It has a thickness of 8-12 nm, so it is impossible to examine it in a light microscope. The structure of the membrane is studied using an electron microscope.

The plasma membrane is formed by two layers of lipids - a bilipid layer, or a bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, with lipids located in biological membranes with their heads outward and their tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are on the surface of the membrane (external or internal), others permeate the membrane.

Plasma Membrane Functions

The membrane protects the contents of the cell from damage, maintains the shape of the cell, selectively passes the necessary substances into the cell and removes metabolic products, and also provides a link between the cells.

The barrier, delimiting function of the membrane provides a double layer of lipids. It prevents the contents of the cell from spreading, mixing with the environment or intercellular fluid, and prevents the entry of hazardous substances into the cell.

A number of the most important functions of the cytoplasmic membrane are carried out due to the proteins immersed in it. With the help of receptor proteins, it can perceive various irritations to its surface. Transport proteins form the thinnest channels through which potassium, calcium, and other small-diameter ions pass into and out of the cell. Proteins - provide vital processes in itself.

Large food particles, unable to pass through thin membrane channels, enter the cell by phagocytosis or pinocytosis. The common name for these processes is endocytosis.

How does endocytosis occur - the penetration of large food particles into the cell

The food particle is in contact with the outer membrane of the cell, and an indentation forms in this place. Then a particle surrounded by a membrane enters the cell, digestive is formed, and digestive enzymes penetrate into the resulting bubble.

Blood leukocytes that can capture and digest foreign bacteria are called phagocytes.

In the case of pinocytosis by inverting the membrane, not solid particles are captured, but droplets of liquid with substances dissolved in it. This mechanism is one of the main pathways for the penetration of substances into the cell.

Plant cells coated on top of the membrane with a solid layer of the cell wall are not capable of phagocytosis.

The process opposite to endocytosis is exocytosis. The synthesized substances (for example, hormones) are packed into membrane vesicles, suited to, embedded in it, and the contents of the bubble are ejected from the cell. Thus, the cell can also get rid of unnecessary metabolic products.

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. So, in the absence of such organelles as mitochondria, the supply of energy extracted from nutrients will immediately decrease by 95%.

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

Lipids  insoluble in water, therefore, in the cell they create a barrier that impedes 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 the following chapters.

Cell (or plasma) membrane  It 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%.

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

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 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 a natural property to 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, which is in contact with the extracellular fluid, and the inner, which is in contact with the intracellular fluid.

The middle of the lipid layer  impervious to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, alcohol, on the contrary, easily penetrate this region of the membrane.

Molecules  cholesterol, which is part of the membrane, by nature also refers to lipids, since their steroid group has a high solubility in fats. These molecules are as if dissolved in a lipid bilayer. Their main purpose is the regulation of 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 surface, not reaching the other.

Many integral proteins  form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intracellular and extracellular fluid. Due to the selectivity of the action 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 opposite direction to diffusion; such transport is called active. 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 the 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 signal transmission from the receptor into the cell using a secondary mediator. Thus, the integrated proteins embedded in the cell membrane involve it in the process of transmitting information about the external environment into the cell.

Peripheral Membrane Protein Molecules  often associated with integral proteins. Most peripheral proteins are enzymes or play the role of a controller for transport of substances through membrane pores.

Cytoplasm  - the mandatory part of the cell, enclosed between the plasma membrane and the nucleus; subdivided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). The chemical composition of the cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm has an alkaline reaction. A characteristic feature of the cytoplasm of a eukaryotic cell is constant movement ( cyclosis) It is found, first of all, by the movement of cell organelles, for example, chloroplasts. If the movement of the cytoplasm ceases, the cell dies, because, only being in constant motion, it can perform its functions.

Hyaloplasm ( cytosol) is a colorless, mucous, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it ensures the interconnection of the nucleus and all organoids. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol  - more liquid hyaloplasm and gel  - a thicker hyaloplasm. Mutual transitions are possible between them: the gel turns into sol and vice versa.

Cytoplasm functions:

1. combining all the components of the cell into a single system,
2. environment for the passage of many biochemical and physiological processes,
3. environment for the existence and functioning of organelles.

Cell membranes

Cell membranes  restrict eukaryotic cells. In each cell membrane, at least two layers can be distinguished. The inner layer is adjacent to the cytoplasm and is represented plasma membrane (synonyms - plasmalemma, cell membrane, cytoplasmic membrane), over which the outer layer is formed. In an animal cell, it is thin and is called glycocalyx  (formed by glycoproteins, glycolipids, lipoproteins), in a plant cell - thick, called cell wall  (formed by cellulose).

All biological membranes have common structural features and properties. Currently generally accepted liquid-mosaic model of the membrane structure. The basis of the membrane is a lipid bilayer, formed mainly by phospholipids. Phospholipids - triglycerides in which one fatty acid residue is replaced by a phosphoric acid residue; the portion of the molecule in which the phosphoric acid residue is located is called the hydrophilic head; the regions in which the fatty acid residues are located are called hydrophobic tails. Phospholipids are arranged in a strictly ordered manner in the membrane: the hydrophobic tails of the molecules are facing each other, and the hydrophilic heads are facing outward, toward the water.

In addition to lipids, proteins are included in the membrane (on average, ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, reception and conversion of signals from the environment, etc.). Distinguish: 1) peripheral proteins  (located on the outer or inner surface of the lipid bilayer), 2) semi-integrated proteins  (immersed in a lipid bilayer at various depths), 3) integral or transmembrane proteins  (penetrate the membrane through, in contact with both the external and internal environment of the cell). Integral proteins in some cases are called channel-forming, or channel, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them pass).

A is a hydrophilic head of a phospholipid; B - hydrophobic tails of a phospholipid; 1 - hydrophobic sections of proteins E and F; 2 - hydrophilic sections of protein F; 3 - a branched oligosaccharide chain attached to a lipid in a glycolipid molecule (glycolipids are less common than glycoproteins); 4 - a branched oligosaccharide chain attached to a protein in a glycoprotein molecule; 5 - hydrophilic channel (functions as a pore through which ions and some polar molecules can pass).

The membrane may include carbohydrates (up to 10%). The carbohydrate component of the membranes is represented by oligosaccharide or polysaccharide chains linked to protein molecules (glycoproteins) or lipids (glycolipids). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide the receptor functions of the membrane. In animal cells, glycoproteins form a supmembrane complex - glycocalyx, which has a thickness of several tens of nanometers. Many cell receptors are located in it, with its help cell adhesion occurs.

Molecules of proteins, carbohydrates and lipids are mobile, able to move in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.

Membrane Functions

Membranes perform these functions:

1. separation of cellular contents from the environment,
2. regulation of metabolism between the cell and the environment,
3. cell division into departments ("compartments"),
4. the location of the "enzymatic conveyors",
5. communication between cells in the tissues of multicellular organisms (adhesion),
6. signal recognition.

The most important membrane property  - selective permeability, i.e. membranes are well permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, which ensures the metabolism between the cell and the environment. The process of passage of substances through the cell membrane is called transport of substances. Distinguish: 1) passive transport  - the process of passage of substances, going without energy; 2) active transport  - the process of passage of substances, which goes with energy.

At passive transport  substances move from a region with a higher concentration to a region with a lower concentration, i.e. by concentration gradient. In any solution, there are solvent molecules and solute. The process of moving the molecules of the solute is called diffusion, the movement of solvent molecules is called osmosis. If a molecule is charged, then its transport is also affected by the electric gradient. Therefore, they often talk about the electrochemical gradient, combining both gradients together. The speed of transport depends on the magnitude of the gradient.

The following types of passive transport can be distinguished: 1) simple diffusion  - transport of substances directly through the lipid bilayer (oxygen, carbon dioxide); 2) diffusion through membrane channels  - transport through channel-forming proteins (Na +, K +, Ca 2+, Cl -); 3) light diffusion - transport of substances using special transport proteins, each of which is responsible for the movement of certain molecules or groups of related molecules (glucose, amino acids, nucleotides); four) osmosis  - transport of water molecules (in all biological systems, the solvent is precisely water).

Necessity active transport  arises when it is necessary to ensure the transfer through the membrane of molecules against the electrochemical gradient. This transport is carried out by special carrier proteins, the activity of which requires energy. ATP molecules serve as an energy source. Active transport includes: 1) Na + / K + pump (sodium-potassium pump), 2) endocytosis, 3) exocytosis.

Work Na + / K + pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in the external environment. The concentration of K + inside the cell should be significantly higher than outside, and Na + - vice versa. It should be noted that Na + and K + can freely diffuse through the membrane pores. The Na + / K + pump counteracts the equalization of the concentrations of these ions and actively pumps Na + from the cell, and K + into the cell. The Na + / K + pump is a transmembrane protein capable of conformational changes, as a result of which it can attach both K + and Na +. The cycle of the Na + / K + pump can be divided into the following phases: 1) Na + addition from the inside of the membrane, 2) phosphorylation of the pump protein, 3) Na + release in the extracellular space, 4) K + addition from the outside of the membrane 5) dephosphorylation of the pump protein; 6) K + release in the intracellular space. Almost a third of all the energy necessary for the life of the cell is spent on the sodium-potassium pump. For one cycle of operation, the pump pumps 3Na + out of the cell and pumps 2K +.

Endocytosis  - the process of absorption by the cell of large particles and macromolecules. There are two types of endocytosis: 1) phagocytosis  - capture and absorption of large particles (cells, parts of cells, macromolecules) and 2) pinocytosis - capture and absorption of liquid material (solution, colloidal solution, suspension). The phenomenon of phagocytosis is open I.I. Mechnikov in 1882. With endocytosis, the plasma membrane forms an indentation, its edges merge, and structures in the cytoplasm delimiting structures delimited from the cytoplasm by a single membrane occur. Many protozoa, some white blood cells, are capable of phagocytosis. Pinocytosis is observed in the intestinal epithelial cells, in the endothelium of the blood capillaries.

Exocytosis  - the reverse process of endocytosis: the removal of various substances from the cell. With exocytosis, the vesicle membrane merges with the outer cytoplasmic membrane, the contents of the vesicle are discharged outside the cell, and its membrane is included in the outer cytoplasmic membrane. In this way, hormones are removed from the cells of the endocrine glands, in the simplest - undigested food remnants.

Go to lecture number 5  “Cell theory. Types of Cell Organization

Go to lecture number 7  “Eukaryotic cell: structure and function of organelles”

A cell membrane is a structure that covers a cell from the outside. It is also called cytolemma or plasmolemma.

This formation is built of a bilipid layer (bilayer) with proteins embedded in it. The carbohydrates that make up the plasma membrane are in a bound state.

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

Cell membrane and its types

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

By 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.

The structure of the cell membrane

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

Proteins of the cytoplasmic membrane are glycoproteins. According to the chemical composition, proteins are included that are included in the lipid layer completely (throughout), - integral proteins. Also peripheral, not reaching one of the surfaces of the plasmolemma.

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

The main 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 is facing outward and consists of two layers: the outer, directed into the extracellular environment, and the inner, delimiting the intracellular contents.

Water-soluble areas are called hydrophilic heads. Areas with fatty acid are directed inside 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 impervious to glucose and urea, hydrophobic substances freely pass here: carbon dioxide, oxygen, alcohol. Cholesterol is important, the content of the latter determines the viscosity of the plasma membrane.

The functions of the outer membrane of the cell

Feature specifications are briefly listed in the table:

What does the cell membrane matter?

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 plasmolemma divide the cell into compartments (compartments), responsible for the performance of certain functions. Specifically arranged membranes corresponding to the liquid-mosaic pattern ensure cell integrity.

Table number 2

Question 1 (8)

Cell membrane  (or cytolemma, or plasmalemma, or plasma membrane) separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles in which certain environmental conditions are maintained.

Cell or plasma membrane functions

The membrane provides:

1) Selective penetration into the cell and from it of molecules and ions necessary to perform specific cell functions;
  2) Selective transport of ions through the membrane, supporting the transmembrane difference in electric potential;
  3) The specificity of intercellular contacts.

Due to the presence in the membrane of numerous receptors that accept chemical signals - hormones, mediators and other biologically active substances, it is able to change the metabolic activity of the cell. Membranes provide specifics of immune manifestations due to the presence of antigens on them - structures that cause the formation of antibodies that can specifically bind to these antigens.
  The nucleus and organelles of the cell are also separated from the cytoplasm by membranes that prevent the free movement of water and substances dissolved in it from the cytoplasm in them and vice versa. This creates the conditions for the separation of biochemical processes taking place in various compartments (compartments) inside the cell.

Cell membrane structure

Cell membrane  - an elastic structure with a thickness of 7 to 11 nm (Fig. 1.1). It consists mainly of lipids and proteins. From 40 to 90% of all lipids are phospholipids - phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin and phosphatidylinositol. An important component of the membrane are glycolipids represented by cerebrizides, sulfatides, gangliosides and cholesterol.

The basic structure of the cell membrane is a double layer of phospholipid molecules. Due to hydrophobic interactions, the carbohydrate chains of lipid molecules are held near each other in an extended state. Groups of phospholipid molecules of both layers interact with protein molecules immersed in the lipid membrane. Due to the fact that most of the lipid components of the bilayer are in a liquid state, the membrane has mobility and makes wave-like movements. Its sections, as well as proteins immersed in the lipid bilayer, are mixed from one part to another. The mobility (fluidity) of cell membranes facilitates the transport of substances through the membrane.

Cell membrane proteins  represented mainly by glycoproteins.

Distinguish

integral proteinspenetrating the entire thickness of the membrane and

peripheral proteinsattached only to the surface of the membrane, mainly to its inner part.

Peripheral proteins  almost all function as enzymes (acetylcholinesterase, acid and alkaline phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.

Integral proteins  provide selective ion exchange through membrane channels between extracellular and intracellular fluid, and also act as proteins - carriers of large molecules.

Membrane receptors and antigens can be represented by both integral and peripheral proteins.

Proteins adjacent to the membrane from the cytoplasmic side are cell cytoskeleton. They can attach to membrane proteins.

So, protein strip 3  (band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeleton molecules - spectrin through the low molecular weight protein ankyrin

Spectrin  is the main cytoskeleton protein that makes up the two-dimensional network to which actin attaches.

Actin  forms microfilaments, which are the contractile apparatus of the cytoskeleton.

Cytoskeleton  allows the cell to exhibit flexible elastic properties, provides additional membrane strength.

Most integral proteins are glycoproteins.. Their carbohydrate part protrudes from the cell membrane. Many glycoproteins have a large negative charge due to the significant content of sialic acid (for example, a glycophorin molecule). This provides the surface of most cells with a negative charge, contributing to the repulsion of other negatively charged objects. Carbohydrate protrusions of glycoproteins are carriers of antigens of blood groups, other antigenic determinants of the cell, they act as receptors that bind hormones. Glycoproteins form adhesive molecules that cause cells to attach to one another, i.e. close intercellular contacts.