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 cytoplasmic 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.
As a result, a lipid bilayer is formed 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 of one) differ from each other in the percentage of different types of 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 in various 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 nerve to muscle cells.
Cytoplasmic membrane (plasmalemma)- the main part of the surface apparatus, universal for all cells. Its thickness is about 10 nm. The plasmalemma limits the cytoplasm and protects it from external influences, and takes part in metabolic processes between the cell and the extracellular environment.
The main components of the membrane are lipids and proteins. Lipids make up about 40% of the mass of membranes. Phospholipids predominate among them.
Phospholipid molecules are arranged in a double layer (lipid bilayer). As you already know, each phospholipid molecule is formed by a polar hydrophilic head and non-polar hydrophobic tails. In the cytoplasmic membrane, the hydrophilic heads face the outer and inner sides of the membrane, and the hydrophobic tails face the inside of the membrane (Fig. 30).
In addition to lipids, membranes contain two types of proteins: integral and peripheral. Integral proteins are more or less deeply immersed in the membrane or penetrate through it. Peripheral proteins are located on the outer and inner surfaces of the membrane, and many of them ensure the interaction of the plasmalemma with supramembrane and intracellular structures.
Oligo- and polysaccharide molecules can be located on the outer surface of the cytoplasmic membrane. They covalently bind to membrane lipids and proteins, forming glycolipids and glycoproteins. In animal cells, such a carbohydrate layer covers the entire surface of the plasma membrane, forming a supramembrane complex. It is called glycocalyx(from lat. glycis — sweet, kalyum- thick skin).
Functions of the cytoplasmic membrane. The plasma membrane performs a number of functions, the most important of which are barrier, receptor and transport.
Barrier function. The cytoplasmic membrane surrounds the cell on all sides, playing the role of a barrier - an obstacle between the complexly organized intracellular contents and the extracellular environment. The barrier function is provided, first of all, by the lipid bilayer, which does not allow the cell contents to spread and prevents the penetration of foreign substances into the cell.
Receptor function. The cytoplasmic membrane contains proteins that are capable of changing their spatial structure in response to various environmental factors and thus transmitting signals into the cell. Consequently, the cytoplasmic membrane provides cell irritability (the ability to perceive stimuli and respond to them in a certain way), exchanging information between the cell and the environment.
Some receptor proteins of the cytoplasmic membrane are able to recognize certain substances and specifically bind to them. Such proteins may be involved in the selection of necessary molecules entering cells.
Receptor proteins include, for example, antigen recognition receptors of lymphocytes, hormone and neurotransmitter receptors, etc. In the implementation of receptor function, in addition to membrane proteins, elements of the glycocalyx play an important role.
The diversity and specificity of sets of receptors on the surface of cells leads to the creation of a complex system of markers that make it possible to distinguish s.self:/ cells (of the same individual or the same species) from s.foreign:/ cells. Thanks to this, cells can interact with each other (for example, conjugation in bacteria, tissue formation in animals).
Specific receptors that respond to various physical factors can be localized in the cytoplasmic membrane. For example, in the plasmalemma of light-sensitive animal cells there is a special photoreceptor system, the key role in the functioning of which is played by the visual pigment rhodopsin. With the help of photoreceptors, the light signal is converted into a chemical signal, which, in turn, leads to the emergence of a nerve impulse.
Transport function. One of the main functions of the plasmalemma is to ensure the transport of substances both into the cell and out of it into the extracellular environment. There are several main methods of transport of substances across the cytoplasmic membrane: simple diffusion, facilitated diffusion, active transport and transport in membrane packaging (Fig. 31).
With simple diffusion, spontaneous movement of substances across a membrane is observed from an area where the concentration of these substances is higher to an area where their concentration is lower. By simple diffusion, small molecules (for example, H 2 0, 0 2, CO 2, urea) and ions can pass through the plasmalemma. As a rule, nonpolar substances are transported directly through the lipid bilayer, and polar molecules and ions are transported through channels formed by special membrane proteins. Simple diffusion occurs relatively slowly. To accelerate diffuse transport, there are membrane carrier proteins. They selectively bind to one or another ion or molecule and transport them across the membrane. This type of transport is called facilitated diffusion. The rate of substance transfer during facilitated diffusion is many times higher than during simple diffusion.
Diffusion (simple and facilitated) are types of passive transport. It is characterized by the fact that substances are transported through the membrane without energy expenditure and only in the direction where there is a lower concentration of these substances.
Active transport is the transfer of substances across a membrane from an area of low concentration of these substances to an area of higher concentration. For this purpose, the membrane contains special pumps that operate using energy (see Fig. 31). Most often, ATP energy is used to operate membrane pumps.
One of the most common membrane pumps is the sodium-potassium AT Phase (Na + /K + - AT Phase). It removes Na + ions from the cell and pumps K + ions into it. To work, Na + /K + -ATPase uses the energy released during the hydrolysis of ATP. Thanks to this pump, the difference between the concentrations of Na + and K + in the cell and the extracellular environment is maintained, which underlies many bioelectrical and transport processes.
As a result of active transport with the help of membrane pumps, the content of Mgr +, Ca 2+ and other ions in the cell is also regulated.
By active transport, not only ions, but also monosaccharides, amino acids, and other low-molecular substances can move across the cytoplasmic membrane.
A unique and relatively well-studied type of membrane transport is membrane-packed transport. Depending on the direction in which substances are transported (into or out of the cell), two types of this transport are distinguished - endocytosis and exocytosis.
Endocytosis (Greek. endon- inside, kitos- cell, cell) - absorption of external particles by a cell through the formation of membrane vesicles. During endocytosis, a certain area of the plasmalemma envelops extracellular material and captures it, enclosing it in a membrane package (Fig. 32).
There are such types of endocytosis as phagocytosis (capture and absorption of solid particles) and pinocytosis (absorption of liquid).
Through endocytosis, heterotrophic protists feed, the body’s defense reactions (absorption of foreign particles by leukocytes), etc.
Exocytosis (from Greek. exo- outside) - transportation of substances enclosed in membrane packaging from the cell to the external environment. For example, the Golgi complex vesicle moves to the cytoplasmic membrane and fuses with it, and the contents of the vesicle are released into the extracellular environment. In this way, cells secrete digestive enzymes, hormones and other substances.
1. Is it possible to see the plasmalemma with a light microscope? What are the chemical composition and structure of the cytoplasmic membrane?
2. What is a glycocalyx? What cells is it characteristic of?
3. List and explain the main functions of the plasmalemma.
4. In what ways can substances be transported across a membrane? What is the fundamental difference between passive transport and active transport?
5. How do the processes of phagocytosis and pinocytosis differ? What are the similarities between these processes?
6. Compare the different types of transport of substances into the cell. Indicate their similarities and differences.
7. What functions could not be performed by the cytoplasmic membrane if it did not contain proteins? Justify your answer.
8. Some substances (for example, diethyl ether, chloroform) penetrate biological membranes even faster than water, although their molecules are much larger than water molecules. What is this connected with?
- § 1. Content of chemical elements in the body. Macro- and microelements
- § 2. Chemical compounds in living organisms. Inorganic substances
- § 10. History of the discovery of the cell. Creation of cell theory
- § 15. Endoplasmic reticulum. Golgi complex. Lysosomes
- § 24. General characteristics of metabolism and energy conversion
Chapter 1. Chemical components of living organisms
Chapter 2. Cell - structural and functional unit of living organisms
Chapter 3. Metabolism and energy conversion in the body
Chapter 4. Structural organization and regulation of functions in living organisms
General information about the eukaryotic cell
Each eukaryotic cell has a separate nucleus, which contains genetic material delimited from the matrix by the nuclear membrane (this is the main difference from prokaryotic cells). The genetic material is concentrated mainly in the form of chromosomes, which have a complex structure and consist of strands of DNA and protein molecules. Cell division occurs through mitosis (and for germ cells, meiosis). Eukaryotes include both unicellular and multicellular organisms.
There are several theories of the origin of eukaryotic cells, one of them is endosymbiontic. An aerobic cell of the bacterial-like type penetrated into the heterotrophic anaerobic cell, which served as the basis for the appearance of mitochondria. Spirochete-like cells began to penetrate these cells, which gave rise to the formation of centrioles. The hereditary material was separated from the cytoplasm, a nucleus appeared, and mitosis appeared. Some eukaryotic cells were invaded by cells such as blue-green algae, which gave rise to chloroplasts. This is how the plant kingdom subsequently arose.
The sizes of human body cells vary from 2-7 microns (for platelets) to gigantic sizes (up to 140 microns for an egg).
The shape of the cells is determined by the function they perform: nerve cells are stellate due to the large number of processes (axons and dendrites), muscle cells are elongated because they must contract, red blood cells can change their shape as they move through small capillaries.
The structure of eukaryotic cells of animal and plant organisms is largely similar. Each cell is bounded on the outside by a cell membrane, or plasmalemma. It consists of a cytoplasmic membrane and a layer glycocalyx(10-20 nm thick), which covers it from the outside. The components of the glycocalyx are complexes of polysaccharides with proteins (glycoproteins) and fats (glycolipids).
The cytoplasmic membrane is a complex of a bilayer of phospholipids with proteins and polysaccharides.
In the cell they secrete nucleus and cytoplasm. The cell nucleus consists of a membrane, nuclear sap, nucleolus and chromatin. The nuclear envelope consists of two membranes separated by a perinuclear space and is permeated with pores.
The basis of the nuclear juice (matrix) is made up of proteins: filamentous, fibrillar (supporting function), globular, heteronuclear RNA and mRNA (result of processing).
Nucleolus is the structure where the formation and maturation of ribosomal RNA (r-RNA) occurs.
Chromatin in the form of clumps, it is scattered in the nucleoplasm and is a nitrogen-phase form of chromosome existence.
The cytoplasm contains the main substance (matrix, hyaloplasm), organelles and inclusions.
Organelles can be of general importance and special (in cells that perform specific functions: microvilli of the intestinal absorptive epithelium, myofibrils of muscle cells, etc.).
Organelles of general importance are the endoplasmic reticulum (smooth and rough), the Golgi complex, mitochondria, ribosomes and polysomes, lysosomes, peroxisomes, microfibrils and microtubules, centrioles of the cell center.
Plant cells also contain chloroplasts, in which photosynthesis occurs.
The elementary membrane consists of a bilayer of lipids in complex with proteins (glycoproteins: proteins + carbohydrates, lipoproteins: fats + proteins). Lipids include phospholipids, cholesterol, glycolipids (carbohydrates + fats), and lipoproteins. Each fat molecule has a polar hydrophilic head and a non-polar hydrophobic tail. In this case, the molecules are oriented so that the heads face outward and inside the cell, and the non-polar tails face inside the membrane itself. This achieves selective permeability for substances entering the cell.
There are peripheral proteins (they are located only on the inner or outer surface of the membrane), integral (they are firmly embedded in the membrane, immersed in it, and are able to change their position depending on the state of the cell). Functions of membrane proteins: receptor, structural (maintain the shape of the cell), enzymatic, adhesive, antigenic, transport.
The structure of the elementary membrane is liquid-mosaic: fats make up a liquid-crystalline frame, and proteins are mosaically built into it and can change their position.
The most important function: promotes compartmentation - the division of cell contents into separate cells that differ in the details of their chemical or enzymatic composition. This achieves high orderliness of the internal contents of any eukaryotic cell. Compartmentation promotes spatial separation of processes occurring in the cell. A separate compartment (cell) is represented by some membrane organelle (for example, a lysosome) or its part (cristae delimited by the inner membrane of mitochondria).
Other features:
1) barrier (delimitation of the internal contents of the cell);
2) structural (giving a certain shape to cells in accordance with the functions they perform);
3) protective (due to selective permeability, reception and antigenicity of the membrane);
4) regulatory (regulation of selective permeability for various substances (passive transport without energy consumption according to the laws of diffusion or osmosis and active transport with energy consumption by pinocytosis, endo- and exocytosis, sodium-potassium pump, phagocytosis));
5) adhesive function (all cells are connected to each other through specific contacts (tight and loose));
6) receptor (due to the work of peripheral membrane proteins). There are nonspecific receptors that perceive several stimuli (for example, cold and heat thermoreceptors), and specific ones that perceive only one stimulus (receptors of the light-perceiving system of the eye);
7) electrogenic (change in the electrical potential of the cell surface due to the redistribution of potassium and sodium ions (the membrane potential of nerve cells is 90 mV));
8) antigenic: associated with glycoproteins and polysaccharides of the membrane. On the surface of each cell there are protein molecules that are specific only to this type of cell. With their help, the immune system is able to distinguish between its own and foreign cells.
Cytoplasmic membrane
Image of a cell membrane. The small blue and white balls correspond to the hydrophilic heads of the lipids, and the lines attached to them correspond to the hydrophobic tails. The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - chains of oligosaccharides forming the glycocalyx
A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end in the outer or inner lipid layer), surface (located on the outer or adjacent to the inner sides of the membrane). Some proteins are the points of contact between the cell membrane and the cytoskeleton inside the cell, and the cell wall (if there is one) outside. Some of the integral proteins function as ion channels, various transporters and receptors.
Functions of biomembranes
- barrier - ensures regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of a membrane to different atoms or molecules depends on their size, electrical charge and chemical properties. Selective permeability ensures that the cell and cellular compartments are separated from the environment and supplied with the necessary substances.
- transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of the appropriate pH and ionic concentration in the cell, which are necessary for the functioning of cellular enzymes.
Particles that for some reason are not able to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane inside is hydrophobic and does not allow hydrophilic substances to pass through, or due to their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis.
During passive transport, substances cross the lipid bilayer without energy consumption, by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through.
Active transport requires energy as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K+) into the cell and pumps sodium ions (Na+) out of it.
- matrix - ensures a certain relative position and orientation of membrane proteins, their optimal interaction;
- mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play a major role in ensuring mechanical function, and in animals, the intercellular substance.
- energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;
- receptor - some proteins sitting in the membrane are receptors (molecules with the help of which the cell perceives certain signals).
For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemical substances that ensure the conduction of nerve impulses) also bind to special receptor proteins in target cells.
- enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
- implementation of generation and conduction of biopotentials.
With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K+ ion inside the cell is much higher than outside, and the concentration of Na+ is much lower, which is very important, since this ensures the maintenance of the potential difference on the membrane and the generation of a nerve impulse.
- cell marking - there are antigens on the membrane that act as markers - “labels” that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of “antennas”. Because of the myriad configurations of side chains, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, in the formation of organs and tissues. This also allows the immune system to recognize foreign antigens.
Structure and composition of biomembranes
Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. Cholesterol gives the membrane rigidity by occupying the free space between the hydrophobic tails of lipids and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, and those with a high cholesterol content are more rigid and fragile. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from the cell and into the cell. An important part of the membrane consists of proteins that penetrate it and are responsible for the various properties of membranes. Their composition and orientation differ in different membranes.
Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.
Membrane organelles
These are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to double membranes - nucleus, mitochondria, plastids. The outside of the cell is bounded by the so-called plasma membrane. The structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.
Selective permeability
Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves, to a certain extent, actively regulate this process - some substances pass through, but others do not. There are four main mechanisms for the entry of substances into the cell or out of the cell: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, i.e. do not require energy consumption; The last two are active processes associated with energy consumption.
The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through, forming a kind of passage. The elements K, Na and Cl have their own channels. Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open and a sudden influx of sodium ions into the cell occurs. In this case, an imbalance of membrane potential occurs. After which the membrane potential is restored. Potassium channels are always open, allowing ions to slowly enter the cell
Any living cell is separated from the environment by a thin membrane of a special structure - the cytoplasmic membrane (CPM). Eukaryotes have numerous intracellular membranes that separate the organelle space from the cytoplasm, while for most prokaryotes the CPM is the only cell membrane. In some bacteria and archaea, it can penetrate into the cytoplasm, forming outgrowths and folds of various shapes.
The CPM of any cells are built according to a single plan and consist of phospholipids (Fig. 3.5, A). In bacteria, they contain two fatty acids, usually with 16-18 carbon atoms in the chain and with saturated or one unsaturated bonds, connected by an ester bond to two hydroxyl groups of glycerol. The fatty acid composition of bacteria can vary in response to environmental changes, particularly temperature. When the temperature decreases, the amount of unsaturated fatty acids in the composition of phospholipids increases, which significantly affects the fluidity of the membrane. Some fatty acids may be branched or contain a cyclopropane ring. The third OH group of glycerol is connected to the phosphoric acid residue and through it to the head group. The head groups of phospholipids may have different chemical natures in different prokaryotes (phosphatidylethanolamine, phosphatidylglycerol, cardiolipin, phosphatidylserine, lecithin, etc.), but they are simpler in structure than in eukaryotes. For example, at E. coli, they are represented by 75% phosphatidylethanolamine, 20% phosphatidylglycerol, the rest consist of cardiolipin (diphosphatidylglycerol), phosphatidylserine and trace amounts of other compounds. Other bacteria have more complex types of membrane lipids. Some cells form glycolipids such as monogalactosyl diglyceride. Archaeal membrane lipids differ from eukaryotic and bacterial ones. Instead of fatty acids, they contain higher isoprenoid alcohols attached to glycerol by a simple rather than an ester bond.
Rice. 3.5.
A- phospholipid; b- bilayer membrane
O O o O o o
Such molecules make up a membrane bilayer, where the hydrophobic parts face inward, and the hydrophilic parts face outward, into the environment and into the cytoplasm (Fig. 3.5, b). Numerous proteins are embedded in or intersect the bilayer and can diffuse within the membrane, sometimes forming complex complexes. Membrane proteins have a number of important functions, including the conversion and storage of metabolic energy, regulation of the absorption and release of all nutrients and metabolic products. In addition, they recognize and transmit many signals reflecting changes in the environment and trigger the corresponding cascade of reactions leading to a cellular response. This organization of membranes is well explained by the liquid crystal model with a mosaic interspersed with membrane proteins (Fig. 3.6).
Rice. 3.6.
Most biological membranes have a thickness of 4 to 7 nm. Cell membranes are clearly visible in a transmission electron microscope when contrasted with heavy metals. In electron micrographs they look like three-layer formations: two outer dark layers show the position of the polar groups of lipids, and the light middle layer shows the hydrophobic inner space (Fig. 3.7).
Another technique for studying membranes is to obtain cleaved cells frozen at liquid nitrogen temperature and contrast the resulting surfaces by sputtering heavy metals
(platinum, gold, silver). The resulting preparations are viewed under a scanning electron microscope. In this case, one can see the surface of the membrane and the mosaic membrane proteins included in it, which do not extend through the membrane, but are connected by special hydrophobic anchor regions to the hydrophobic region of the bilayer.
Rice. 3.7.
The CPM has the property of selective permeability, preventing the free movement of most substances in and out of the cell, and also plays a significant role in cell growth and division, movement, and the export of surface and extracellular proteins and carbohydrates (exopolysaccharides). If a cell is placed in an environment with a higher or lower osmotic pressure than inside the cytoplasm, then water will leave the cell or water will enter it. This reflects the property of water to equalize solution gradients. In this case, the cytoplasm contracts or expands (the phenomenon of plasmolysis/deplasmolysis). Most bacteria, however, do not change their shape in such experiments due to the presence of a rigid cell wall.
The CPM regulates the flow of nutrients and metabolites. The presence of a hydrophobic layer formed by membrane lipids prevents the passage of any polar molecules and macromolecules through it. This property allows cells, which generally exist in dilute solutions, to retain useful macromolecules and metabolic precursors. The cell membrane is also designed to carry out a transport function. Typically, prokaryotes have a large number of very specific transport systems. Transport is an integral part of the overall bioenergetics of the cell, which creates and uses various ionic gradients through the CPM to transport substances and form other gradients necessary for the cell. The CPM plays a significant role in cell movement, growth and division. Many metabolic processes are concentrated in the membrane of prokaryotes. Membrane proteins perform important functions: they participate in the transformation and storage of energy, regulate the absorption and release of all nutrients and metabolic products, recognize and transmit signals about changes in the environment.