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.
Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions cell membranes:
1. The plasma membrane is a barrier that maintains the different composition of the extra- and intracellular environment.
2. Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.
3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in the membranes.
Structure and composition of membranes
The basis of the membrane is a double lipid layer, the formation of which involves phospholipids and glycolipids. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inward, and the hydrophilic groups face outward and are in contact with the aqueous environment. Protein molecules are, as it were, “dissolved” in the lipid bilayer.
Structure of membrane lipids
Membrane lipids are amphiphilic molecules, because the molecule has both a hydrophilic region (polar heads) and a hydrophobic region, represented by hydrocarbon radicals of fatty acids, which spontaneously form a bilayer. Membranes contain three main types of lipids - phospholipids, glycolipids and cholesterol.
The lipid composition is different. The content of a particular lipid is apparently determined by the variety of functions performed by these lipids in membranes.
Phospholipids. All phospholipids can be divided into two groups - glycerophospholipids and sphingophospholipids. Glycerophospholipids are classified as phosphatidic acid derivatives. The most common glycerophospholipids are phosphatidylcholines and phosphatidylethanolamines. Sphingophospholipids are based on the amino alcohol sphingosine.
Glycolipids. In glycolipids, the hydrophobic part is represented by the alcohol ceramide, and the hydrophilic part is represented by a carbohydrate residue. Depending on the length and structure of the carbohydrate part, cerebrosides and gangliosides are distinguished. The polar “heads” of glycolipids are located on the outer surface of plasma membranes.
Cholesterol (CS). CS is present in all membranes of animal cells. Its molecule consists of a rigid hydrophobic core and a flexible hydrocarbon chain. The single hydroxyl group at the 3-position is the “polar head”. For an animal cell, the average molar ratio of cholesterol/phospholipids is 0.3-0.4, but in the plasma membrane this ratio is much higher (0.8-0.9). The presence of cholesterol in membranes reduces the mobility of fatty acids, reduces the lateral diffusion of lipids and therefore can affect the functions of membrane proteins.
Membrane properties:
1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into cells due to the small size of their molecules and weak interaction with solvents. Molecules of a lipid nature, such as steroid hormones, also easily penetrate the bilayer.
2. Liquidity. Membranes are characterized by liquidity (fluidity), the ability of lipids and proteins to move. Two types of phospholipid movements are possible: somersault (called “flip-flop” in the scientific literature) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with energy consumption. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the surface of the membrane. The speed of movement of molecules depends on the microviscosity of the membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in the lipid composition. Microviscosity is lower if unsaturated fatty acids predominate in the lipid composition, and higher if the content of saturated fatty acids is high.
3. Membrane asymmetry. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, and phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous structure called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes they regulate - adenylate cyclase, phospholipase C - on the inner surface, etc.
Membrane proteins
Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. Proteins account for 30 to 70% of the mass of membranes. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the major histocompatibility system, receptors for various molecules.
Based on their localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins cross the membrane once (glycophorin), others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor cross the bilayer 7 times.
Peripheral proteins and domains of integral proteins, located on the outer surface of all membranes, are almost always glycosylated. Oligosaccharide residues protect the protein from proteolysis and are also involved in ligand recognition or adhesion.
In 1972, the theory was put forward that a partially permeable membrane surrounds the cell and performs a number of vital 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 channel and transporter protein: Nutrients such as glucose or amino acids must enter the cell, and some metabolic products must leave the cell.
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 under 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 in the aqueous environment of the cytoplasm reach a critical concentration, they combine into micelles, which are more stable in the 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 the modern concept, 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. An important feature of the membrane structure 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 fluid mosaic model describes the structure of the cell membrane as follows: flexible and elastic, with a consistency similar to vegetable oil, so that all individual molecules simply float in a liquid medium, and they are all capable of moving laterally within this membrane. 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 interactions of membranes, structures necessary for the organization and performance of biological functions are created. Many structural and functional features are common to bacteria and enveloped viruses. All biological membranes are built on a lipid bilayer, which results in a number of common 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 membrane is an ultra-fine structure that forms the surfaces of organelles and the cell as a whole. All membranes have a similar structure and are connected into one system.
Chemical composition
Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:
- phospholipids;
- galactolipids;
- sulfolipids.
They also contain nucleic acids, polysaccharides and other substances.
Physical properties
At normal temperatures, the membranes are in a liquid crystalline state and constantly fluctuate. Their viscosity is close to that of vegetable oil.
The membrane is recoverable, durable, elastic and porous. Membrane thickness is 7 - 14 nm.
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The membrane is impermeable to large molecules. Small molecules and ions can pass through the pores and the membrane itself under the influence of concentration differences on different sides of the membrane, as well as with the help of transport proteins.
Model
Typically, the structure of membranes is described using a fluid mosaic model. The membrane has a framework - two rows of lipid molecules, tightly adjacent to each other, like bricks.
Rice. 1. Sandwich-type biological membrane.
On both sides the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the surface of the membrane.
According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:
- transmembrane;
- submerged;
- superficial.
Proteins provide the main property of the membrane - its selective permeability to various substances.
Membrane types
All cell membranes according to localization can be divided into the following types:
- external;
- nuclear;
- organelle membranes.
The outer cytoplasmic membrane, or plasmolemma, is the boundary of the cell. Connecting with the elements of the cytoskeleton, it maintains its shape and size.
Rice. 2. Cytoskeleton.
The nuclear membrane, or karyolemma, is the boundary of the nuclear contents. It is constructed of two membranes, very similar to the outer one. The outer membrane of the nucleus is connected to the membranes of the endoplasmic reticulum (ER) and, through pores, to the inner membrane.
ER membranes penetrate the entire cytoplasm, forming surfaces on which the synthesis of various substances, including membrane proteins, takes place.
Organelle membranes
Most organelles have a membrane structure.
The walls are built from one membrane:
- Golgi complex;
- vacuoles;
- lysosomes
Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, and the inner one forms many folds.
Features of photosynthetic membranes of chloroplasts are built-in chlorophyll molecules.
Animal cells have a carbohydrate layer on the surface of their outer membrane called the glycocalyx.
Rice. 3. Glycocalyx.
The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmalemma.
Table "Structure of the cell membrane"
What have we learned?
We looked at the structure and functions of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by the fluid mosaic model. According to this model, protein molecules are built into the bilayer of viscous lipids.
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