Membranes perform a large number of different functions:
membranes determine the shape of an organelle or cell;
barrier: control the exchange of soluble substances (for example, Na +, K +, Cl - ions) between the internal and external compartments;
energy: ATP synthesis on the inner membranes of mitochondria and photosynthesis in the membranes of chloroplasts; form a surface for flow chemical reactions(phosphorylation on mitochondrial membranes);
are a structure that ensures the recognition of chemical signals (hormone and neurotransmitter receptors are located on the membrane);
play a role in intercellular interaction and promote cell movement.
Transport through the membrane. The membrane has selective permeability to soluble substances, which is necessary for:
separation of the cell from the extracellular environment;
ensuring the penetration into the cell and retention of necessary molecules (such as lipids, glucose and amino acids), as well as the removal of metabolic products (including unnecessary ones) from the cell;
maintaining a transmembrane ion gradient.
Intracellular organelles may also have a selectively permeable membrane. For example, in lysosomes the membrane maintains a concentration of hydrogen ions (H+) 1000-10000 times higher than in the cytosol.
Transport across the membrane can be passive, lightened or active.
Passive transport- this is the movement of molecules or ions along a concentration or electrochemical gradient. This may be simple diffusion, as in the case of penetration of gases (for example O 2 and CO 2) or simple molecules (ethanol) through the plasma membrane. In simple diffusion, small molecules dissolved in the extracellular fluid are successively dissolved in the membrane and then in the intracellular fluid. This process is nonspecific, and the rate of penetration through the membrane is determined by the degree of hydrophobicity of the molecule, that is, its fat solubility. The rate of diffusion through the lipid bilayer is directly proportional to hydrophobicity as well as to the transmembrane concentration gradient or electrochemical gradient.
Facilitated diffusion is the rapid movement of molecules across a membrane with the help of specific membrane proteins called permeases. This process is specific; it proceeds faster than simple diffusion, but has a transport speed limitation.
Facilitated diffusion is usually characteristic of water-soluble substances. Most (if not all) membrane transporters are proteins. The specific mechanism of transporter functioning during facilitated diffusion has not been sufficiently studied. They can, for example, provide transfer by rotational movement in the membrane. IN Lately Information has appeared that carrier proteins, upon contact with the transported substance, change their conformation, as a result of which a kind of “gate” or channel opens in the membrane. These changes occur due to the energy released when the transported substance binds to the protein. Relay-type transfers are also possible. In this case, the carrier itself remains motionless, and the ions migrate along it from one hydrophilic bond to another.
The antibiotic gramicidin can serve as a model for this type of vector. In the lipid layer of the membrane, its long linear molecule takes the shape of a helix and forms a hydrophilic channel through which the K ion can migrate along a gradient.
Experimental evidence was obtained for the existence of natural channels in biological membranes Oh. Transport proteins are highly specific for the substance transported through the membrane, resembling enzymes in many properties. They exhibit greater sensitivity to pH, are competitively inhibited by compounds similar in structure to the transported substance, and non-competitively by agents that change specifically functional groups of proteins.
Facilitated diffusion differs from ordinary diffusion not only in speed, but also in its ability to saturate. The increase in the rate of transfer of substances occurs in proportion to the increase in the concentration gradient only up to certain limits. The latter is determined by the “power” of the carrier.
Active transport is the movement of ions or molecules across a membrane against a concentration gradient due to the energy of ATP hydrolysis. There are three main types of active ion transport:
sodium-potassium pump - Na + /K + -adenosine triphosphatase (ATPase), which transports Na + out and K + in;
calcium (Ca 2+) pump - Ca 2+ -ATPase, which transports Ca 2+ from the cell or cytosol to the sarcoplasmic reticulum;
proton pump - H + -ATPase. The ion gradients created by active transport can be used for the active transport of other molecules, such as some amino acids and sugars (secondary active transport).
Cotransport is the transport of an ion or molecule coupled with the transfer of another ion. Simport- simultaneous transfer of both molecules in one direction; antiport- simultaneous transfer of both molecules in opposite directions. If transport is not associated with the transfer of another ion, this process is called uniport. Cotransport is possible both during facilitated diffusion and during active transport.
Glucose can be transported by facilitated diffusion using the symport type. Cl - and HCO 3 - ions are transported across the red blood cell membrane by facilitated diffusion by a carrier called band 3, an antiport type. In this case, Cl - and HCO 3 - are transferred in opposite directions, and the direction of transfer is determined by the prevailing concentration gradient.
Active transport of ions against a concentration gradient requires energy released during the hydrolysis of ATP to ADP: ATP ADP + P (inorganic phosphate). Active transport, as well as facilitated diffusion, is characterized by: specificity, limitation of the maximum speed (that is, the kinetic curve reaches a plateau) and the presence of inhibitors. An example is the primary active transport carried out by Na + /K + - ATPase. For the functioning of this enzyme antiport system, the presence of Na +, K + and magnesium ions is necessary. It is present in virtually all animal cells, and its concentration is especially high in excitable tissues (for example, nerves and muscles) and in cells that actively participate in the movement of Na + across the plasma membrane (for example, in the renal cortex and salivary glands) .
The ATPase enzyme itself is an oligomer consisting of 2 -subunits of 110 kDa and 2 glycoprotein -subunits of 55 kDa each.. during the hydrolysis of ATP, reversible phosphorylation of a certain aspartate residue on the -subunit occurs with the formation of -aspartamyl phosphate.. Phosphorylation requires Na + and Mg 2+ , but not K + , whereas dephosphorylation requires K + , but not Na + or Mg 2+ . Two conformational states of the protein complex with different energy levels are described, which are usually designated E 1 and E 2, therefore ATPase is also called type E vector 1 - E 2 . Cardiac glycosides, e.g. digoxin And ouabain, inhibit ATPase activity. Due to its excellent solubility in water, ouabain is widely used in experimental studies to study the sodium pump.
The generally accepted idea of how Na + /K + - ATPase works is as follows. Na and ATP ions join the ATPase molecule in the presence of Mg 2+. The binding of Na ions triggers the hydrolysis reaction of ATP, which results in the formation of ADP and the phosphorylated form of the enzyme. Phosphorylation induces a transition of the enzymatic protein to a new conformational state and the Na-bearing region or regions turn towards external environment. Here, Na + is exchanged for K + , since the phosphorylated form of the enzyme is characterized by a high affinity for K ions. The reverse transition of the enzyme to its original conformation is initiated by the hydrolytic elimination of the phosphoryl group in the form of inorganic phosphate and is accompanied by the release of K + into the internal space of the cell. The dephosphorylated active site of the enzyme is able to attach a new ATP molecule, and the cycle repeats.
The amounts of K and Na ions entering the cell as a result of the pump are not equal. For three removed Na ions, there are two introduced K ions with the simultaneous hydrolysis of one ATP molecule. The opening and closing of the channel on opposite sides of the membrane and the alternating change in the efficiency of Na and K binding are provided by the energy of ATP hydrolysis. The transported ions - Na and K - are cofactors of this enzymatic reaction. Theoretically, it is possible to imagine a variety of pumps operating on this principle, although only a few of them are currently known.
Glucose transport. Glucose transport can occur by type of either facilitated diffusion or active transport, and in the first case it proceeds as uniport, in the second - as symport. Glucose can be transported into red blood cells by facilitated diffusion. The Michaelis constant (Km) for glucose transport into red blood cells is approximately 1.5 mmol/L (that is, at this glucose concentration, about 50% of the available permease molecules will be bound to glucose molecules). Since the concentration of glucose in human blood is 4-6 mmol/l, its absorption by red blood cells occurs at almost maximum speed. The specificity of the permease is already manifested in the fact that the L-isomer is almost not transported into erythrocytes, unlike D-galactose and D-mannose, but higher concentrations are required to achieve half-saturation of the transport system. Once inside the cell, glucose undergoes phosphorylation and is no longer able to leave the cell. Glucose permease is also called D-hexose permease. It is an integral membrane protein with a molecular weight of 45 kDa.
Glucose can also be transported by a Na + -dependent symport system found in the plasma membranes of a number of tissues, including renal tubules and intestinal epithelium. In this case, one glucose molecule is transported by facilitated diffusion against the concentration gradient, and one Na ion is transported along the concentration gradient. The entire system ultimately functions through the pumping function of Na + /K + - ATPase. Thus, symport is a secondary active transport system. Amino acids are transported in a similar way.
Ca 2+ pump is an active transport system of the E 1 - E 2 type, consisting of an integral membrane protein, which, during the transfer of Ca 2+, is phosphorylated at the aspartate residue. During the hydrolysis of each ATP molecule, two Ca 2+ ions are transferred. In eukaryotic cells, Ca 2+ can bind to a calcium-binding protein called calmodulin, and the entire complex binds to the Ca 2+ pump. Ca 2+ -binding proteins also include troponin C and parvalbumin.
Ca ions, like Na ions, are actively removed from cells by Ca 2+ -ATPase. The membranes of the endoplasmic reticulum contain particularly large amounts of calcium pump protein. The chain of chemical reactions leading to ATP hydrolysis and Ca 2+ transfer can be written in the form of the following equations:
2Ca n + ATP + E 1 Ca 2 - E - P + ADP
Ca 2 - E - P 2Ca ext + PO 4 3- + E 2
Where is San - Ca2+ located outside;
Ca ext - Ca 2+ located inside;
E 1 and E 2 are different conformations of the transporter enzyme, the transition of which from one to another is associated with the use of ATP energy.
The system for the active removal of H + from the cytoplasm is supported by two types of reactions: the activity of the electron transport chain (redox chain) and ATP hydrolysis. Both redox and hydrolytic H + pumps are located in membranes capable of converting light or chemical energy into H + energy (that is, the plasma membranes of prokaryotes, the conjugating membranes of chloroplasts and mitochondria). As a result of the work of H + ATPase and/or the redox chain, protons are translocated, and a proton motive force (H +) appears on the membrane. The electrochemical gradient of hydrogen ions, as studies show, can be used for coupled transport (secondary active transport) large number metabolites - anions, amino acids, sugars, etc.
Associated with the activity of the plasma membrane are those that ensure the absorption of solid and liquid substances with a large molecular weight by the cell, - phagocytosis And pinocytosis(from Gerch. phagos- There is , pinos- drink, cytos- cell). The cell membrane forms pockets, or invaginations, that draw in substances from the outside. Then such invaginations are detached and surround a droplet of the external environment (pinocytosis) or solid particles (phagocytosis) with a membrane. Pinocytosis is observed in a wide variety of cells, especially in those organs where absorption processes occur.
1 – polar head of the phospholipid molecule
2 – fatty acid tail of the phospholipid molecule
3 – integral protein
4 – peripheral protein
5 – semi-integral protein
6 – glycoprotein
7 - glycolipid
The outer cell membrane is inherent in all cells (animal and plant), has a thickness of about 7.5 (up to 10) nm and consists of lipid and protein molecules.
Currently, the fluid-mosaic construction model is widespread. cell membrane. According to this model, lipid molecules are arranged in two layers, with their water-repellent ends (hydrophobic - fat-soluble) facing each other, and their water-soluble (hydrophilic) ends facing the periphery. Protein molecules are embedded in the lipid layer. Some of them are located on the outer or inner surface of the lipid part, others are partially submerged or penetrate the membrane through.
Functions of membranes :
Protective, border, barrier;
Transport;
Receptor - carried out due to proteins - receptors, which have a selective ability to certain substances (hormones, antigens, etc.), enter into interaction with them chemical interactions, conduct signals inside the cell;
Participate in the formation of intercellular contacts;
Provide movement of some cells (amoeba movement).
Animal cells have a thin layer of glycocalyx on top of the outer cell membrane. It is a complex of carbohydrates with lipids and carbohydrates with proteins. The glycocalyx is involved in intercellular interactions. The cytoplasmic membranes of most cell organelles have exactly the same structure.
U plant cells outside the cytoplasmic membrane. located cell wall, consisting of cellulose.
Transport of substances across the cytoplasmic membrane .
There are two main mechanisms for substances entering or exiting the cell:
1.Passive transport.
2.Active transport.
Passive transport of substances occurs without energy consumption. An example of such transport is diffusion and osmosis, in which the movement of molecules or ions occurs from an area of high concentration to an area of lower concentration, for example, water molecules.
Active transport - in this type of transport, molecules or ions penetrate the membrane against a concentration gradient, which requires energy. An example of active transport is the sodium-potassium pump, which actively pumps sodium out of the cell and absorbs potassium ions from the external environment, transporting them into the cell. The pump is a special membrane protein that drives ATP.
Active transport ensures the maintenance of constant cell volume and membrane potential.
Transport of substances can be carried out by endocytosis and exocytosis.
Endocytosis is the penetration of substances into the cell, exocytosis is from the cell.
During endocytosis, the plasma membrane forms invaginations or protrusions, which then envelop the substance and, when released, turn into vesicles.
There are two types of endocytosis:
1) phagocytosis - absorption of solid particles (phagocyte cells),
2) pinocytosis - absorption of liquid material. Pinocytosis is characteristic of amoeboid protozoa.
By exocytosis, various substances are removed from the cells: they are removed from the digestive vacuoles undigested remains food, their liquid secretion is removed from the secretory cells.
Cytoplasm –(cytoplasm + nucleus form protoplasm). Cytoplasm consists of a watery ground substance (cytoplasmic matrix, hyaloplasm, cytosol) and various organelles and inclusions contained in it.
Inclusions– waste products of cells. There are 3 groups of inclusions - trophic, secretory (gland cells) and special (pigment) significance.
Organelles – These are permanent structures of the cytoplasm that perform certain functions in the cell.
Organelles are isolated general meaning and special. Specials are found in most cells, but are present in significant quantities only in cells that perform a specific function. These include microvilli of intestinal epithelial cells, cilia of the epithelium of the trachea and bronchi, flagella, myofibrils (providing muscle contraction, etc.).
Organelles of general importance include the ER, Golgi complex, mitochondria, ribosomes, lysosomes, centrioles of the cell center, peroxisomes, microtubules, microfilaments. In plant cells there are plastids and vacuoles. Organelles of general importance can be divided into organelles having a membrane and non-membrane structure.
Organelles with a membrane structure are either double-membrane or single-membrane. Mitochondria and plastids are classified as double-membrane cells. Single-membrane cells include the endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and vacuoles.
Organelles that do not have membranes: ribosomes, cell center, microtubules, microfilaments.
Mitochondria – these are organelles of round or oval shape. They consist of two membranes: internal and external. The inner membrane has projections called cristae, which divide the mitochondria into compartments. The compartments are filled with a substance - matrix. The matrix contains DNA, mRNA, tRNA, ribosomes, calcium and magnesium salts. Autonomous protein biosynthesis occurs here. The main function of mitochondria is the synthesis of energy and its accumulation in ATP molecules. New mitochondria are formed in the cell as a result of the division of old ones.
Plastids – organelles found primarily in plant cells. They come in three types: chloroplasts, which contain a green pigment; chromoplasts (red, yellow, orange pigments); leucoplasts (colorless).
Chloroplasts, thanks to the green pigment chlorophyll, are able to synthesize organic matter from inorganic, using solar energy.
Chromoplasts give bright colors to flowers and fruits.
Leukoplasts are able to accumulate reserve nutrients: starch, lipids, proteins, etc.
Endoplasmic reticulum ( EPS ) is a complex system of vacuoles and channels that are bounded by membranes. There are smooth (agranular) and rough (granular) EPS. Smooth does not have ribosomes on its membrane. It contains the synthesis of lipids, lipoproteins, accumulation and removal of toxic substances from the cell. Granular ER has ribosomes on its membranes in which proteins are synthesized. The proteins then enter the Golgi complex and from there out.
Golgi complex (Golgi apparatus) It is a stack of flattened membrane sacs - cisterns and an associated system of bubbles. A stack of cisternae is called a dictyosome.
Functions of the Golgi complex : protein modification, polysaccharide synthesis, substance transport, cell membrane formation, lysosome formation.
Lysosomes – They are membrane-surrounded vesicles containing enzymes. They carry out intracellular breakdown of substances and are divided into primary and secondary. Primary lysosomes contain enzymes in an inactive form. After entering the organelles various substances Enzymes are activated and the digestion process begins - these are secondary lysosomes.
Peroxisomes have the appearance of bubbles bounded by one membrane. They contain enzymes that break down hydrogen peroxide, which is toxic to cells.
Vacuoles – These are organelles of plant cells containing cell sap. IN cell sap There may be spare nutrients, pigments, and waste products. Vacuoles participate in the creation of turgor pressure and in the regulation of water-salt metabolism.
Ribosomes – organelles consisting of large and small subunits. They can be located either on the ER or located freely in the cell, forming polysomes. They consist of rRNA and protein and are formed in the nucleolus. Protein biosynthesis occurs in ribosomes.
Cell center – found in animal cells, fungi, lower plants and is absent in higher plants. It consists of two centrioles and a radiate sphere. The centriole has the appearance of a hollow cylinder, the wall of which consists of 9 triplets of microtubules. When cells divide, they form mitotic spindle threads, which ensure the separation of chromatids in anaphase of mitosis and homologous chromosomes during meiosis.
Microtubules – tubular formations of various lengths. They are part of centrioles, mitotic spindles, flagella, cilia, perform a supporting function, and promote the movement of intracellular structures.
Microfilaments – filamentous thin formations located throughout the cytoplasm, but there are especially many of them under the cell membrane. Together with microtubules, they form the cell cytoskeleton, determine the flow of cytoplasm, intracellular movements of vesicles, chloroplasts and other organelles.
Cell evolution
There are two stages in the evolution of a cell:
1. Chemical.
2.Biological.
The chemical stage began about 4.5 billion years ago. Under the influence ultraviolet radiation, radiation, lightning discharges (energy sources), the formation of simple chemical compounds– monomers, and then more complex ones – polymers and their complexes (carbohydrates, lipids, proteins, nucleic acids).
The biological stage of cell formation begins with the appearance of probionts - isolated complex systems capable of self-reproduction, self-regulation and natural selection. Probionts appeared 3-3.8 billion years ago. The first prokaryotic cells, bacteria, originated from probionts. Eukaryotic cells evolved from prokaryotes (1-1.4 billion years ago) in two ways:
1) Through symbiosis of several prokaryotic cells - this is a symbiotic hypothesis;
2) By invagination of the cell membrane. The essence of the invagination hypothesis is that the prokaryotic cell contained several genomes attached to the cell wall. Then invagination occurred - invagination, unlacing of the cell membrane, and these genomes turned into mitochondria, chloroplasts, and the nucleus.
Cell differentiation and specialization .
Differentiation is the formation of different types of cells and tissues during the development of a multicellular organism. One hypothesis links differentiation to gene expression during individual development. Expression is the process of turning on certain genes into work, which creates conditions for the targeted synthesis of substances. Therefore, tissues develop and specialize in one direction or another.
Related information.
Cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from the external environment, but is also part of most cellular organelles and the nucleus, in turn separating them from the hyaloplasm (cytosol) - the viscous-liquid part of the cytoplasm. Let's agree to call cyto plasma membrane the one that separates the contents of the cell from the external environment. The remaining terms denote all membranes.
The structure of the cellular (biological) membrane is based on a double layer of lipids (fats). The formation of such a layer is associated with the characteristics of their molecules. Lipids do not dissolve in water, but condense in it in their own way. One part of a single lipid molecule is a polar head (it is attracted to water, i.e. hydrophilic), and the other is a pair of long non-polar tails (this part of the molecule is repelled by water, i.e. hydrophobic). This structure of molecules causes them to “hide” their tails from the water and turn their polar heads towards the water.
The result is a lipid bilayer in which the nonpolar tails are inward (facing each other) and the polar heads are outward (toward the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.
In cell membranes, phospholipids predominate among the lipids (they belong to complex lipids). Their heads contain a phosphoric acid residue. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (related to sterols). The latter imparts rigidity to the membrane, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).
Due to electrostatic interaction, some protein molecules are attached to the charged lipid heads, which become surface membrane proteins. Other proteins interact with nonpolar tails, are partially buried in the bilayer, or penetrate through it.
Thus, the cell membrane consists of a bilayer of lipids, surface (peripheral), embedded (semi-integral) and permeating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.
This fluid mosaic model of membrane structure was put forward in the 70s of the XX century. Previously, a sandwich model of structure was assumed, according to which the lipid bilayer is located inside, and on the inside and outside the membrane is covered with continuous layers of surface proteins. However, the accumulation of experimental data refuted this hypothesis.
The thickness of membranes in different cells is about 8 nm. Membranes (even different sides one) differ from each other in percentage various types lipids, proteins, enzymatic activity etc. Some membranes are more liquid and more permeable, others are more dense.
Cell membrane breaks easily merge due to the physicochemical properties of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are anchored by the cytoskeleton) move.
Functions of the cell membrane
Most proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are located in a certain sequence so that the reaction products catalyzed by one enzyme pass to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow the enzymes to float along the lipid bilayer.
The cell membrane performs a delimiting (barrier) function from the environment and at the same time transport functions. We can say that this is its most important purpose. The cytoplasmic membrane, having strength and selective permeability, maintains the constancy of the internal composition of the cell (its homeostasis and integrity).
In this case, the transport of substances occurs different ways. Transport along a concentration gradient involves the movement of substances from an area with a higher concentration to an area with a lower one (diffusion). For example, gases (CO 2 , O 2 ) diffuse.
There is also transport against a concentration gradient, but with energy consumption.
Transport can be passive and facilitated (when it is helped by some kind of carrier). Passive diffusion across the cell membrane is possible for fat-soluble substances.
There are special proteins that make membranes permeable to sugars and other water-soluble substances. Such carriers bind to transported molecules and pull them through the membrane. This is how glucose is transported inside red blood cells.
Threading proteins combine to form a pore for the movement of certain substances across the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. Transfer occurs due to a change in protein conformation, resulting in the formation of channels in the membrane. An example is the sodium-potassium pump.
The transport function of the eukaryotic cell membrane is also realized through endocytosis (and exocytosis). Thanks to these mechanisms, large molecules of biopolymers, even whole cells, enter the cell (and out of it). Endo- and exocytosis are not characteristic of all eukaryotic cells (prokaryotes do not have it at all). Thus, endocytosis is observed in protozoa and lower invertebrates; in mammals, leukocytes and macrophages absorb harmful substances and bacteria, i.e. endocytosis performs a protective function for the body.
Endocytosis is divided into phagocytosis(cytoplasm envelops large particles) and pinocytosis(capturing droplets of liquid with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the surface of cells are surrounded by a membrane. A vesicle (phagocytic or pinocytic) is formed, which then moves into the cell.
Exocytosis is the removal of substances from the cell (hormones, polysaccharides, proteins, fats, etc.) by the cytoplasmic membrane. These substances are contained in membrane vesicles that fit the cell membrane. Both membranes merge and the contents appear outside the cell.
The cytoplasmic membrane performs a receptor function. To do this, structures are located on its outer side that can recognize a chemical or physical stimulus. Some of the proteins that penetrate the plasmalemma are connected from the outside to polysaccharide chains (forming glycoproteins). These are peculiar molecular receptors that capture hormones. When a particular hormone binds to its receptor, it changes its structure. This in turn triggers the cellular response mechanism. In this case, channels can open, and certain substances can begin to enter or exit the cell.
The receptor function of cell membranes has been well studied based on the action of the hormone insulin. When insulin binds to its glycoprotein receptor, the catalytic intracellular part of this protein (adenylate cyclase enzyme) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or suppresses various enzymes of cellular metabolism.
The receptor function of the cytoplasmic membrane also includes recognition of neighboring cells of the same type. Such cells are attached to each other by various intercellular contacts.
In tissues, with the help of intercellular contacts, cells can exchange information with each other using specially synthesized low-molecular substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that free space is occupied.
Intercellular contacts can be simple (the membranes of different cells are adjacent to each other), locking (invaginations of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers that penetrate the cytoplasm). In addition, there is a variant of intercellular contacts due to mediators (intermediaries) - synapses. In them, the signal is transmitted not only chemically, but also electrically. Synapses transmit signals between nerve cells, as well as from nervous to muscular.
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 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 necessary cage substances. This phenomenon was called pinocytosis.
The outer membranes of different cells differ significantly in both chemical composition their proteins and lipids, and by 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 they are carried out Various types intercellular contacts, i.e. communication between individual cells.
Many types of cells are characterized by the presence on their surface large quantity protrusions, folds, 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. |
