The structure and role of the cell membrane. Structure and functions of the cell membrane
Cell- a self-regulating structural and functional unit of tissues and organs. The cellular theory of the structure of organs and tissues was developed by Schleiden and Schwann in 1839. Subsequently, with the help of electron microscopy and ultracentrifugation, it was possible to clarify the structure of all the main organelles of animal and plant cells (Fig. 1).
Rice. 1. Scheme of the structure of an animal cell
The main parts of a cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.
The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows necessary nutrients and chemical elements to penetrate into the cell, and excess products to leave it. The plasma membrane consists of two layers of lipid molecules containing specific proteins. The main membrane lipids are phospholipids. They contain phosphorus, a polar head and two non-polar tails of long-chain fatty acids. Membrane lipids include cholesterol and cholesteryl esters. In accordance with the liquid mosaic model of structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. Each type of membrane of any animal cell has its own relatively constant lipid composition.
Membrane proteins are divided into two types according to their structure: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types of membrane proteins: transport proteins, enzymes, receptors and structural proteins. Some membrane proteins have enzymatic activity, others bind certain substances and facilitate their transport into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores consisting of several protein subunits that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. The cytoskeleton provides mechanical strength to the cell membrane. In various membranes, proteins account for from 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.
The membrane also contains carbohydrates that can be covalently bound to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outer side of the membrane there are receptor sites that bind various hormones. Other specific areas of the membrane cannot recognize and bind certain proteins and various biologically active compounds that are foreign to these cells.
The internal space of the cell is filled with cytoplasm, in which most enzyme-catalyzed reactions of cellular metabolism take place. The cytoplasm consists of two layers: the internal one, called endoplasm, and the peripheral one, ectoplasm, which has a high viscosity and is devoid of granules. The cytoplasm contains all the components of a cell or organelle. The most important of the cell organelles are the endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.
Endoplasmic reticulum is a system of interconnected channels and cavities that penetrate the entire cytoplasm. It ensures the transport of substances from the environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site of lipid synthesis in the cell.
Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to the outer surface of the membranes of the endoplasmic reticulum and nuclear membrane. They interact with messenger and transport RNA, and protein synthesis occurs in them. They synthesize proteins that enter the cisternae or the Golgi apparatus and are then released outside. Ribosomes, freely located in the cytoplasm, synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Ribosomes synthesize various functional proteins: carrier proteins, enzymes, receptors, cytoskeletal proteins.
Golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that enter here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merge with it, and the substances contained in the vesicles are removed from the cell through the process of exocytosis.
Lysosomes - membrane-surrounded particles measuring 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.
Peroxisomes formed from smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is broken down under the influence of peroxidases and catalase.
Mitochondria contain outer and inner membranes and are the “energy station” of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP synthesis occurs in them, oxidation of Krebs cycle substrates and many biochemical reactions occur. ATP molecules produced in mitochondria diffuse to all parts of the cell. Mitochondria contain a small amount of DNA, RNA, and ribosomes, and with their participation, the renewal and synthesis of new mitochondria occurs.
Microfilaments They are thin protein filaments consisting of myosin and actin and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures within cells.
Microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives cells their characteristic appearance and shape and serves as a site for attachment of intracellular organelles and various bodies. In nerve cells, bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the mitotic spindle functions during cell division. They play the role of motor elements in villi and flagella in eukaryotes.
Core is the main structure of the cell, participates in the transmission of hereditary characteristics and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. There is a nucleolus inside it. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The remaining parts of the nucleus contain chromatin, consisting of DNA, RNA and a number of specific proteins.
Functions of the cell membrane
Cell membranes play a crucial role in the regulation of intracellular and intercellular metabolism. They have selective permeability. Their specific structure allows them to provide barrier, transport and regulatory functions.
Barrier function manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.
Regulatory function membranes is to regulate intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in enzyme activity.
Transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or using active transport.
Diffusion - movement of a gas or soluble substance along a concentration and electrochemical gradient. The rate of diffusion depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, and the electrical and concentration gradients for charged particles. Simple diffusion occurs through the lipid bilayer or through channels. Charged particles move according to an electrochemical gradient, and uncharged particles move according to a chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc. penetrate through the lipid layer of the membrane by simple diffusion. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and ungated channels. Depending on the selectivity, a distinction is made between ion-selective cables, which allow only one ion to pass through, and channels that do not have selectivity. The channels have an orifice and a selective filter, and the controlled channels have a gate mechanism.
Facilitated diffusion - a process in which substances are transported across a membrane using special membrane transport proteins. In this way, amino acids and monosaccharides penetrate into the cell. This type of transport happens very quickly.
Osmosis - movement of water through the membrane from a solution with a lower to a solution with a higher osmotic pressure.
Active transport - transport of substances against a concentration gradient using transport ATPases (ion pumps). This transfer occurs with the expenditure of energy.
Na + /K + -, Ca 2+ - and H + -pumps have been studied to a greater extent. The pumps are located on cell membranes.
A type of active transport is endocytosis And exocytosis. Using these mechanisms, larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through channels are transported. This transport is more common in intestinal epithelial cells, renal tubules, and vascular endothelium.
At In endocytosis, cell membranes form invaginations into the cell, which, when released, turn into vesicles. During exocytosis, vesicles with their contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.
Structure and functions of the cell membrane
To understand the processes that ensure the existence of electrical potentials in living cells, you first need to understand the structure of the cell membrane and its properties.
Currently, the most widely accepted is the liquid mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972. The membrane is based on a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the surrounding aquatic environment (Fig. 2).
Membrane proteins are localized on the surface of the membrane or can be embedded to varying depths in the hydrophobic zone. Some proteins span the membrane, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they participate in the formation of ion channels, play the role of membrane pumps and transporters of various substances, and can also perform a receptor function.
The main functions of the cell membrane: barrier, transport, regulatory, catalytic.
The barrier function is to limit the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and maintain a relatively constant content of various substances inside the cells. Thus, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.
Rice. 2. Three-dimensional diagram of the liquid-mosaic model of the Singer-Nicholson membrane
Depicted are globular integral proteins embedded in a lipid bilayer. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains that are involved in the recognition of cells among each other and in intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent sections of the “tails”. The internal sections of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)
The membrane contains channels through which ions penetrate. Channels can be voltage dependent or potential independent. Voltage-dependent channels open when the potential difference changes, and potential independent(hormone-regulated) open when receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(deep in the channel) and inactivation(on the channel surface). The gate can be in one of three states:
- open state (both types of gates are open);
- closed state (activation gate closed);
- inactivation state (inactivation gate closed).
Another characteristic feature of membranes is the ability to selectively transport inorganic ions, nutrients, and various metabolic products. There are systems of passive and active transfer (transport) of substances. Passive transport occurs through ion channels with or without the help of carrier proteins, and its driving force is the difference in the electrochemical potential of ions between the intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and the chemical nature of the groups lining the walls of the channel and its mouth.
Currently, the most well studied channels are those that are selectively permeable to Na + , K + , Ca 2+ ions and also to water (the so-called aquaporins). The diameter of ion channels, according to various studies, is 0.5-0.7 nm. The channel capacity can vary; 10 7 - 10 8 ions per second can pass through one ion channel.
Active transport occurs with the expenditure of energy and is carried out by so-called ion pumps. Ion pumps are molecular protein structures embedded in a membrane that transport ions toward a higher electrochemical potential.
The pumps operate using the energy of ATP hydrolysis. Currently, Na+/K+ - ATPase, Ca 2+ - ATPase, H + - ATPase, H + /K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na +, K +, Ca 2+ ions, respectively, have been well studied , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). The molecular mechanism of active transport is not fully understood.
The cell membrane, also called plasmalemma, cytolemma or plasma membrane, is a molecular structure, elastic in nature, which consists of various proteins and lipids. It separates the contents of any cell from the external environment, thereby regulating its protective properties, and also ensures the exchange between the external environment and the immediate internal contents of the cell.
Plasma membrane
The plasmalemma is a partition located inside, directly behind the membrane. It divides the cell into certain compartments, which are directed to compartments or organelles. They contain specialized environmental conditions. The cell wall completely covers the entire cell membrane. It looks like a double layer of molecules.
Basic information
The composition of the plasmalemma is phospholipids or, as they are also called, complex lipids. Phospholipids have several parts: a tail and a head. Experts call hydrophobic and hydrophilic parts: depending on the structure of the animal or plant cell. The areas called the head face the inside of the cell, and the tails face the outside. Plasmalemmas are invariable in structure and are very similar in different organisms; Most often, the exception may be archaea, whose partitions consist of various alcohols and glycerol.
Plasmalemma thickness approximately 10 nm.
There are partitions that are located on the outside or outside the part adjacent to the membrane - they are called superficial. Some types of protein can be unique contact points for the cell membrane and membrane. Inside the cell there is a cytoskeleton and an outer wall. Certain types of integral protein can be used as channels in ion transport receptors (in parallel with nerve endings).
If you use an electron microscope, you can obtain data on the basis of which you can construct a diagram of the structure of all parts of the cell, as well as the main components and membranes. The upper apparatus will consist of three subsystems:
- complex supramembrane inclusion;
- the supporting-contractile apparatus of the cytoplasm, which will have a submembrane part.
This apparatus includes the cytoskeleton of the cell. Cytoplasm with organelles and a nucleus is called the nuclear apparatus. The cytoplasmic or, in other words, plasma cell membrane is located under the cell membrane.
The word "membrane" comes from the Latin word membrum, which can be translated as "skin" or "sheath". The term was proposed more than 200 years ago and was more often used to refer to the edges of the cell, but during the period when the use of various electronic equipment began, it was established that plasma cytolemmas make up many different elements of the membrane.
Elements are most often structural, such as:
- mitochondria;
- lysosomes;
- plastids;
- partitions.
One of the first hypotheses regarding the molecular composition of the plasmalemma was put forward in 1940 by a British scientific institute. Already in 1960, William Roberts proposed the “Elementary Membrane” hypothesis to the world. She assumed that all cell plasmalemmas consist of certain parts and, in fact, are formed according to a general principle for all kingdoms of organisms.
In the early seventies of the 20th century, a lot of data was discovered, on the basis of which in 1972, scientists from Australia proposed a new mosaic-liquid model of cell structure.
Structure of the plasma membrane
The 1972 model is generally recognized to this day. That is, in modern science, various scientists working with the shell rely on the theoretical work “Structure of the biological membrane of the liquid-mosaic model.”
Protein molecules are associated with the lipid bilayer and completely penetrate the entire membrane - integral proteins (one of the common names is transmembrane proteins).
The shell contains various carbohydrate components that will look like a polysaccharide or saccharide chain. The chain, in turn, will be connected by lipids and protein. Chains connected by protein molecules are called glycoproteins, and by lipid molecules - glycosides. Carbohydrates are located on the outside of the membrane and function as receptors in animal cells.
Glycoprotein - represent a complex of supra-membrane functions. It is also called glycocalyx (from the Greek words glyk and kalix, which means “sweet” and “cup”). The complex promotes cell adhesion.
Functions of the plasma membrane
Barrier
Helps separate the internal components of the cell mass from those substances that are external. It protects the body from the entry of various substances that would be foreign to it, and helps maintain intracellular balance.
Transport
The cell has its own “passive transport” and uses it to reduce energy consumption. The transport function operates in the following processes:
- endocytosis;
- exocytosis;
- sodium and potassium metabolism.
On the outer side of the membrane there is a receptor, at the site of which mixing of hormones and various regulatory molecules occurs.
Passive transport- a process in which a substance passes through a membrane without expending energy. In other words, the substance is delivered from an area of the cell with a high concentration to the side where the concentration will be lower.
There are two types:
- Simple diffusion- is inherent in small neutral molecules H2O, CO2 and O2 and some hydrophobic organic substances with low molecular weight and, accordingly, pass through membrane phospholipids without problems. These molecules can penetrate the membrane until the concentration gradient is stable and unchanged.
- Facilitated diffusion- characteristic of various hydrophilic molecules. They can also pass through the membrane according to a concentration gradient. However, the process will be carried out with the help of various proteins, which will form specific channels of ionic compounds in the membrane.
Active transport- this is the movement of various components through the membrane wall as opposed to a gradient. Such transfer requires significant expenditure of energy resources in the cell. Most often, active transport is the main source of energy consumption.
There are several varieties active transport with the participation of carrier proteins:
- Sodium-potassium pump. Receipt of necessary minerals and trace elements by the cell.
- Endocytosis- a process in which the cell captures solid particles (phagocytosis) or various droplets of any liquid (pinocytosis).
- Exocytosis- a process in which certain particles are released from a cell into the external environment. The process is a counterbalance to endocytosis.
The term "endocytosis" comes from the Greek words "enda" (from within) and "ketosis" (cup, container). The process characterizes the capture of external compounds by the cell and is carried out during the production of membrane vesicles. This term was coined in 1965 by Christian Bayles, a professor of cytology in Belgium, who studied the uptake of various substances by mammalian cells, as well as phagocytosis and pinocytosis.
Phagocytosis
Occurs when a cell captures certain solid particles or living cells. And pinocytosis is the process by which droplets of liquid are captured by a cell. Phagocytosis (from the Greek words "devourer" and "receptacle") is the process by which very small living objects are captured and absorbed, as well as solid parts of various single-celled organisms.
The discovery of the process belongs to the physiologist from Russia - Vyacheslav Ivanovich Mechnikov, who determined the process itself, while he conducted various tests with starfish and tiny daphnia.
The nutrition of unicellular heterotrophic organisms is based on their ability to digest and also capture various particles.
Mechnikov described the algorithm for the absorption of bacteria by amoeba and the general principle of phagocytosis:
- adhesion - sticking of bacteria to the cell membrane;
- absorption;
- formation of a vesicle with a bacterial cell;
- uncorking the bottle.
Based on this, the process of phagocytosis consists of the following stages:
- The absorbed particle is attached to the membrane.
- Surrounding the absorbed particle with a membrane.
- Formation of a membrane vesicle (phagosome).
- Detachment of a membrane vesicle (phagosome) into the interior of the cell.
- Combination of phagosome and lysosome (digestion), as well as internal movement of particles.
Complete or partial digestion can be observed.
In the case of partial digestion, a residual body is most often formed, which will remain inside the cell for some time. Those residues that are undigested are removed (evacuated) from the cell by exocytosis. During the process of evolution, this phagocytosis predisposition function gradually became separated and passed from various single-celled cells to specialized cells (such as the digestive cell in coelenterates and sponges), and then to specialized cells in mammals and humans.
Lymphocytes and leukocytes in the blood are predisposed to phagocytosis. The process of phagocytosis itself requires large amounts of energy and is directly combined with the activity of the outer cell membrane and lysosome, where digestive enzymes are located.
Pinocytosis
Pinocytosis is the capture of a fluid containing various substances by the cell surface. The discovery of the phenomenon of pinocytosis belongs to the scientist Fitzgerald Lewis. This event took place in 1932.
Pinocytosis is one of the main mechanisms in which high-molecular compounds, for example, various glycoproteins or soluble proteins, enter the cell. Pinocytotic activity, in turn, is impossible without the physiological state of the cell and depends on its composition and the composition of the environment. We can observe the most active pinocytosis in amoeba.
In humans, pinocytosis is observed in intestinal cells, blood vessels, renal tubules, and also in growing oocytes. In order to depict the process of pinocytosis, which will be carried out using human leukocytes, a protrusion of the plasma membrane can be made. In this case, the parts will be unlaced and separated. The process of pinocytosis requires energy.
Stages of the pinocytosis process:
- Thin growths appear on the outer cellular plasmalemma, which surround droplets of liquid.
- This section of the outer shell becomes thinner.
- Formation of a membrane vesicle.
- The wall is breaking through (failing).
- The vesicle moves in the cytoplasm and can merge with various vesicles and organelles.
Exocytosis
The term comes from the Greek words “exo” - external, external and “cytosis” - vessel, cup. The process involves the release of certain particles by the cell into the external environment. The process of exocytosis is the opposite of pinocytosis.
During the process of ecocytosis, bubbles of intracellular fluid emerge from the cell and move to the outer membrane of the cell. The contents inside the vesicles can be released outside, and the cell membrane merges with the membrane of the vesicles. Thus, most macromolecular connections will occur in this manner.
Exocytosis performs a number of tasks:
- delivery of molecules to the outer cell membrane;
- transportation throughout the cell of substances that will be needed for growth and increasing the membrane area, for example, certain proteins or phospholipids;
- releasing or connecting various parts;
- removal of harmful and toxic products that appear during metabolism, for example, hydrochloric acid secreted by cells of the gastric mucosa;
- transport of pepsinogen, as well as signaling molecules, hormones or neurotransmitters.
Specific functions of biological membranes:
- generation of an impulse occurring at the nerve level, inside the neuron membrane;
- synthesis of polypeptides, as well as lipids and carbohydrates of the rough and smooth reticulum of the endoplasmic reticulum;
- change in light energy and its conversion into chemical energy.
Video
From our video you will learn a lot of interesting and useful things about the structure of a 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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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.
![](https://i0.wp.com/animals-world.ru//wp-content/uploads/2018/02/transport-cherez-membranu.jpg)
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. |
Cell membrane - molecular structure that consists of lipids and proteins. Its main properties and functions:
- separation of the contents of any cell from the external environment, ensuring its integrity;
- control and establishment of exchange between the environment and the cell;
- intracellular membranes divide the cell into special compartments: organelles or compartments.
The word "membrane" in Latin means "film". If we talk about the cell membrane, then it is a combination of two films that have different properties.
The biological membrane includes three types of proteins:
- Peripheral – located on the surface of the film;
- Integral – completely penetrate the membrane;
- Semi-integral - one end penetrates into the bilipid layer.
What functions does the cell membrane perform?
1. The cell wall is a durable cell membrane that is located outside the cytoplasmic membrane. It performs protective, transport and structural functions. Present in many plants, bacteria, fungi and archaea.
2. Provides a barrier function, that is, selective, regulated, active and passive metabolism with the external environment.
3. Capable of transmitting and storing information, and also takes part in the reproduction process.
4. Performs a transport function that can transport substances into and out of the cell through the membrane.
5. The cell membrane has one-way conductivity. Thanks to this, water molecules can pass through the cell membrane without delay, and molecules of other substances penetrate selectively.
6. With the help of the cell membrane, water, oxygen and nutrients are obtained, and through it the products of cellular metabolism are removed.
7. Performs cellular metabolism through membranes, and can perform them using 3 main types of reactions: pinocytosis, phagocytosis, exocytosis.
8. The membrane ensures the specificity of intercellular contacts.
9. The membrane contains numerous receptors that are capable of perceiving chemical signals - mediators, hormones and many other biological active substances. So it has the power to change the metabolic activity of the cell.
10. Basic properties and functions of the cell membrane:
- Matrix
- Barrier
- Transport
- Energy
- Mechanical
- Enzymatic
- Receptor
- Protective
- Marking
- Biopotential
What function does the plasma membrane perform in a cell?
- Delimits the contents of the cell;
- Carries out the entry of substances into the cell;
- Provides removal of a number of substances from the cell.
Cell membrane structure
Cell membranes include lipids of 3 classes:
- Glycolipids;
- Phospholipids;
- Cholesterol.
Basically, the cell membrane consists of proteins and lipids, and has a thickness of no more than 11 nm. From 40 to 90% of all lipids are phospholipids. It is also important to note glycolipids, which are one of the main components of the membrane.
The structure of the cell membrane is three-layered. In the center there is a homogeneous liquid bilipid layer, and proteins cover it on both sides (like a mosaic), partially penetrating into the thickness. Proteins are also necessary for the membrane to allow special substances into and out of cells that cannot penetrate the fat layer. For example, sodium and potassium ions.
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