Cell- this is not only liquid, enzymes and other substances, but also highly organized structures called intracellular organelles. Organelles for a cell are no less important than its chemical components. Thus, in the absence of organelles such as mitochondria, the supply of energy extracted from nutrients, will immediately decrease by 95%.

Most organelles in a cell are covered membranes consisting mainly of lipids and proteins. There are membranes of cells, endoplasmic reticulum, mitochondria, lysosomes, and Golgi apparatus.

Lipids are insoluble in water, so they create a barrier in the cell that prevents the movement of water and water-soluble substances from one compartment to another. Protein molecules, however, make the membrane permeable to various substances through specialized structures called pores. Many other membrane proteins are enzymes that catalyze numerous chemical reactions, which will be discussed in subsequent chapters.

Cell (or plasma) membrane is a thin, flexible and elastic structure with a thickness of only 7.5-10 nm. It consists mainly of proteins and lipids. The approximate ratio of its components is as follows: proteins - 55%, phospholipids - 25%, cholesterol - 13%, other lipids - 4%, carbohydrates - 3%.

Lipid layer cell membrane prevents water penetration. The basis of the membrane is a lipid bilayer - a thin lipid film consisting of two monolayers and completely covering the cell. Proteins are located throughout the membrane in the form of large globules.

Lipid bilayer consists mainly of phospholipid molecules. One end of such a molecule is hydrophilic, i.e. soluble in water (a phosphate group is located on it), the other is hydrophobic, i.e. soluble only in fats (it contains a fatty acid).

Due to the fact that the hydrophobic part of the molecule phospholipid repels water, but is attracted to similar parts of the same molecules, phospholipids have natural property attach to each other in the thickness of the membrane, as shown in Fig. 2-3. The hydrophilic part with the phosphate group forms two membrane surfaces: the outer one, which is in contact with the extracellular fluid, and the inner one, which is in contact with the intracellular fluid.

Middle of the lipid layer impermeable to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, and alcohol, on the contrary, easily penetrate this area of ​​the membrane.

Molecules cholesterol, which is part of the membrane, also belongs to lipids by nature, since their steroid group is highly soluble in fats. These molecules seem to be dissolved in the lipid bilayer. Their main purpose is to regulate the permeability (or impermeability) of membranes for water-soluble components of body fluids. In addition, cholesterol is the main regulator of membrane viscosity.

Cell membrane proteins. In the figure, globular particles are visible in the lipid bilayer - these are membrane proteins, most of which are glycoproteins. There are two types of membrane proteins: (1) integral, which penetrate the membrane through; (2) peripheral, which protrude only above one of its surfaces, without reaching the other.

Many integral proteins form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intra- and extracellular fluid. Due to the selectivity of the channels, some substances diffuse better than others.

Other integral proteins function as carrier proteins, transporting substances for which the lipid bilayer is impermeable. Sometimes carrier proteins act in the direction opposite to diffusion; such transport is called active transport. Some integral proteins are enzymes.

Integral membrane proteins can also serve as receptors for water-soluble substances, including peptide hormones, since the membrane is impermeable to them. The interaction of a receptor protein with a specific ligand leads to conformational changes in the protein molecule, which, in turn, stimulates the enzymatic activity of the intracellular segment of the protein molecule or the transmission of a signal from the receptor into the cell using a second messenger. Thus, integral proteins embedded in the cell membrane involve it in the process of transmitting information about the external environment into the cell.

Molecules of peripheral membrane proteins often associated with integral proteins. Most peripheral proteins are enzymes or play the role of dispatcher of the transport of substances through membrane pores.

Biological membranes.
The term “membrane” (Latin membrana - skin, film) began to be used more than 100 years ago to designate a cell boundary that serves, on the one hand, as a barrier between the contents of the cell and the external environment, and on the other, as a semi-permeable partition through which water can pass. and some substances. However, the functions of the membrane are not limited to this, since biological membranes form the basis of the structural organization of the cell.
Membrane structure. According to this model, the main membrane is a lipid bilayer in which the hydrophobic tails of the molecules face inward and the hydrophilic heads face outward. Lipids are represented by phospholipids - derivatives of glycerol or sphingosine. Proteins are associated with the lipid layer. Integral (transmembrane) proteins penetrate the membrane through and are firmly associated with it; peripheral ones do not penetrate and are less firmly connected to the membrane. Functions of membrane proteins: maintaining membrane structure, receiving and converting signals from the environment. environment, transport of certain substances, catalysis of reactions occurring on membranes. The membrane thickness ranges from 6 to 10 nm.

Membrane properties:
1. Fluidity. The membrane is not a rigid structure; most of its constituent proteins and lipids can move in the plane of the membrane.
2. Asymmetry. The composition of the outer and inner layers of both proteins and lipids is different. In addition, the plasma membranes of animal cells have a layer of glycoproteins on the outside (glycocalyx, which performs signaling and receptor functions, and is also important for uniting cells into tissues)
3. Polarity. The outer side of the membrane carries a positive charge, while the inner side carries a negative charge.
4. Selective permeability. The membranes of living cells, in addition to water, allow only certain molecules and ions of dissolved substances to pass through. (The use of the term “semi-permeability” in relation to cell membranes is not entirely correct, since this concept implies that the membrane allows only solvent molecules to pass through, while retaining all molecules and ions of dissolved substances.)

The outer cell membrane (plasmalemma) is an ultramicroscopic film 7.5 nm thick, consisting of proteins, phospholipids and water. An elastic film that is well wetted by water and quickly restores its integrity after damage. It has a universal structure, typical of all biological membranes. The borderline position of this membrane, its participation in the processes of selective permeability, pinocytosis, phagocytosis, excretion of excretory products and synthesis, in interaction with neighboring cells and protection of the cell from damage makes its role extremely important. Animal cells outside the membrane are sometimes covered with a thin layer consisting of polysaccharides and proteins - the glycocalyx. In plant cells, outside the cell membrane there is a strong cell wall that creates external support and maintains the shape of the cell. It consists of fiber (cellulose), a water-insoluble polysaccharide.

Cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from external environment, but is also included in the composition 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 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 nerve to muscle cells.

Cell membrane - molecular structure that consists of lipids and proteins. Its main properties and functions:

• separation of the contents of any cell from the external environment, ensuring its integrity;
• control and establishment of exchange between the environment and the cell;
• intracellular membranes divide the cell into special compartments: organelles or compartments.

The word "membrane" in Latin means "film". If we talk about the cell membrane, then it is a combination of two films that have different properties.

Biological membrane includes three types of proteins:

1. Peripheral – located on the surface of the film;
2. Integral – completely penetrate the membrane;
3. Semi-integral - one end penetrates into the bilipid layer.

What functions does the cell membrane perform?

1. The cell wall is a durable cell membrane that is located outside the cytoplasmic membrane. It performs protective, transport and structural functions. Present in many plants, bacteria, fungi and archaea.

2. Provides a barrier function, that is, selective, regulated, active and passive metabolism with the external environment.

3. Capable of transmitting and storing information, and also takes part in the reproduction process.

4. Performs a transport function that can transport substances into and out of the cell through the membrane.

5. The cell membrane has one-way conductivity. Thanks to this, water molecules can pass through the cell membrane without delay, and molecules of other substances penetrate selectively.

6. With the help of the cell membrane, water, oxygen and nutrients are obtained, and through it the products of cellular metabolism are removed.

7. Performs cellular metabolism through membranes, and can perform them using 3 main types of reactions: pinocytosis, phagocytosis, exocytosis.

8. The membrane ensures the specificity of intercellular contacts.

9. The membrane contains numerous receptors that are capable of perceiving chemical signals - mediators, hormones and many other biological active substances. So it has the power to change the metabolic activity of the cell.

10. Basic properties and functions of the cell membrane:

• Matrix
• Barrier
• Transport
• Energy
• Mechanical
• Enzymatic
• Receptor
• Protective
• Marking
• Biopotential

What function does the plasma membrane perform in a cell?

1. Delimits the contents of the cell;
2. Carries out the entry of substances into the cell;
3. Provides removal of a number of substances from the cell.

Cell membrane structure

Cell membranes include lipids of 3 classes:

• Glycolipids;
• Phospholipids;
• Cholesterol.

Basically, the cell membrane consists of proteins and lipids, and has a thickness of no more than 11 nm. From 40 to 90% of all lipids are phospholipids. It is also important to note glycolipids, which are one of the main components of the membrane.

The structure of the cell membrane is three-layered. In the center there is a homogeneous liquid bilipid layer, and proteins cover it on both sides (like a mosaic), partially penetrating into the thickness. Proteins are also necessary for the membrane to allow special substances into and out of cells that cannot penetrate the fat layer. For example, sodium and potassium ions.

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Cell structure - video