cell membrane, that is, the lipid bilayer surrounding the entire cell, plays these roles. Inside it there are organelles, which also have membranes. Their roles should be outlined.
Functions of the plasma membrane: diagram
The following organelles differ in the presence of membranes in the cell: nucleus, rough and smooth endoplasmic reticulum, Golgi complex, mitochondria, chloroplasts, lysosomes. In each ofthese organelles, the membrane plays a crucial role. You can consider it using the example of a tabular scheme.
| Organella and membrane | Function | Biological role |
| Nucleus, nuclear membrane | Mechanical | Mechanical functions of the plasma membrane of the cytoplasm of the nucleus allow it to maintain its shape, prevent the appearance of structural damage |
| Barrier | Separation of nucleoplasm and cytoplasm | |
| Transportation | Has transport pores for the exit of ribosomes and messenger RNA from the nucleus and the entry of nutrients, amino acids and nitrogenous bases into the interior | |
| Mitochondrion, mitochondrial membrane | Mechanical | Maintaining the shape of the mitochondria, preventing mechanical damage |
| Transportation | Ions and energy substrates are transferred through the membrane | |
| Electrogenic | Provides the generation of transmembrane potential, which is the basis of energy production in the cell | |
| Chloroplasts, plastid membrane | Mechanical | Supports the shape of plastids, prevents their mechanical damage |
| Transportation | Provides transport of substances | |
| Endoplasmic reticulum, membrane of the network | Mechanical and environment-forming | Provides the presence of a cavity where the processes of protein synthesis and their post-translational modification take place |
| Golgi apparatus, membrane of vesicles and cisterns | Mechanical and environment-forming | Role see above |
| Lysosomes, lysosomal membrane |
Mechanical Barrier |
Maintaining the shape of the lysosome, preventing mechanical damage and the release of enzymes into the cytoplasm, limiting it from lytic complexes |
Animal cell membranes
These are the functions of the plasma membrane in the cell, where it plays an important role for every organelle. Moreover, a number of functions should be combined into one - into a protective one. In particular, the barrier and mechanical functions are combined into a protective one. Moreover, the functions of the plasma membrane in a plant cell are almost identical to those in an animal and bacterial cell.
The animal cell is the most complex and highly differentiated. Much more integral, semi-integral and surface proteins are located here. In general, in multicellular organisms, the membrane structure is always more complex than in unicellular ones. And what functions the plasma membrane of a particular cell performs determines whether it will be classified as epithelial, connective orexcitable tissue.
