Outer cell membrane. What functions does the outer cell membrane perform? The structure of the outer cell membrane

The cell membrane (plasma membrane) is a thin, semi-permeable membrane that surrounds cells.

Function and role of the cell membrane

Its function is to protect the integrity of the interior by allowing some essential substances into the cell and preventing others from entering.

It also serves as the basis for attachment to some organisms and to others. Thus, the plasma membrane also provides the shape of the cell. Another function of the membrane is to regulate cell growth through balance and.

During endocytosis, lipids and proteins are removed from cell membrane as the substances are absorbed. During exocytosis, vesicles containing lipids and proteins fuse with the cell membrane, increasing cell size. , and fungal cells have plasma membranes. Internal ones, for example, are also enclosed in protective membranes.

Cell membrane structure

The plasma membrane is mainly composed of a mixture of proteins and lipids. Depending on the location and role of the membrane in the body, lipids can make up 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help give the membrane flexibility, proteins control and maintain the cell's chemistry and assist in the transport of molecules across the membrane.

Membrane lipids

Phospholipids are the main component of plasma membranes. They form a lipid bilayer in which the hydrophilic (water-attracted) head regions spontaneously organize to face the aqueous cytosol and extracellular fluid, while the hydrophobic (water-repelled) tail regions face away from the cytosol and extracellular fluid. The lipid bilayer is semipermeable, allowing only some molecules to diffuse across the membrane.

Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps maintain the rigidity of cell membranes by preventing phospholipids from packing too tightly together. Cholesterol is absent in plant cell membranes.

Glycolipids are located on the outer surface of cell membranes and are connected to them by a carbohydrate chain. They help the cell recognize other cells in the body.

Membrane proteins

The cell membrane contains two types of associated proteins. Proteins of the peripheral membrane are external and are associated with it by interacting with other proteins. Integral membrane proteins are introduced into the membrane and most pass through. Parts of these transmembrane proteins are located on both sides of it.

Plasma membrane proteins have a number of different functions. Structural proteins provide support and shape to cells. Membrane receptor proteins help cells communicate with their external environment using hormones, neurotransmitters, and other signaling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes by facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane, helping in the exchange and transport of molecules.

Organelle membranes

Some cellular organelles are also surrounded by protective membranes. core,

By functional features The cell membrane can be divided into 9 functions it performs.
Functions of the cell membrane:
1. Transport. Transports substances from cell to cell;
2. Barrier. Has selective permeability, ensures the necessary metabolism;
3. Receptor. Some proteins found in the membrane are receptors;
4. Mechanical. Ensures the autonomy of the cell and its mechanical structures;
5. Matrix. Ensures optimal interaction and orientation of matrix proteins;
6. Energy. Membranes contain energy transfer systems during cellular respiration in mitochondria;
7. Enzymatic. Membrane proteins are sometimes enzymes. For example, intestinal cell membranes;
8. Marking. The membrane contains antigens (glycoproteins) that allow cell identification;
9. Generating. Carries out the generation and conduction of biopotentials.

You can see what a cell membrane looks like using the example of the structure of an animal cell or plant cell.

The figure shows the structure of the cell membrane.
The components of the cell membrane include various cell membrane proteins (globular, peripheral, surface), as well as cell membrane lipids (glycolipid, phospholipid). Also in the structure of the cell membrane there are carbohydrates, cholesterol, glycoprotein and protein alpha helix.

Cell membrane composition

The main composition of the cell membrane includes:
1. Proteins - responsible for various properties of the membrane;
2. Lipids three types(phospholipids, glycolipids and cholesterol) responsible for membrane rigidity.
Cell membrane proteins:
1. Globular protein;
2. Surface protein;
3. Peripheral protein.

The main purpose of the cell membrane

The main purpose of the cell membrane:
1. Regulate the exchange between the cell and the environment;
2. Separate the contents of any cell from the external environment, thereby ensuring its integrity;
3. Intracellular membranes divide the cell into specialized closed compartments - organelles or compartments in which certain environmental conditions are maintained.

Cell membrane structure

The structure of the cell membrane is a two-dimensional solution of globular integral proteins dissolved in a liquid phospholipid matrix. This model of membrane structure was proposed by two scientists Nicholson and Singer in 1972. Thus, the basis of the membranes is a bimolecular lipid layer, with an ordered arrangement of molecules, as you could see in.

Cell membrane- this is the cell membrane that performs following functions: separation of the contents of the cell and the external environment, selective transport of substances (exchange with the environment external to the cell), place of occurrence of some biochemical reactions, association of cells into tissues and reception.

Cell membranes are divided into plasma (intracellular) and external. The main property of any membrane is semi-permeability, that is, the ability to allow only certain substances to pass through. This allows for selective exchange between the cell and the external environment or exchange between cell compartments.

Plasma membranes are lipoprotein structures. Lipids spontaneously form a bilayer (double layer), and membrane proteins “float” in it. The membranes contain several thousand different proteins: structural, transporters, enzymes, etc. Between the protein molecules there are pores through which hydrophilic substances pass (the lipid bilayer prevents their direct penetration into the cell). Glycosyl groups (monosaccharides and polysaccharides) are attached to some molecules on the surface of the membrane, which are involved in the process of cell recognition during tissue formation.

Membranes vary in thickness, usually ranging from 5 to 10 nm. The thickness is determined by the size of the amphiphilic lipid molecule and is 5.3 nm. A further increase in membrane thickness is due to the size of membrane protein complexes. Depending on external conditions (cholesterol is the regulator), the structure of the bilayer can change so that it becomes more dense or liquid - the speed of movement of substances along the membranes depends on this.

Cell membranes include: plasma membrane, karyolemma, membranes of the endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, inclusions, etc.

Lipids are insoluble in water (hydrophobicity), but soluble in organic solvents and fats (lipophilicity). The composition of lipids in different membranes is not the same. For example, the plasma membrane contains a lot of cholesterol. The most common lipids in the membrane are phospholipids (glycerophosphatides), sphingomyelins (sphingolipids), glycolipids and cholesterol.

Phospholipids, sphingomyelins, glycolipids consist of two functional various parts: hydrophobic nonpolar, which does not carry charges - “tails” consisting of fatty acids, and hydrophilic, containing charged polar “heads” - alcohol groups (for example, glycerol).

The hydrophobic part of the molecule usually consists of two fatty acids. One of the acids is saturated, and the second is unsaturated. This determines the ability of lipids to spontaneously form bilayer (bilipid) membrane structures. Membrane lipids perform the following functions: barrier, transport, protein microenvironment, electrical resistance of the membrane.

Membranes differ from each other in their set of protein molecules. Many membrane proteins consist of regions rich in polar (charge-bearing) amino acids and regions with nonpolar amino acids (glycine, alanine, valine, leucine). Such proteins in the lipid layers of membranes are located so that their non-polar sections are, as it were, immersed in the “fat” part of the membrane, where the hydrophobic sections of lipids are located. The polar (hydrophilic) part of these proteins interacts with the lipid heads and faces the aqueous phase.

Biological membranes have common properties:

membranes - closed systems, which do not allow the contents of the cell and its compartments to mix. Violation of the integrity of the membrane can lead to cell death;

superficial (planar, lateral) mobility. In membranes there is a continuous movement of substances across the surface;

membrane asymmetry. The structure of the outer and surface layers is chemically, structurally and functionally heterogeneous.

The outside of the cage is covered 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 substances necessary for the cell in a similar way. 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.

Associated with the outer membrane endoplasmic reticulum cells. 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)