The plasma membrane is a lipid bilayer with proteins, ion channels and receptor molecules embedded in its thickness. This is a mechanical barrier that separates the cytoplasm of the cell from the extracellular space, while being the only link with the external environment. Therefore, plasmolemma is one of the most important cell structures, and its functions allow it to exist and interact with other cell groups.
Overview of Cytolemma Functions
The plasma membrane in the form in which it is present in the 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 organisms, the cytolemma is somewhat different, although its functions are still the same. They can be divided into three groups: delimiting, transport and communicative.
The group of demarcation 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 cytolemma include the receptor. On the surface of the membrane there is a totality of receptor complexes, through which the cell is involved 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
The barrier functions of the plasma membrane are multiple. It protects the internal environment of the cell with the established concentration of chemicals from its changes. In solutions, a diffusion process occurs, that is, an independent equalization of 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 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 against strong acidic and strong alkaline media. The plasmolemma is constructed in such a way that the hydrophobic ends of the lipid molecules are turned outward. Therefore, it often distinguishes between intracellular and extracellular environments with different pH values. This is necessary for cell 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. Moreover, it is believed that karyolemma is inextricably linked with the membrane of the endoplasmic reticulum. Therefore, the whole system is considered as one repository of hereditary information, a protein synthesizing system and a cluster of post-translational modification of protein molecules. A membrane of the endoplasmic reticulum is necessary to maintain the shape of the transport intracellular channels along which protein, lipid and carbohydrate molecules move.
The mitochondrial membrane protects the mitochondria, and the plastid membrane protects the chloroplasts. The lysosomal membrane also plays the role of a barrier: within the lysosome, an aggressive pH environment and reactive oxygen species can damage structures within the cell if they penetrate there. The membrane is a universal barrier, at the same time allowing lysosomes to "digest" solid particles and limiting the site of action of enzymes.
The mechanical function of the plasma membrane
The mechanical functions of the plasma membrane are also heterogeneous. First, plasmolemma supports the cellular form. Secondly, it limits the deformability of the cell, but does not prevent the change in shape and fluidity. At the same time, membrane strengthening 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 glycocalyx.
In bacteria, it is glycoprotein, in plants - cellulose, in fungi - chitin. Diatoms and completely embed silica (silicon oxide) in their cell wall, which significantly increases the strength and mechanical resistance of the cell. Moreover, every body needs a cell wall just for this. And the plasmolemma itself has a much lower strength than the layer of proteoglycans, cellulose or chitin. There is no doubt that cytolemma plays a mechanical role.
The mechanical functions of the plasma membrane also allow mitochondria, chloroplasts, lysosomes, the nucleus and the endoplasmic reticulum to function inside the cell and protect themselves from subthreshold damage. This is characteristic of any cell having 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 a 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 include transport and reception. Both of these qualities are characteristic of the plasma membrane and karyolemma. The membrane of organelles does not always have receptors or is penetrated by transport channels, and karyolemma and cytolemma have these formations. It is through them that these communicative functions are implemented.
Transport is realized by two possible mechanisms: with the consumption of energy, that is, in an active way, and without the costs, by simple diffusion. However, the cell can transport substances through phagocytosis or pinocytosis. This is realized by trapping a cloud of liquid or solid particle by protrusions of the cytoplasm. Then the cell, as if by hands, captures a particle or a drop of liquid, drawing it inward and forming a cytoplasmic layer around it.
Active transport, diffusion
Active transport is an example of the selective absorption of electrolytes or nutrients. Through specific channels, represented by protein molecules consisting of several subunits, a substance or hydrated ion penetrates 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.
Lipid solubility
Highly differentiated cells, for example, nerve, endocrine or muscle, use these ion channels to generate resting and acting potentials. It is formed due to the osmotic and electrochemical differences, and tissues receive the ability to contract, generate or conduct momentum, 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 for the regulation of vital functions, protection and movement of the whole organism.
Some substances can even penetrate through the membrane, but this is typical only for lipophilic fat-soluble molecules. They simply dissolve in the bilayer of the membrane, easily entering the cytoplasm. Such a transport mechanism 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 of receptor (integral) molecules on the surface of the plasma membrane. 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 the functions of the cytoplasmic membrane
The most obvious way to highlight the functions of the plasma membrane is the table, which indicates its biological role for the cell as a whole.
Structure | Function | Biological role |
Cytoplasmic membrane in the form of a lipid bilayer with outwardly hydrophobic ends equipped with receptor complexes of integral and surface proteins | Mechanical | It supports the cellular form, protects against mechanical subthreshold effects, maintains cellular integrity |
Transport | Carries droplets of liquid, solid particles, macromolecules and hydrated ions into the cell with or without energy |
Receptor | Has on its surface receptor molecules that serve to transfer information to the nucleus |
Adhesive | Due to protrusions of the cytoplasm, neighboring cells form contacts between themselves |
Electrogenic | Provides conditions for the generation of action potential and resting 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 expressed as a diagram.
The functions of the plasma membrane: scheme
The following organelles are distinguished by the presence of membranes in the cell: 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 on the example of a tabular diagram.
Organelle and membrane | Function | Biological role |
Core, nuclear membrane | Mechanical | The mechanical functions of the plasma membrane of the cytoplasm of the nucleus allow it to maintain its shape and prevent the appearance of structural damage |
Barrier | Separation of nucleoplasm and cytoplasm |
Transport | It has transport pores for the exit of ribosomes and messenger RNA from the nucleus and the ingestion of nutrients, amino acids and nitrogen bases |
Mitochondria, mitochondrial membrane | Mechanical | Maintaining mitochondria, obstructing mechanical damage |
Transport | Ions and energy substrates are transmitted through the membrane |
Electrogenic | Provides the generation of transmembrane potential, which underlies the generation of energy in the cell |
Chloroplasts, plastid membrane | Mechanical | Maintains the shape of plastids, prevents their mechanical damage |
Transport | Provides transport of substances |
Endoplasmic reticulum, network membrane | Mechanical and environment-forming | Provides a cavity where the processes of protein synthesis and their post-translational modification |
Golgi apparatus, membrane of vesicles and tanks | 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 in 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 each organelle. Moreover, a number of functions should be combined into one - in the protective. In particular, the 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-integral and surface proteins. 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 attributed to epithelial, connective or excitable tissue.