The cell membrane is the structure that covers the outside of the cell. It is also called cytolemma or plasmalemma.
This formation is built from a bilipid layer (bilayer) with proteins built into it. The carbohydrates that make up the plasmalemma are in a bound state.
The distribution of the main components of the plasmalemma is as follows: more than half of the chemical composition is proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.
Cell membrane and its types
The cell membrane is a thin film, the basis of which is made up of layers of lipoproteins and proteins.
According to localization, membrane organelles are distinguished, which have some features in plant and animal cells:
- mitochondria;
- core;
- endoplasmic reticulum;
- Golgi complex;
- lysosomes;
- chloroplasts (in plant cells).
There is also an inner and outer (plasmolemma) cell membrane.
Structure of the cell membrane
The cell membrane contains carbohydrates that cover it in the form of a glycocalyx. This is a supra-membrane structure that performs a barrier function. The proteins located here are in a free state. Unbound proteins participate in enzymatic reactions, providing extracellular breakdown of substances.
Proteins of the cytoplasmic membrane are represented by glycoproteins. Based on their chemical composition, proteins that are completely included in the lipid layer (along its entire length) are classified as integral proteins. Also peripheral, not reaching one of the surfaces of the plasmalemma.
The former function as receptors, binding to neurotransmitters, hormones and other substances. Insertion proteins are necessary for the construction of ion channels through which the transport of ions and hydrophilic substrates occurs. The latter are enzymes that catalyze intracellular reactions.
Basic properties of the plasma membrane
The lipid bilayer prevents the penetration of water. Lipids are hydrophobic compounds represented in the cell by phospholipids. The phosphate group faces outward and consists of two layers: the outer one, directed to the extracellular environment, and the inner one, delimiting the intracellular contents.
Water-soluble areas are called hydrophilic heads. The fatty acid sites are directed into the cell, in the form of hydrophobic tails. The hydrophobic part interacts with neighboring lipids, which ensures their attachment to each other. The double layer has selective permeability in different areas.
So, in the middle the membrane is impermeable to glucose and urea; hydrophobic substances pass through here freely: carbon dioxide, oxygen, alcohol. Cholesterol is important; the content of the latter determines the viscosity of the plasmalemma.
Functions of the outer cell membrane
The characteristics of the functions are briefly listed in the table:
| Membrane function | Description |
| Barrier role | The plasmalemma performs a protective function, protecting the contents of the cell from the effects of foreign agents. Thanks to the special organization of proteins, lipids, and carbohydrates, the semipermeability of the plasmalemma is ensured. |
| Receptor function | Biologically active substances are activated through the cell membrane in the process of binding to receptors. Thus, immune reactions are mediated through the recognition of foreign agents by the cell receptor apparatus localized on the cell membrane. |
| Transport function | The presence of pores in the plasmalemma allows you to regulate the flow of substances into the cell. The transfer process occurs passively (without energy consumption) for compounds with low molecular weight. Active transport is associated with the expenditure of energy released during the breakdown of adenosine triphosphate (ATP). This method takes place for the transfer of organic compounds. |
| Participation in digestive processes | Substances are deposited on the cell membrane (sorption). Receptors bind to the substrate, moving it into the cell. A bubble is formed, lying freely inside the cell. Merging, such vesicles form lysosomes with hydrolytic enzymes. |
| Enzymatic function | Enzymes are essential components of intracellular digestion. Reactions requiring the participation of catalysts occur with the participation of enzymes. |
What is the importance of the cell membrane
The cell membrane is involved in maintaining homeostasis due to the high selectivity of substances entering and exiting the cell (in biology this is called selective permeability).
Outgrowths of the plasmalemma divide the cell into compartments (compartments) responsible for performing certain functions. Specifically designed membranes corresponding to the fluid-mosaic pattern ensure the integrity of the cell.
The outside of the cell is covered with a plasma membrane (or outer cell membrane) about 6-10 nm thick.
The cell membrane is a dense film of proteins and lipids (mainly phospholipids). Lipid molecules are arranged in an orderly manner - perpendicular to the surface, in two layers, so that their parts that interact intensively with water (hydrophilic) are directed outward, and their parts inert to water (hydrophobic) are directed inward.
Protein molecules are located in a non-continuous layer on the surface of the lipid framework on both sides. Some of them are immersed in the lipid layer, and some pass through it, forming areas permeable to water. These proteins perform various functions - some of them are enzymes, others are transport proteins involved in the transfer of certain substances from the environment to the cytoplasm and in the opposite direction.
Basic functions of the cell membrane
One of the main properties of biological membranes is selective permeability (semi-permeability)- some substances pass through them with difficulty, others easily and even towards higher concentrations. Thus, for most cells, the concentration of Na ions inside is significantly lower than in the environment. The opposite relationship is typical for K ions: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to penetrate the cell, and K ions always tend to exit. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump, which pumps Na ions out of the cell and simultaneously pumps K ions inside.
The tendency of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.
In many cells, substances are also absorbed by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression into which the captured particle falls. This recess increases, and, surrounded by a section of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoebas and some other protozoa, as well as leukocytes (phagocytes). Cells absorb liquids containing substances necessary for the cell in a similar way. This phenomenon was called pinocytosis.
The outer membranes of different cells differ significantly both in the chemical composition of their proteins and lipids, and in their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.
The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, various types of intercellular contacts are carried out, i.e. communication between individual cells.
Many types of cells are characterized by the presence on their surface of a large number of protrusions, folds, and microvilli. They contribute to both a significant increase in cell surface area and improved metabolism, as well as stronger connections between individual cells and each other.
Plant cells have thick membranes on the outside of the cell membrane, clearly visible under an optical microscope, consisting of fiber (cellulose). They create a strong support for plant tissues (wood).
Some animal cells also have a number of external structures located on top of the cell membrane and have a protective nature. An example is the chitin of insect integumentary cells.
Functions of the cell membrane (briefly)
| Function | Description |
|---|---|
| Protective Barrier | Separates internal cell organelles from the external environment |
| Regulatory | Regulates the metabolism between the internal contents of the cell and the external environment |
| Dividing (compartmentalization) | Division of the internal space of the cell into independent blocks (compartments) |
| Energy | - Energy accumulation and transformation; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion. |
| Receptor (informational) | Participates in the formation of arousal and its conduction. |
| Motor | Carries out the movement of the cell or its individual parts. |
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.
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 the 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.
The branch of biology called cytology studies the structure of organisms, as well as plants, animals and humans. Scientists have found that the contents of the cell, which is located inside it, are built quite complex. It is surrounded by the so-called surface apparatus, which includes the outer cell membrane, supra-membrane structures: the glycocalyx and also microfilaments, pelicule and microtubules that form its submembrane complex.
In this article we will study the structure and functions of the outer cell membrane, which is part of the surface apparatus of various types of cells.
What functions does the outer cell membrane perform?
As described earlier, the outer membrane is part of the surface apparatus of each cell, which successfully separates its internal contents and protects cellular organelles from adverse environmental conditions. Another function is to ensure metabolism between cellular contents and tissue fluid, so the outer cell membrane transports molecules and ions entering the cytoplasm, and also helps remove waste and excess toxic substances from the cell.
Structure of the cell membrane
The membranes, or plasma membranes, of different types of cells differ greatly from each other. Mainly, by their chemical structure, as well as the relative content of lipids, glycoproteins, proteins and, accordingly, the nature of the receptors located in them. The external one, which is determined primarily by the individual composition of glycoproteins, takes part in the recognition of environmental stimuli and in the reactions of the cell itself to their actions. Some types of viruses can interact with proteins and glycolipids of cell membranes, as a result of which they penetrate the cell. Herpes and influenza viruses can be used to build their protective shell.
And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and dissolve it at the point of contact using a special enzyme. Then a viral DNA molecule passes into the resulting hole.
Features of the structure of the plasma membrane of eukaryotes
Let us recall that the outer cell membrane performs the function of transport, that is, the transfer of substances in and out of it into the external environment. To carry out such a process, a special structure is required. Indeed, the plasmalemma is a permanent, universal system of surface apparatus. This is a thin (2-10 Nm), but quite dense multilayer film that covers the entire cell. Its structure was studied in 1972 by scientists such as D. Singer and G. Nicholson, and they also created a fluid-mosaic model of the cell membrane.
The main chemical compounds that form it are ordered molecules of proteins and certain phospholipids, which are embedded in a liquid lipid medium and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the non-polar hydrophobic “tails” of which are located inside the membrane, and the polar hydrophilic heads are facing the cell cytoplasm and the intercellular fluid.
The lipid layer is penetrated by large protein molecules that form hydrophilic pores. It is through them that aqueous solutions of glucose and mineral salts are transported. Some protein molecules are found on both the outer and inner surfaces of the plasmalemma. Thus, on the outer cell membrane in the cells of all organisms with nuclei there are carbohydrate molecules linked by covalent bonds to glycolipids and glycoproteins. The carbohydrate content in cell membranes ranges from 2 to 10%.
The structure of the plasmalemma of prokaryotic organisms
The outer cell membrane in prokaryotes performs similar functions to the plasma membranes of cells of nuclear organisms, namely: perception and transmission of information coming from the external environment, transport of ions and solutions into and out of the cell, protection of the cytoplasm from foreign reagents from the outside. It can form mesosomes - structures that arise when the plasma membrane is invaginated into the cell. They may contain enzymes involved in metabolic reactions of prokaryotes, for example, DNA replication and protein synthesis.
Mesosomes also contain redox enzymes, and photosynthetics contain bacteriochlorophyll (in bacteria) and phycobilin (in cyanobacteria).
The role of outer membranes in intercellular contacts
Continuing to answer the question of what functions the outer cell membrane performs, let us dwell on its role in. In plant cells, pores are formed in the walls of the outer cell membrane, which pass into the cellulose layer. Through them, the cytoplasm of the cell can exit to the outside; such thin channels are called plasmodesmata.
Thanks to them, the connection between neighboring plant cells is very strong. In human and animal cells, the contact points between adjacent cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and are also found in cardiomyocytes.
Auxiliary formations of the plasmalemma
Understanding how plant cells differ from animal cells is helped by studying the structural features of their plasma membranes, which depend on the functions of the outer cell membrane. Above it in animal cells there is a layer of glycocalyx. It is formed by polysaccharide molecules associated with proteins and lipids of the outer cell membrane. Thanks to the glycocalyx, adhesion (sticking together) occurs between cells, leading to the formation of tissues, therefore it takes part in the signaling function of the plasmalemma - recognizing environmental stimuli.
How is the passive transport of certain substances carried out across cell membranes?
As mentioned earlier, the outer cell membrane is involved in the process of transporting substances between the cell and the external environment. There are two types of transport through the plasmalemma: passive (diffusion) and active transport. The first includes diffusion, facilitated diffusion and osmosis. The movement of substances along a concentration gradient depends, first of all, on the mass and size of molecules passing through the cell membrane. For example, small nonpolar molecules easily dissolve in the middle lipid layer of the plasmalemma, move through it and end up in the cytoplasm.
Large molecules of organic substances penetrate into the cytoplasm with the help of special carrier proteins. They have species specificity and, when connecting with a particle or ion, passively transfer them across the membrane along a concentration gradient without energy expenditure (passive transport). This process underlies such a property of the plasmalemma as selective permeability. During the process, the energy of ATP molecules is not used, and the cell saves it for other metabolic reactions.
Active transport of chemical compounds through the plasmalemma
Since the outer cell membrane ensures the transfer of molecules and ions from the external environment into the cell and back, it becomes possible to remove dissimilation products, which are toxins, outside, that is, into the intercellular fluid. occurs against a concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATPases, which are also enzymes.
An example of such transport is the sodium-potassium pump (sodium ions move from the cytoplasm into the external environment, and potassium ions are pumped into the cytoplasm). Epithelial cells of the intestines and kidneys are capable of it. Varieties of this transfer method are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the outer cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, for example, leukocytes, are capable of the processes of pino- and phagocytosis.
Bioelectric processes in cell membranes
It has been established that there is a potential difference between the outer surface of the plasma membrane (it is positively charged) and the wall layer of the cytoplasm, which is negatively charged. It was called the resting potential, and it is inherent in all living cells. And nervous tissue not only has a resting potential, but is also capable of conducting weak biocurrents, which is called the process of excitation. The outer membranes of nerve cells-neurons, receiving irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. And the near-wall layer of the cytoplasm, due to an excess of cations, receives a positive charge. This explains why the outer cell membrane of the neuron is recharged, which causes the conduction of nerve impulses that underlie the excitation process.