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 located on the inside, while 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 the 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 a special system that plays 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. In the first option, a flexible outer membrane creates a small depression 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 they are made of helps form support for plant tissues, such as 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 surface area of the cell and an improvement in 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.
It has a thickness of 8-12 nm, so it is impossible to examine it with a light microscope. The structure of the membrane is studied using an electron microscope.
The plasma membrane is formed by two layers of lipids - a bilipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes lipids are located with their heads outward and tails inward.
Numerous protein molecules are immersed in the bilipid layer. Some of them are located on the surface of the membrane (external or internal), others penetrate the membrane.
Functions of the plasma membrane
The membrane protects the contents of the cell from damage, maintains the shape of the cell, selectively allows necessary substances into the cell and removes metabolic products, and also ensures communication between cells.
The barrier, delimiting function of the membrane is provided by a double layer of lipids. It prevents the contents of the cell from spreading, mixing with the environment or intercellular fluid, and prevents the penetration of dangerous substances into the cell.
A number of the most important functions of the cytoplasmic membrane are carried out by proteins immersed in it. With the help of receptor proteins, it can perceive various irritations on its surface. Transport proteins form the finest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins provide vital processes in the body itself.
Large food particles that are unable to pass through thin membrane channels enter the cell by phagocytosis or pinocytosis. The general name for these processes is endocytosis.
How does endocytosis occur - the penetration of large food particles into the cell?
The food particle comes into contact with the outer membrane of the cell, and an invagination forms at this point. Then the particle, surrounded by a membrane, enters the cell, a digestive vesicle is formed, and digestive enzymes penetrate into the resulting vesicle.
White blood cells that can capture and digest foreign bacteria are called phagocytes.
In the case of pinocytosis, the invagination of the membrane captures not solid particles, but droplets of liquid with substances dissolved in it. This mechanism is one of the main ways for substances to enter the cell.
Plant cells covered with a hard layer of cell wall on top of the membrane are not capable of phagocytosis.
The reverse process of endocytosis is exocytosis. Synthesized substances (for example, hormones) are packaged in membrane vesicles, approach the membrane, are built into it, and the contents of the vesicle are released from the cell. In this way, the cell can get rid of unnecessary metabolic products.
9.5.1. One of the main functions of membranes is participation in the transfer of substances. This process is achieved through three main mechanisms: simple diffusion, facilitated diffusion and active transport (Figure 9.10). Remember the most important features of these mechanisms and examples of the substances transported in each case.
Figure 9.10. Mechanisms of transport of molecules across the membrane
Simple diffusion- transfer of substances through the membrane without the participation of special mechanisms. Transport occurs along a concentration gradient without energy consumption. By simple diffusion, small biomolecules are transported - H2O, CO2, O2, urea, hydrophobic low-molecular substances. The rate of simple diffusion is proportional to the concentration gradient.
Facilitated diffusion- transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along a concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, and some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transported substance, all molecules of the carrier take part in the transfer and the transport speed reaches a maximum value.
Active transport- also requires the participation of special transport proteins, but transport occurs against the concentration gradient and therefore requires energy expenditure. Using this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons are transported through the mitochondrial membrane. Active transport of substances is characterized by saturation kinetics.
9.5.2. An example of a transport system that carries out active transport of ions is Na+,K+-adenosine triphosphatase (Na+,K+-ATPase or Na+,K+-pump). This protein is located deep in the plasma membrane and is capable of catalyzing the reaction of ATP hydrolysis. The energy released during the hydrolysis of 1 ATP molecule is used to transfer 3 Na+ ions from the cell to the extracellular space and 2 K+ ions in the opposite direction (Figure 9.11). As a result of the action of Na+,K+-ATPase, a concentration difference is created between the cell cytosol and the extracellular fluid. Since the transfer of ions is not equivalent, an electrical potential difference occurs. Thus, an electrochemical potential arises, which consists of the energy of the difference in electrical potentials Δφ and the energy of the difference in the concentrations of substances ΔC on both sides of the membrane.
Figure 9.11. Na+, K+ pump diagram.
9.5.3. Transport of particles and high molecular weight compounds across membranes
Along with the transport of organic substances and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb high-molecular compounds into the cell and remove high-molecular compounds from it by changing the shape of the biomembrane. This mechanism is called vesicular transport.
Figure 9.12. Types of vesicular transport: 1 - endocytosis; 2 - exocytosis.
During the transfer of macromolecules, sequential formation and fusion of membrane-surrounded vesicles (vesicles) occurs. Based on the direction of transport and the nature of the substances transported, the following types of vesicular transport are distinguished:
Endocytosis(Figure 9.12, 1) - transfer of substances into the cell. Depending on the size of the resulting vesicles, they are distinguished:
A) pinocytosis — absorption of liquid and dissolved macromolecules (proteins, polysaccharides, nucleic acids) using small bubbles (150 nm in diameter);
b) phagocytosis — absorption of large particles, such as microorganisms or cell debris. In this case, large vesicles called phagosomes with a diameter of more than 250 nm are formed.
Pinocytosis is characteristic of most eukaryotic cells, while large particles are absorbed by specialized cells - leukocytes and macrophages. At the first stage of endocytosis, substances or particles are adsorbed on the surface of the membrane; this process occurs without energy consumption. At the next stage, the membrane with the adsorbed substance deepens into the cytoplasm; the resulting local invaginations of the plasma membrane are detached from the cell surface, forming vesicles, which then migrate into the cell. This process is connected by a system of microfilaments and is energy dependent. The vesicles and phagosomes that enter the cell can merge with lysosomes. Enzymes contained in lysosomes break down substances contained in vesicles and phagosomes into low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported into the cytosol, where they can be used by the cell.
Exocytosis(Figure 9.12, 2) - transfer of particles and large compounds from the cell. This process, like endocytosis, occurs with the absorption of energy. The main types of exocytosis are:
A) secretion - removal from the cell of water-soluble compounds that are used or affect other cells of the body. It can be carried out both by unspecialized cells and by cells of the endocrine glands, the mucous membrane of the gastrointestinal tract, adapted for the secretion of the substances they produce (hormones, neurotransmitters, proenzymes) depending on the specific needs of the body.
Secreted proteins are synthesized on ribosomes associated with the membranes of the rough endoplasmic reticulum. These proteins are then transported to the Golgi apparatus, where they are modified, concentrated, sorted, and then packaged into vesicles, which are released into the cytosol and subsequently fuse with the plasma membrane so that the contents of the vesicles are outside the cell.
Unlike macromolecules, small secreted particles, such as protons, are transported out of the cell using the mechanisms of facilitated diffusion and active transport.
b) excretion - removal from the cell of substances that cannot be used (for example, during erythropoiesis, removal from reticulocytes of the mesh substance, which is aggregated remains of organelles). The mechanism of excretion appears to be that the excreted particles are initially trapped in a cytoplasmic vesicle, which then fuses with the plasma membrane.
The cell membrane is one of the most important organelles, serving as a kind of barrier between a given cell and the external environment. Scientific names are plasmalemma, cytolemma or plasma membrane. It is through it that the cell interacts with the external environment, through it nutrients get in, and what has already been processed is released out. The plasmalemma has a rather complex structure and also performs many functions in the body. This article will discuss in detail the cell membrane and its structure.
This organoid was discovered relatively recently, only at the beginning of the twentieth century. The discovery was made by German scientists - Gorter and Grendel. Throughout the previous century, scientists actively studied the cytolemma; various theories about its structure were put forward, which were refuted over time, and new ones took their place. And only in the seventies were scientists able to reliably determine its structure.
So what does the cell membrane consist of? Through numerous studies, it was found that it consists of three layers. The upper and lower layers are non-continuous sections of associations of protein molecules, and the inner layer, on the contrary, is continuous, consisting of fats; it is the main one, due to which isolation from the external environment is ensured. The fat layer includes two rows of lipids (otherwise it is called bilipid).
The following types of lipids are present in the cytolemma:
- phospholipids (fats and phosphorus);
- glycolipids (fats and carbohydrates);
- cholesterol
The protein outer and inner layers serve to ensure that substances that cannot penetrate inside through the fat layer can get there through these layers, that is, they represent “crossings” for water-soluble substances.
So, the cell membrane is formed by three levels, two of which are peculiar transporters for substances that cannot penetrate through the third level, which is the main one, this is the barrier that isolates the internal contents, but at the same time also ensures connection with other cells, after all, it is through it that the main amount of nutrients enters.
It is also important to understand that the cell membrane and the cell wall are different organelles. There are many differences, and they are significant; the wall is located above the cytolemma and serves as protection from mechanical damage and pressure. The functions of the cytolemma, in turn, are different.
Watch a video about the cell membrane and its functions.
The functions of the cell membrane include:
- Barrier. Serves as a natural filter for molecules that are about to penetrate inside, it allows only those that meet certain parameters to pass through.
- Protective. Since most animals do not have a cell wall, the plasmalemma also serves as protection against mechanical stress and prevents damage. The cell membrane in a plant cell does not perform a similar function, since plant cells have a complex structured wall that can protect them.
- Matrix. Responsible for the arrangement of internal organelles relative to each other to maintain the internal balance necessary for full activity.
- Transport. It completely controls the exchange of necessary substances with the external environment, and thanks to its special features helps those that are necessary for life, but at the same time cannot penetrate inside on their own.
- Enzymatic. Necessary for the production of enzymes necessary, for example, for digesting food.
- Receptor. Necessary for receiving signals about what is happening in the external environment.
- Marking. Each cell is unique, and the cells are able to recognize each other, this is necessary in order to interact with each other. Recognition occurs due to the structure of the cytolemma, which is not repeated.
The cytolemma of any living creature performs essentially the same number of functions, with only slight variations, regardless of whether the cytolemma is being considered: an animal, a human, an insect, or the cell membrane of a plant.
Conclusions about the plasmalemma
Having examined the structure and functions of this organelle, one can notice that the cell membrane has features that are not characteristic of other components of the cell. Its discovery at the beginning of the last century contributed to the further development of medicine and served as the key to understanding many human diseases, as well as methods of treating them.
The cell membrane is characteristic of the cells of every organism. It serves as protection and also performs very important functions, because various substances penetrate inside through it. In order for this organelle to function normally, and, consequently, for the cell as a whole to function normally, it is necessary that the body maintain conditions that do not interfere with its activity.
As is known, the membrane is plasmatic; its structure consists of many channels, thanks to which exchange with the external environment is ensured. Scientists have found that for normal functioning, in particular to ensure that the cell does not begin to degenerate into a cancerous one, it is necessary that the channels of the plasmalemma work properly, do not become clogged, and do not allow inappropriate molecules to pass through.
- proper nutrition;
- regular walks in the fresh air;
- maintaining the body's water balance.
This is amazing, but it is precisely such a seemingly insignificant organelle that can greatly influence a person’s well-being and health. Therefore, the discovery of the plasmalemma was a huge step forward for biological science.
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Cell- this is not only liquid, enzymes and other substances, but also highly organized structures called intracellular organelles. Organelles for a cell are no less important than its chemical components. Thus, in the absence of organelles such as mitochondria, the supply of energy extracted from nutrients will immediately decrease by 95%.
Most organelles in a cell are covered membranes consisting mainly of lipids and proteins. There are membranes of cells, endoplasmic reticulum, mitochondria, lysosomes, and Golgi apparatus.
Lipids are insoluble in water, so they create a barrier in the cell that prevents the movement of water and water-soluble substances from one compartment to another. Protein molecules, however, make the membrane permeable to various substances through specialized structures called pores. Many other membrane proteins are enzymes that catalyze numerous chemical reactions, which will be discussed in subsequent chapters.
Cell (or plasma) membrane is a thin, flexible and elastic structure with a thickness of only 7.5-10 nm. It consists mainly of proteins and lipids. The approximate ratio of its components is as follows: proteins - 55%, phospholipids - 25%, cholesterol - 13%, other lipids - 4%, carbohydrates - 3%.
Lipid layer of the cell membrane prevents water penetration. The basis of the membrane is a lipid bilayer - a thin lipid film consisting of two monolayers and completely covering the cell. Proteins are located throughout the membrane in the form of large globules.
Schematic representation of a cell membrane, reflecting its main elements- phospholipid bilayer and a large number of protein molecules protruding above the surface of the membrane.
Carbohydrate chains are attached to proteins on the outer surface
and to additional protein molecules inside the cell (not shown in the figure).
Lipid bilayer consists mainly of phospholipid molecules. One end of such a molecule is hydrophilic, i.e. soluble in water (a phosphate group is located on it), the other is hydrophobic, i.e. soluble only in fats (it contains a fatty acid).
Due to the fact that the hydrophobic part of the molecule phospholipid repels water, but is attracted to similar parts of the same molecules, phospholipids have a natural property of attaching to each other in the thickness of the membrane, as shown in Fig. 2-3. The hydrophilic part with the phosphate group forms two membrane surfaces: the outer one, which is in contact with the extracellular fluid, and the inner one, which is in contact with the intracellular fluid.
Middle of the lipid layer impermeable to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, and alcohol, on the contrary, easily penetrate this area of the membrane.
Molecules cholesterol, which is part of the membrane, also belongs to lipids by nature, since their steroid group is highly soluble in fats. These molecules seem to be dissolved in the lipid bilayer. Their main purpose is to regulate the permeability (or impermeability) of membranes for water-soluble components of body fluids. In addition, cholesterol is the main regulator of membrane viscosity.
Cell membrane proteins. In the figure, globular particles are visible in the lipid bilayer - these are membrane proteins, most of which are glycoproteins. There are two types of membrane proteins: (1) integral, which penetrate the membrane through; (2) peripheral, which protrude only above one of its surfaces, without reaching the other.
Many integral proteins form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intra- and extracellular fluid. Due to the selectivity of the channels, some substances diffuse better than others.
Other integral proteins function as carrier proteins, transporting substances for which the lipid bilayer is impermeable. Sometimes carrier proteins act in the direction opposite to diffusion; such transport is called active transport. Some integral proteins are enzymes.
Integral membrane proteins can also serve as receptors for water-soluble substances, including peptide hormones, since the membrane is impermeable to them. The interaction of a receptor protein with a specific ligand leads to conformational changes in the protein molecule, which, in turn, stimulates the enzymatic activity of the intracellular segment of the protein molecule or the transmission of a signal from the receptor into the cell using a second messenger. Thus, integral proteins embedded in the cell membrane involve it in the process of transmitting information about the external environment into the cell.
Molecules of peripheral membrane proteins often associated with integral proteins. Most peripheral proteins are enzymes or play the role of dispatcher of the transport of substances through membrane pores.