The cell membrane is the structure that covers the outside of the cell. It is also called cytolemma or plasmalemma.

This formation is built from a bilipid layer (bilayer) with proteins built into it. The carbohydrates that make up the plasmalemma are in a bound state.

The distribution of the main components of the plasmalemma is as follows: more than half of the chemical composition is proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.

Cell membrane and its types

The cell membrane is a thin film, the basis of which is made up of layers of lipoproteins and proteins.

According to localization, membrane organelles are distinguished, which have some features in plant and animal cells:

• mitochondria;
• core;
• endoplasmic reticulum;
• Golgi complex;
• lysosomes;
• chloroplasts (in plant cells).

There is also an inner and outer (plasmolemma) cell membrane.

Structure of the cell membrane

The cell membrane contains carbohydrates that cover it in the form of a glycocalyx. This is a supra-membrane structure that performs a barrier function. The proteins located here are in a free state. Unbound proteins participate in enzymatic reactions, providing extracellular breakdown of substances.

Proteins of the cytoplasmic membrane are represented by glycoproteins. By chemical composition secrete proteins included in the lipid layer completely (along its entire length) - integral proteins. Also peripheral, not reaching one of the surfaces of the plasmalemma.

The former function as receptors, binding to neurotransmitters, hormones and other substances. Insertion proteins are necessary for the construction of ion channels through which the transport of ions and hydrophilic substrates occurs. The latter are enzymes that catalyze intracellular reactions.

Basic properties of the plasma membrane

The lipid bilayer prevents the penetration of water. Lipids are hydrophobic compounds represented in the cell by phospholipids. The phosphate group faces outward and consists of two layers: the outer one, directed to the extracellular environment, and the inner one, delimiting the intracellular contents.

Water-soluble areas are called hydrophilic heads. The fatty acid sites are directed into the cell, in the form of hydrophobic tails. The hydrophobic part interacts with neighboring lipids, which ensures their attachment to each other. The double layer has selective permeability in different areas.

So, in the middle the membrane is impermeable to glucose and urea; hydrophobic substances pass through here freely: carbon dioxide, oxygen, alcohol. Cholesterol is important; the content of the latter determines the viscosity of the plasmalemma.

Functions of the outer cell membrane

The characteristics of the functions are briefly listed in the table:

Membrane function	Description
Barrier role	The plasmalemma performs a protective function, protecting the contents of the cell from the effects of foreign agents. Thanks to the special organization of proteins, lipids, and carbohydrates, the semipermeability of the plasmalemma is ensured.
Receptor function	Activation occurs biologically through the cell membrane active substances in the process of binding to receptors. Thus, immune reactions are mediated through the recognition of foreign agents by the cell receptor apparatus localized on the cell membrane.
Transport function	The presence of pores in the plasmalemma allows you to regulate the flow of substances into the cell. The transfer process occurs passively (without energy consumption) for compounds with low molecular weight. Active transport is associated with the expenditure of energy released during the breakdown of adenosine triphosphate (ATP). This method takes place for the transfer of organic compounds.
Participation in digestive processes	Substances are deposited on the cell membrane (sorption). Receptors bind to the substrate, moving it into the cell. A bubble is formed, lying freely inside the cell. Merging, such vesicles form lysosomes with hydrolytic enzymes.
Enzymatic function	Enzymes are essential components of intracellular digestion. Reactions requiring the participation of catalysts occur with the participation of enzymes.

What is the importance of the cell membrane

The cell membrane is involved in maintaining homeostasis due to the high selectivity of substances entering and leaving the cell (in biology this is called selective permeability).

Outgrowths of the plasmalemma divide the cell into compartments (compartments) responsible for performing certain functions. Specifically designed membranes corresponding to the fluid-mosaic pattern ensure the integrity of the cell.

Cell membrane

Image cell membrane. The small blue and white balls correspond to the hydrophobic heads of the phospholipids, and the lines attached to them correspond to the hydrophilic 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 into the outer or inner lipid layer), surface (located on the outer or adjacent to internal sides membranes). 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

• 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 the separation of the cell and cellular compartments from the environment and supply them with the necessary substances.
• transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of optimal ion concentrations in the cell that are necessary for the functioning of cellular enzymes.
  Particles that for any reason are unable 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.
  In passive transport, substances cross the lipid bilayer without expending energy along a concentration gradient 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). Greater role in ensuring mechanical function have cell walls, and in animals - 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 located 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 ( chemicals, ensuring the conduction of nerve impulses) also bind to special receptor proteins of 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 side chain configurations, 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 free space between the hydrophobic tails of the 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 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 their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; 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 right 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 potassium ions to slowly enter the cell.

See also

Literature

• Antonov V.F., Smirnova E.N., Shevchenko E.V. Lipid membranes during phase transitions. - M.: Science, 1994.
• Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M.: Mir, 1997. - ISBN 5-03-002419-0
• Ivanov V. G., Berestovsky T. N. Lipid bilayer of biological membranes. - M.: Nauka, 1982.
• Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, corrected and expanded. - M.: Moscow University Publishing House, 2004. -

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.

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 with outside membranes 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 arranged 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, which is stabilized by 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 internal composition cells (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 concentration (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 by the cytoplasmic membrane (hormones, polysaccharides, proteins, fats, etc.). These substances are contained in membrane vesicles that approach 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 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 include nucleic acids, polysaccharides and other substances.

Physical properties

At normal temperature The membranes are in a liquid crystalline state and constantly oscillate. 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, occurs.

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 the photosynthetic membranes of chloroplasts are built-in chlorophyll molecules.

Animal cells have on the surface outer membrane carbohydrate layer called 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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