Based on its functional characteristics, the cell membrane can be divided into 9 functions it performs.
Functions of the cell membrane:
1. Transport. Transports substances from cell to cell;
2. Barrier. Has selective permeability, ensures the necessary metabolism;
3. Receptor. Some proteins found in the membrane are receptors;
4. Mechanical. Ensures the autonomy of the cell and its mechanical structures;
5. Matrix. Ensures optimal interaction and orientation of matrix proteins;
6. Energy. Membranes contain energy transfer systems during cellular respiration in mitochondria;
7. Enzymatic. Membrane proteins are sometimes enzymes. For example, intestinal cell membranes;
8. Marking. The membrane contains antigens (glycoproteins) that allow cell identification;
9. Generating. Carries out the generation and conduction of biopotentials.

You can see what a cell membrane looks like using the example of the structure of an animal cell or plant cell.

The figure shows the structure of the cell membrane.
The components of the cell membrane include various cell membrane proteins (globular, peripheral, surface), as well as cell membrane lipids (glycolipid, phospholipid). Also in the structure of the cell membrane there are carbohydrates, cholesterol, glycoprotein and protein alpha helix.

Cell membrane composition

The main composition of the cell membrane includes:
1. Proteins - responsible for various properties of the membrane;
2. Three types of lipids (phospholipids, glycolipids and cholesterol) responsible for membrane rigidity.
Cell membrane proteins:
1. Globular protein;
2. Surface protein;
3. Peripheral protein.

The main purpose of the cell membrane

The main purpose of the cell membrane:
1. Regulate the exchange between the cell and the environment;
2. Separate the contents of any cell from external environment thereby ensuring its integrity;
3. Intracellular membranes divide the cell into specialized closed compartments - organelles or compartments in which certain environmental conditions are maintained.

Cell membrane structure

The structure of the cell membrane is a two-dimensional solution of globular integral proteins dissolved in a liquid phospholipid matrix. This model of membrane structure was proposed by two scientists Nicholson and Singer in 1972. Thus, the basis of the membranes is a bimolecular lipid layer, with an ordered arrangement of molecules, as you could see in.

Biological membranes.
The term “membrane” (Latin membrana - skin, film) began to be used more than 100 years ago to designate a cell boundary that serves, on the one hand, as a barrier between the contents of the cell and the external environment, and on the other, as a semi-permeable partition through which water can pass. and some substances. However, the functions of the membrane are not limited to this, since biological membranes form the basis of the structural organization of the cell.
Membrane structure. According to this model, the main membrane is a lipid bilayer in which the hydrophobic tails of the molecules face inward and the hydrophilic heads face outward. Lipids are represented by phospholipids - derivatives of glycerol or sphingosine. Proteins are associated with the lipid layer. Integral (transmembrane) proteins penetrate the membrane through and are firmly associated with it; peripheral ones do not penetrate and are less firmly connected to the membrane. Functions of membrane proteins: maintaining membrane structure, receiving and converting signals from the environment. environment, transport of certain substances, catalysis of reactions occurring on membranes. The membrane thickness ranges from 6 to 10 nm.

Membrane properties:
1. Fluidity. The membrane is not a rigid structure; most of its constituent proteins and lipids can move in the plane of the membrane.
2. Asymmetry. The composition of the outer and inner layers of both proteins and lipids is different. In addition, the plasma membranes of animal cells have a layer of glycoproteins on the outside (glycocalyx, which performs signaling and receptor functions, and is also important for uniting cells into tissues)
3. Polarity. The outer side of the membrane carries a positive charge, while the inner side carries a negative charge.
4. Selective permeability. The membranes of living cells, in addition to water, allow only certain molecules and ions of dissolved substances to pass through. (The use of the term “semi-permeability” in relation to cell membranes is not entirely correct, since this concept implies that the membrane allows only solvent molecules to pass through, while retaining all molecules and ions of dissolved substances.)

The outer cell membrane (plasmalemma) is an ultramicroscopic film 7.5 nm thick, consisting of proteins, phospholipids and water. An elastic film that is well wetted by water and quickly restores its integrity after damage. It has a universal structure, typical of all biological membranes. The borderline position of this membrane, its participation in the processes of selective permeability, pinocytosis, phagocytosis, excretion of excretory products and synthesis, in interaction with neighboring cells and protection of the cell from damage makes its role extremely important. Animal cells outside the membrane are sometimes covered with a thin layer consisting of polysaccharides and proteins - the glycocalyx. In plant cells, outside the cell membrane there is a strong cell wall that creates external support and maintains the shape of the cell. It consists of fiber (cellulose), a water-insoluble polysaccharide.

In 1972, a theory was put forward according to which a partially permeable membrane surrounds the cell and performs a number of vital functions. important tasks, and the structure and function of cell membranes are significant issues regarding the proper functioning of all cells in the body. became widespread in the 17th century, along with the invention of the microscope. It became known that plant and animal tissues consist of cells, but due to the low resolution of the device, it was impossible to see any barriers around the animal cell. In the 20th century, the chemical nature of the membrane was studied in more detail, and it was found that it is based on lipids.

Structure and functions of cell membranes

The cell membrane surrounds the cytoplasm of living cells, physically separating intracellular components from the external environment. Fungi, bacteria and plants also have cell walls that provide protection and prevent the passage of large molecules. Cell membranes also play a role in the formation of the cytoskeleton and the attachment of other vital particles to the extracellular matrix. This is necessary in order to hold them together, forming the tissues and organs of the body. Features of the structure of the cell membrane include permeability. The main function is protection. The membrane consists of a phospholipid layer with embedded proteins. This part is involved in processes such as cell adhesion, ionic conductance and signaling systems and serves as an attachment surface for several extracellular structures, including the wall, glycocalyx and internal cytoskeleton. The membrane also maintains cell potential by acting as a selective filter. It is selectively permeable to ions and organic molecules and controls the movement of particles.

Biological mechanisms involving the cell membrane

1. Passive diffusion: Some substances (small molecules, ions), such as carbon dioxide (CO2) and oxygen (O2), can penetrate the plasma membrane by diffusion. The shell acts as a barrier for certain molecules and ions, they can concentrate on either side.

2. Transmembrane protein of channels and transporters: nutrients, such as glucose or amino acids, must enter the cell, and some metabolic products must leave it.

3. Endocytosis is the process by which molecules are taken up. A slight deformation (invagination) is created in the plasma membrane in which the substance to be transported is ingested. This requires energy and is thus a form of active transport.

4. Exocytosis: Occurs in various cells to remove undigested remains of substances brought by endocytosis to secrete substances such as hormones and enzymes and transport the substance completely across the cell barrier.

Molecular structure

The cell membrane is a biological membrane consisting primarily of phospholipids and separating the contents of the entire cell from the external environment. The formation process occurs spontaneously when normal conditions. To understand this process and correctly describe the structure and functions of cell membranes, as well as properties, it is necessary to evaluate the nature of phospholipid structures, which are characterized by structural polarization. When phospholipids are aquatic environment cytoplasm reaches a critical concentration, they combine into micelles, which are more stable in an aqueous environment.

Membrane properties

• Stability. This means that once formed, membrane disintegration is unlikely.
• Strength. The lipid shell is reliable enough to prevent the passage of a polar substance; both solutes (ions, glucose, amino acids) and much larger molecules (proteins) cannot pass through the formed boundary.
• Dynamic character. This is perhaps the most important property when considering the structure of the cell. The cell membrane can undergo various deformations, can fold and bend without being destroyed. Under special circumstances, for example, during vesicle fusion or budding, it can be disrupted, but only temporarily. At room temperature its lipid components are in constant, chaotic movement, forming a stable fluid boundary.

Liquid mosaic model

Speaking about the structure and functions of cell membranes, it is important to note that in modern idea the membrane as a liquid mosaic model was considered in 1972 by scientists Singer and Nicholson. Their theory reflects three main features of the membrane structure. Integrals promote a mosaic pattern for the membrane, and they are capable of lateral in-plane movement due to the variable nature of lipid organization. Transmembrane proteins are also potentially mobile. Important feature The structure of the membrane is its asymmetry. What is the structure of a cell? Cell membrane, nucleus, proteins and so on. The cell is the basic unit of life, and all organisms are composed of one or many cells, each of which has a natural barrier separating it from its environment. This outer boundary of the cell is also called the plasma membrane. It is made up of four different types of molecules: phospholipids, cholesterol, proteins and carbohydrates. The liquid mosaic model describes the structure of the cell membrane as follows: flexible and elastic, with a consistency similar to vegetable oil, so all the individual molecules are just floating in a liquid environment, and they are all capable of moving laterally within this shell. A mosaic is something that contains many different pieces. In the plasma membrane it is represented by phospholipids, cholesterol molecules, proteins and carbohydrates.

Phospholipids

Phospholipids constitute the main structure of the cell membrane. These molecules have two different ends: a head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it is attracted to water molecules. The tail is made up of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic; they do not like to mix with water molecules. This process is similar to what happens when you pour vegetable oil into water, that is, it does not dissolve in it. The structural features of the cell membrane are associated with the so-called lipid bilayer, which consists of phospholipids. Hydrophilic phosphate heads are always located where there is water in the form of intracellular and extracellular fluid. The hydrophobic tails of phospholipids in the membrane are organized in such a way that they keep them away from water.

Cholesterol, proteins and carbohydrates

When people hear the word cholesterol, they usually think it's bad. However, cholesterol is actually a very important component of cell membranes. Its molecules consist of four hydrogen rings and carbon atoms. They are hydrophobic and occur among the hydrophobic tails in the lipid bilayer. Their importance lies in maintaining consistency, they strengthen the membranes, preventing crossing. Cholesterol molecules also keep the phospholipid tails from coming into contact and hardening. This ensures fluidity and flexibility. Membrane proteins function as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport substances across the cell membrane.

Carbohydrates, or saccharides, are found only on the extracellular side of the cell membrane. Together they form the glycocalyx. It provides cushioning and protection to the plasma membrane. Based on the structure and type of carbohydrates in the glycocalyx, the body can recognize cells and determine whether they should be there or not.

Membrane proteins

The structure of a cell membrane cannot be imagined without such an important component as protein. Despite this, they can be significantly smaller in size than another important component - lipids. There are three types of major membrane proteins.

• Integral. They completely cover the bilayer, cytoplasm and extracellular environment. They perform transport and signaling functions.
• Peripheral. Proteins are attached to the membrane by electrostatic or hydrogen bonds at their cytoplasmic or extracellular surfaces. They are involved mainly as a means of attachment for integral proteins.
• Transmembrane. They perform enzymatic and signaling functions, and also modulate the basic structure of the lipid bilayer of the membrane.

Functions of biological membranes

The hydrophobic effect, which regulates the behavior of hydrocarbons in water, controls the structures formed by membrane lipids and membrane proteins. Many membrane properties are conferred by the carrier lipid bilayers, which form the basic structure for all biological membranes. Integral membrane proteins are partially hidden in the lipid bilayer. Transmembrane proteins have a specialized organization of amino acids in their primary sequence.

Peripheral membrane proteins are very similar to soluble proteins, but they are also membrane bound. Specialized cell membranes have specialized cell functions. How do the structure and functions of cell membranes affect the body? The functionality of the entire organism depends on how biological membranes are structured. From intracellular organelles, extracellular and intercellular membrane interactions, the structures necessary for organization and execution are created biological functions. Many structural and functional features are common to bacteria and enveloped viruses. All biological membranes are built on a lipid bilayer, which gives rise to a number of general characteristics. Membrane proteins have many specific functions.

• Controlling. Plasma membranes of cells determine the boundaries of interaction between the cell and the environment.
• Transport. The intracellular membranes of cells are divided into several functional units with different internal compositions, each of which is supported by the necessary transport function in combination with permeability control.
• Signal transduction. Membrane fusion provides a mechanism for intracellular vesicular signaling and preventing various types of viruses from freely entering the cell.

Significance and conclusions

The structure of the outer cell membrane affects the entire body. It plays an important role in protecting the integrity by allowing only selected substances to penetrate. It is also a good base for the attachment of the cytoskeleton and cell wall, which helps in maintaining the shape of the cell. Lipids make up about 50% of the membrane mass of most cells, although this varies depending on the type of membrane. The structure of the outer cell membrane of mammals is more complex, containing four main phospholipids. An important property of lipid bilayers is that they behave as two-dimensional liquids in which individual molecules can freely rotate and move laterally. Such fluidity is an important property of membranes, which is determined depending on temperature and lipid composition. Due to its hydrocarbon ring structure, cholesterol plays a role in determining membrane fluidity. biological membranes for small molecules allows the cell to control and maintain its internal structure.

Considering the structure of the cell (cell membrane, nucleus, and so on), we can conclude that the body is a self-regulating system that, without outside help, cannot harm itself and will always look for ways to restore, protect and properly function each cell.

The cell membrane has a rather complex structure, which can be viewed with an electron microscope. Roughly speaking, it consists of a double layer of lipids (fats), in which various peptides (proteins) are embedded in different places. The total thickness of the membrane is about 5-10 nm.

The general structure of the cell membrane is universal for the entire living world. However, animal membranes contain cholesterol inclusions, which determine their rigidity. The difference between membranes different kingdoms organisms mainly concerns supra-membrane formations (layers). So in plants and fungi there is a cell wall above the membrane (on the outside). In plants it consists mainly of cellulose, and in fungi it consists mainly of chitin. In animals, the supra-membrane layer is called the glycocalyx.

Another name for the cell membrane cytoplasmic membrane or plasma membrane.

A deeper study of the structure of the cell membrane reveals many of its features related to the functions it performs.

The lipid bilayer is mainly composed of phospholipids. These are fats, one end of which contains a phosphoric acid residue that has hydrophilic properties (that is, it attracts water molecules). The second end of the phospholipid is chains of fatty acids that have hydrophobic properties (they do not form hydrogen bonds with water).

Phospholipid molecules in the cell membrane are arranged in two rows so that their hydrophobic “ends” are on the inside and their hydrophilic “heads” are on the outside. The result is a fairly strong structure that protects the contents of the cell from the external environment.

Protein inclusions in the cell membrane are distributed unevenly, in addition, they are mobile (since phospholipids in the bilayer have lateral mobility). Since the 70s of the XX century they began to talk about fluid-mosaic structure of the cell membrane.

Depending on how the protein is included in the membrane, three types of proteins are distinguished: integral, semi-integral and peripheral. Integral proteins pass through the entire thickness of the membrane, and their ends protrude on both sides of it. They mainly perform a transport function. In semi-integral proteins, one end is located in the thickness of the membrane, and the second goes outside (from the outer or inner) side. Perform enzymatic and receptor functions. Peripheral proteins are found on the outer or inner surface of the membrane.

The structural features of the cell membrane indicate that it is the main component of the cell surface complex, but not the only one. Its other components are the supra-membrane layer and the sub-membrane layer.

The glycocalyx (the supra-membrane layer of animals) is formed by oligosaccharides and polysaccharides, as well as peripheral proteins and protruding parts of integral proteins. The components of the glycocalyx perform a receptor function.

In addition to the glycocalyx, animal cells also have other supra-membrane formations: mucus, chitin, perilemma (membrane-like).

The supra-membrane structure in plants and fungi is the cell wall.

The submembrane layer of the cell is the surface cytoplasm (hyaloplasm) with the supporting-contractile system of the cell included in it, the fibrils of which interact with proteins included in the cell membrane. Various signals are transmitted through such molecular connections.

The basic structural unit of a living organism is the cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. Due to the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the membrane must be plastic and dense.

History of the discovery and research of the cell membrane

In 1925, Grendel and Gorder conducted a successful experiment to identify the “shadows” of red blood cells, or empty membranes. Despite several serious mistakes, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, and Robertson in 1960. As a result of many years of work and accumulation of arguments, in 1972 Singer and Nicholson created a fluid-mosaic model of the membrane structure. Further experiments and studies confirmed the works of scientists.

Meaning

What is a cell membrane? This word began to be used more than a hundred years ago; translated from Latin it means “film”, “skin”. This is how the cell boundary is designated, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane implies semi-permeability, due to which moisture and nutrients and breakdown products can freely pass through it. This shell can be called the main structural component of the cell organization.

Let's consider the main functions of the cell membrane

1. Separates the internal contents of the cell and components of the external environment.

2. Helps maintain a constant chemical composition of the cell.

3. Regulates proper metabolism.

4. Provides communication between cells.

5. Recognizes signals.

6. Protection function.

"Plasma Shell"

The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film whose thickness ranges from five to seven nanomillimeters. It consists mainly of protein compounds, phospholides, and water. The film is elastic, easily absorbs water, and quickly restores its integrity after damage.

It has a universal structure. This membrane occupies a border position, participates in the process of selective permeability, removal of decay products, and synthesizes them. The relationship with its “neighbors” and reliable protection of the internal contents from damage makes it an important component in such a matter as the structure of the cell. The cell membrane of animal organisms is sometimes covered with a thin layer - the glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall, which serves as support and maintains shape. The main component of its composition is fiber (cellulose) - a polysaccharide that is insoluble in water.

Thus, the outer cell membrane has the function of repair, protection and interaction with other cells.

Structure of the cell membrane

The thickness of this movable shell varies from six to ten nanomillimeters. The cell membrane of a cell has a special composition, the basis of which is a lipid bilayer. Hydrophobic tails, inert to water, are placed with inside, while the hydrophilic heads interacting with water face outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid framework is closely surrounded by proteins, which are arranged in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, areas permeable to water are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that transfer various substances from the external environment to the cytoplasm and back.

The cell membrane is permeated through and closely connected by integral proteins, and the connection with peripheral ones is less strong. These proteins perform an important function, which is to maintain the structure of the membrane, receive and convert signals from the environment, transport substances, and catalyze reactions that occur on membranes.

Compound

The basis of the cell membrane is a bimolecular layer. Thanks to its continuity, the cell has barrier and mechanical properties. At different stages of life, this bilayer can be disrupted. As a result, structural defects of through hydrophilic pores are formed. In this case, absolutely all functions of such a component as the cell membrane can change. The core may suffer from external influences.

Properties

The cell membrane of a cell has interesting features. Due to its fluidity, this membrane is not a rigid structure, and the bulk of the proteins and lipids that make up it move freely on the plane of the membrane.

In general, the cell membrane is asymmetrical, so the composition of the protein and lipid layers differs. Plasma membranes in animal cells, on their outer side, have a glycoprotein layer that performs receptor and signaling functions, and also plays a large role in the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive and the charge on the inside is negative. In addition to all of the above, the cell membrane has selective insight.

This means that, in addition to water, only a certain group of molecules and ions of dissolved substances are allowed into the cell. The concentration of a substance such as sodium in most cells is much lower than in the external environment. Potassium ions have a different ratio: their amount in the cell is much higher than in environment. In this regard, sodium ions tend to penetrate the cell membrane, and potassium ions tend to be released outside. Under these circumstances, the membrane activates special system, which performs a “pumping” role, leveling the concentration of substances: sodium ions are pumped to the surface of the cell, and potassium ions are pumped inside. This feature is one of the most important functions of the cell membrane.

This tendency of sodium and potassium ions to move inward from the surface plays a big role in the transport of sugar and amino acids into the cell. In the process of actively removing sodium ions from the cell, the membrane creates conditions for new intakes of glucose and amino acids inside. On the contrary, in the process of transferring potassium ions into the cell, the number of “transporters” of decay products from inside the cell to the external environment is replenished.

How does cell nutrition occur through the cell membrane?

Many cells take up substances through processes such as phagocytosis and pinocytosis. With the first option of flexible outer membrane a small depression is created in which the captured particle ends up. The diameter of the recess then becomes larger until the enclosed particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoebas, are fed, as well as blood cells - leukocytes and phagocytes. Similarly, cells absorb fluid, which contains the necessary nutrients. This phenomenon is called pinocytosis.

The outer membrane is closely connected to the endoplasmic reticulum of the cell.

Many types of main tissue components have protrusions, folds, and microvilli on the surface of the membrane. Plant cells on the outside of this shell are covered with another, thick and clearly visible under a microscope. The fiber from which they are composed helps to form tissue support plant origin, for example, wood. Animal cells also have a number of external structures that sit on top of the cell membrane. They are exclusively protective in nature, an example of this is chitin contained in the integumentary cells of insects.

In addition to the cellular membrane, there is an intracellular membrane. Its function is to divide the cell into several specialized closed compartments - compartments or organelles, where a certain environment must be maintained.

Thus, it is impossible to overestimate the role of such a component of the basic unit of a living organism as the cell membrane. The structure and functions suggest a significant expansion of the total cell surface area, improvement metabolic processes. This molecular structure consists of proteins and lipids. Separating the cell from the external environment, the membrane ensures its integrity. With its help, intercellular connections are maintained at a fairly strong level, forming tissues. In this regard, we can conclude that the cell membrane plays one of the most important roles in the cell. The structure and functions performed by it differ radically in different cells, depending on their purpose. Through these features, a variety of physiological activities of cell membranes and their roles in the existence of cells and tissues is achieved.