The cell membrane performs. Features of the structure of the cell membrane

Cell structure

Cell theory.

Plan

Cell is the basic structural unit of a living organism.

1.Cell theory.

2. The structure of the cell.

3. Evolution of the cell.

In 1665 R. Hooke first discovered plant cells. In 1674 A. Leeuwenhoek discovered the animal cell. In 1839 T. Schwann and M. Schleiden formulated the cell theory. The main position of the cell theory was that the cell is the structural and functional basis of living systems. But they mistakenly believed that cells are formed from a structureless substance. In 1859 R. Virchow proved that new cells are formed only by dividing the previous ones.

Basic provisions of cell theory :

1) The cell is the structural and functional unit of all living things. All living organisms are made up of cells.

2) All cells are basically similar in chemical composition and metabolic processes.

3) New cells are formed by dividing existing ones.

4) All cells store and implement hereditary information in the same way.

5) The vital activity of a multicellular organism as a whole is due to the interaction of its constituent cells.

According to the structure, 2 types of cells are distinguished:

prokaryotes

eukaryotes

Prokaryotes include bacteria and blue-green algae. Prokaryotes differ from eukaryotes in the following: they do not have membrane organelles present in a eukaryotic cell (mitochondria, endoplasmic reticulum, lysosomes, Golgi complex, chloroplasts).

The most important difference is that they do not have a nucleus surrounded by a membrane. Prokaryotic DNA is represented by one folded circular molecule. Prokaryotes also lack cell center centrioles, so they never divide by mitosis. They are characterized by amitosis - direct rapid division.

Eukaryotic cells are cells of unicellular and multicellular organisms. They consist of three main constituent parts:

The cell membrane that surrounds the cell and separates it from the external environment;

Cytoplasm containing water, mineral salts, organic compounds, organelles and inclusions;

The nucleus that contains the genetic material of the cell.

1 - polar head of the phospholipid molecule

2 - fatty acid tail of the phospholipid molecule

3 - integral protein

4 - peripheral protein

5 - semi-integral protein

6 - glycoprotein

7 - glycolipid

outdoor cell membrane inherent in all cells (animals and plants), has a thickness of about 7.5 (up to 10) nm and consists of lipid and protein molecules.

At present, the fluid-mosaic model of the construction of the cell membrane is widespread. According to this model, lipid molecules are arranged in two layers, with their water-repellent ends (hydrophobic - fat-soluble) facing each other, and water-soluble (hydrophilic) - to the periphery. Protein molecules are embedded in the lipid layer. Some of them are located on the outer or inner surface of the lipid part, others are partially immersed or penetrate the membrane through and through.

Membrane functions :

Protective, border, barrier;

Transport;

Receptor - is carried out at the expense of proteins - receptors, which have a selective ability for certain substances (hormones, antigens, etc.), enter into chemical interactions with them, conduct signals inside the cell;

Participate in the formation of intercellular contacts;

They provide the movement of some cells (amoeboid movement).

Animal cells have a thin layer of glycocalyx on top of the outer cell membrane. It is a complex of carbohydrates with lipids and carbohydrates with proteins. The glycocalyx is involved in intercellular interactions. The cytoplasmic membranes of most cell organelles have exactly the same structure.

In plant cells outside of the cytoplasmic membrane. the cell wall is made up of cellulose.

Transport of substances across the cytoplasmic membrane .

There are two main mechanisms for the entry of substances into the cell or out of the cell to the outside:

1. Passive transport.

2. Active transport.

Passive transport of substances occurs without the expenditure of energy. An example of such transport is diffusion and osmosis, in which the movement of molecules or ions is carried out from a region of high concentration to a region of lower concentration, for example, water molecules.

Active transport - in this type of transport, molecules or ions penetrate the membrane against a concentration gradient, which requires energy. An example of active transport is the sodium-potassium pump, which actively pumps sodium out of the cell and absorbs potassium ions from the external environment, transferring them into the cell. The pump is a special membrane protein that sets it in motion with ATP.

Active transport maintains a constant cell volume and membrane potential.

Substances can be transported by endocytosis and exocytosis.

Endocytosis - the penetration of substances into the cell, exocytosis - out of the cell.

During endocytosis, the plasma membrane forms an invagination or outgrowths, which then envelop the substance and, lacing off, turn into vesicles.

There are two types of endocytosis:

1) phagocytosis - the absorption of solid particles (phagocyte cells),

2) pinocytosis - the absorption of liquid material. Pinocytosis is characteristic of amoeboid protozoa.

By exocytosis, various substances are removed from the cells: undigested food residues are removed from the digestive vacuoles, their liquid secret is removed from the secretory cells.

Cytoplasm -(cytoplasm + nucleus form protoplasm). The cytoplasm consists of a watery ground substance (cytoplasmic matrix, hyaloplasm, cytosol) and various organelles and inclusions in it.

Inclusions– cell waste products. There are 3 groups of inclusions - trophic, secretory (gland cells) and special (pigment) values.

Organelles - These are permanent structures of the cytoplasm that perform certain functions in the cell.

Isolate organelles general meaning and special. Special ones are found in most cells, but are present in significant numbers only in cells that perform a specific function. These include microvilli of intestinal epithelial cells, cilia of the epithelium of the trachea and bronchi, flagella, myofibrils (providing muscle contraction, etc.).

Organelles of general importance include EPS, the Golgi complex, mitochondria, ribosomes, lysosomes, centrioles of the cell center, peroxisomes, microtubules, microfilaments. Plant cells contain plastids and vacuoles. Organelles of general importance can be divided into organelles having a membrane and non-membrane structure.

Organelles having a membrane structure are two-membrane and one-membrane. Two-membrane cells include mitochondria and plastids. To single-membrane - endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, vacuoles.

Membraneless organelles: ribosomes, cell center, microtubules, microfilaments.

Mitochondria – These are round or oval organelles. They consist of two membranes: internal and external. The inner membrane has outgrowths - cristae, which divide the mitochondria into compartments. The compartments are filled with a substance - a matrix. The matrix contains DNA, mRNA, tRNA, ribosomes, calcium and magnesium salts. This is where protein biosynthesis takes place. The main function of mitochondria is the synthesis of energy and its accumulation in ATP molecules. New mitochondria are formed in the cell as a result of the division of old ones.

plastids – organelles found predominantly in plant cells. They are of three types: chloroplasts containing a green pigment; chromoplasts (pigments of red, yellow, orange color); leucoplasts (colorless).

Chloroplasts, thanks to the green pigment chlorophyll, are able to synthesize organic substances from inorganic ones using the energy of the sun.

Chromoplasts give bright colors to flowers and fruits.

Leucoplasts are able to accumulate reserve nutrients: starch, lipids, proteins, etc.

Endoplasmic reticulum ( EPS ) is a complex system of vacuoles and channels that are limited by membranes. There are smooth (agranular) and rough (granular) EPS. Smooth has no ribosomes on its membrane. It contains the synthesis of lipids, lipoproteins, the accumulation and removal of toxic substances from the cell. Granular EPS has ribosomes on membranes in which proteins are synthesized. Then the proteins enter the Golgi complex, and from there out.

Golgi complex (Golgi apparatus) is a stack of flattened membrane sacs - cisterns and a system of bubbles associated with them. The stack of cisterns is called a dictyosome.

Functions of the Golgi complex : protein modification, polysaccharide synthesis, substance transport, cell membrane formation, lysosome formation.

Lysosomes – are membrane-bound vesicles containing enzymes. They carry out intracellular cleavage of substances and are divided into primary and secondary. Primary lysosomes contain enzymes in an inactive form. After entering the organelles various substances enzymes are activated and the process of digestion begins - these are secondary lysosomes.

Peroxisomes have the appearance of bubbles bounded by a single membrane. They contain enzymes that break down hydrogen peroxide, which is toxic to cells.

Vacuoles – These are plant cell organelles that contain cell sap. Cell sap may contain spare nutrients, pigments, and waste products. Vacuoles are involved in the creation of turgor pressure, in the regulation of water-salt metabolism.

Ribosomes – organelles made up of large and small subunits. They can be located either on the ER or located freely in the cell, forming polysomes. They are composed of rRNA and protein and are produced in the nucleolus. Protein synthesis takes place in ribosomes.

Cell Center – found in the cells of animals, fungi, lower plants and absent in higher plants. It consists of two centrioles and a radiant sphere. The centriole has the form of a hollow cylinder, the wall of which consists of 9 triplets of microtubules. When dividing, cells form threads of the mitotic spindle, which ensure the divergence of chromatids in the anaphase of mitosis and homologous chromosomes during meiosis.

microtubules – tubular formations of various lengths. They are part of the centrioles, mitotic spindle, flagella, cilia, perform a supporting function, promote the movement of intracellular structures.

Microfilaments – filamentous thin formations located throughout the cytoplasm, but there are especially many of them under the cell membrane. Together with microtubules, they form the cytoskeleton of the cell, determine the flow of the cytoplasm, intracellular movements of vesicles, chloroplasts, and other organelles.

Short description:

Sazonov V.F. 1_1 The structure of the cell membrane [Electronic resource] // Kinesiologist, 2009-2018: [website]. Date of update: 06.02.2018..__.201_). _The structure and functioning of the cell membrane is described (synonyms: plasmalemma, plasmolemma, biomembrane, cell membrane, outer cell membrane, cell membrane, cytoplasmic membrane). This initial information is necessary both for cytology and for understanding the processes of nervous activity: nervous excitation, inhibition, the work of synapses and sensory receptors.

cell membrane (plasma a lemma or plasma about lemma)

Concept definition

The cell membrane (synonyms: plasmalemma, plasmolemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (i.e. "fat-protein") membrane that separates the cell from the environment and carries out a controlled exchange and communication between the cell and its environment.

The main thing in this definition is not that the membrane separates the cell from the environment, but just that it connects cell with the environment. The membrane is active structure of the cell, it is constantly working.

A biological membrane is an ultrathin bimolecular film of phospholipids encrusted with proteins and polysaccharides. This cellular structure underlies the barrier, mechanical and matrix properties of a living organism (Antonov VF, 1996).

Figurative representation of the membrane

To me, the cell membrane appears as a lattice fence with many doors in it, which surrounds a certain territory. Any small living creatures can freely move back and forth through this fence. But larger visitors can only enter through the doors, and even then not all. Different visitors have keys only to their own doors, and they cannot pass through other people's doors. So, through this fence there are constantly flows of visitors back and forth, because the main function of the membrane-fence is twofold: to separate the territory from the surrounding space and at the same time connect it with the surrounding space. For this, there are many holes and doors in the fence - !

Membrane properties

1. Permeability.

2. Semi-permeability (partial permeability).

3. Selective (synonym: selective) permeability.

4. Active permeability (synonym: active transport).

5. Controlled permeability.

As you can see, the main property of the membrane is its permeability with respect to various substances.

6. Phagocytosis and pinocytosis.

7. Exocytosis.

8. The presence of electrical and chemical potentials, more precisely, the potential difference between the inner and outer sides of the membrane. Figuratively, one can say that "the membrane turns the cell into an "electric battery" by controlling ion flows". Details: .

9. Changes in electrical and chemical potential.

10. Irritability. Special molecular receptors located on the membrane can connect with signal (control) substances, as a result of which the state of the membrane and the entire cell can change. Molecular receptors trigger biochemical reactions in response to the combination of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, while the changes continue inside the cell. It turns out that the membrane transmitted information from the environment to the internal environment of the cell.

11. Catalytic enzymatic activity. Enzymes can be embedded in the membrane or associated with its surface (both inside and outside the cell), and there they carry out their enzymatic activity.

12. Changing the shape of the surface and its area. This allows the membrane to form outgrowths outward or, conversely, invaginations into the cell.

13. The ability to form contacts with other cell membranes.

14. Adhesion - the ability to stick to solid surfaces.

Brief list of membrane properties

• Permeability.
• Endocytosis, exocytosis, transcytosis.
• Potentials.
• Irritability.
• enzymatic activity.
• Contacts.
• Adhesion.

Membrane functions

1. Incomplete isolation of internal content from the external environment.

2. The main thing in the work of the cell membrane is exchange various substances between the cell and the extracellular environment. This is due to such property of the membrane as permeability. In addition, the membrane regulates this exchange by regulating its permeability.

3. Another important function of the membrane is creating a difference in chemical and electrical potentials between its inner and outer sides. Due to this, inside the cell has a negative electrical potential -.

4. Through the membrane is also carried out information exchange between the cell and its environment. Special molecular receptors located on the membrane can bind to control substances (hormones, mediators, modulators) and trigger biochemical reactions in the cell, leading to various changes in the cell or in its structures.

Video:The structure of the cell membrane

Video lecture:Details about the structure of the membrane and transport

Membrane structure

The cell membrane has a universal three-layer structure. Its median fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of individual protein areas. The fat layer is the basis that ensures the isolation of the cell from the environment, isolating it from the environment. By itself, it passes water-soluble substances very poorly, but easily passes fat-soluble ones. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) has to be provided with special protein structures - and.

Below are microphotographs of real cell membranes of contacting cells, obtained using an electron microscope, as well as a schematic drawing showing the three-layered membrane and the mosaic nature of its protein layers. To enlarge an image, click on it.

Separate image of the inner lipid (fatty) layer of the cell membrane, permeated with integral embedded proteins. The upper and lower protein layers are removed so as not to interfere with the consideration of the lipid bilayer

Figure above: An incomplete schematic representation of the cell membrane (cell wall) from Wikipedia.

Note that the outer and inner protein layers have been removed from the membrane here so that we can better see the central fatty double lipid layer. In a real cell membrane, large protein "islands" float above and below along the fatty film (small balls in the figure), and the membrane turns out to be thicker, three-layered: protein-fat-protein . So it's actually like a sandwich of two protein "slices of bread" with a thick layer of "butter" in the middle, ie. has a three-layer structure, not a two-layer one.

In this figure, small blue and white balls correspond to the hydrophilic (wettable) "heads" of the lipids, and the "strings" attached to them correspond to the hydrophobic (non-wettable) "tails". Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. Yellow oval dots inside the membrane are cholesterol molecules Yellow-green chains of beads on the outside of the membrane are oligosaccharide chains that form the glycocalyx. Glycocalyx is like a carbohydrate ("sugar") "fluff" on the membrane, formed by long carbohydrate-protein molecules protruding from it.

Living is a small "protein-fat bag" filled with semi-liquid jelly-like contents, which is penetrated by films and tubes.

The walls of this sac are formed by a double fatty (lipid) film, covered inside and out with proteins - the cell membrane. Therefore, the membrane is said to have three-layer structure : proteins-fats-proteins. Inside the cell there are also many similar fatty membranes that divide its internal space into compartments. Cellular organelles are surrounded by the same membranes: nucleus, mitochondria, chloroplasts. So the membrane is a universal molecular structure inherent in all cells and all living organisms.

On the left - no longer a real, but an artificial model of a piece of a biological membrane: this is an instant snapshot of an adipose phospholipid bilayer (i.e. a double layer) in the process of its molecular dynamics modeling. The calculation cell of the model is shown - 96 PQ molecules ( f osphatidil X oline) and 2304 water molecules, total 20544 atoms.

On the right is a visual model of a single molecule of the same lipid, from which the membrane lipid bilayer is assembled. It has a hydrophilic (water-loving) head at the top, and two hydrophobic (water-fearing) tails at the bottom. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18:0/22:6(n-3)cis PC), but you don't need to memorize it unless you plan to make your teacher swoon with the depth of your knowledge.

You can give a more precise scientific definition of a cell:

is an ordered, structured heterogeneous system of biopolymers limited by an active membrane, participating in a single set of metabolic, energy and information processes, and also maintaining and reproducing the entire system as a whole.

Inside the cell is also penetrated by membranes, and between the membranes there is not water, but a viscous gel / sol of variable density. Therefore, the interacting molecules in the cell do not float freely, as in a test tube with an aqueous solution, but mostly sit (immobilized) on the polymer structures of the cytoskeleton or intracellular membranes. And therefore, chemical reactions take place inside the cell almost like in a solid body, and not in a liquid. The outer membrane that surrounds the cell is also covered in enzymes and molecular receptors, making it a very active part of the cell.

The cell membrane (plasmalemma, plasmolemma) is an active shell that separates the cell from the environment and connects it with the environment. © Sazonov V.F., 2016.

From this definition of a membrane, it follows that it does not simply limit the cell, but actively working linking it to its environment.

The fat that makes up the membranes is special, so its molecules are usually called not just fat, but lipids, phospholipids, sphingolipids. The membrane film is double, i.e. it consists of two films stuck together. Therefore, textbooks write that the base of the cell membrane consists of two lipid layers (or " bilayer", i.e. double layer). For each individual lipid layer, one side can be wetted by water, and the other cannot. So, these films stick together with each other precisely by their non-wetting sides.

bacteria membrane

The shell of a prokaryotic cell of gram-negative bacteria consists of several layers, shown in the figure below.
Layers of the shell of gram-negative bacteria:
1. The inner three-layer cytoplasmic membrane, which is in contact with the cytoplasm.
2. Cell wall, which consists of murein.
3. The outer three-layer cytoplasmic membrane, which has the same system of lipids with protein complexes as the inner membrane.
Communication between Gram-negative bacterial cells and outside world through such a complex three-step structure does not give them an advantage in surviving in harsh conditions compared to gram-positive bacteria that have a less powerful shell. They just as badly tolerate high temperatures, high acidity and pressure drops.

Video lecture:Plasma membrane. E.V. Cheval, Ph.D.

Video lecture:The membrane as a cell boundary. A. Ilyaskin

Importance of Membrane Ion Channels

It is easy to understand that only fat-soluble substances can enter the cell through the membrane fatty film. These are fats, alcohols, gases. For example, in erythrocytes, oxygen and carbon dioxide easily pass in and out directly through the membrane. But water and water-soluble substances (for example, ions) simply cannot pass through the membrane into any cell. This means that they need special holes. But if you just make a hole in the fatty film, then it will immediately tighten back. What to do? A solution was found in nature: it is necessary to make special protein transport structures and stretch them through the membrane. This is how the channels for the passage of fat-insoluble substances are obtained - the ion channels of the cell membrane.

So, in order to give its membrane additional properties of permeability for polar molecules (ions and water), the cell synthesizes special proteins in the cytoplasm, which are then integrated into the membrane. They are of two types: transporter proteins (for example, transport ATPases) and channel-forming proteins (channel formers). These proteins are embedded in the double fatty layer of the membrane and form transport structures in the form of transporters or in the form of ion channels. Various water-soluble substances can now pass through these transport structures, which otherwise cannot pass through the fatty membrane film.

In general, proteins embedded in the membrane are also called integral, precisely because they are, as it were, included in the composition of the membrane and penetrate it through and through. Other proteins, not integral, form, as it were, islands that "float" on the surface of the membrane: either along its outer surface or along its inner one. After all, everyone knows that fat is a good lubricant and it is easy to slide on it!

findings

1. In general, the membrane is three-layered:

1) the outer layer of protein "islands",

2) fatty two-layer "sea" (lipid bilayer), i.e. double lipid film

3) the inner layer of protein "islands".

But there is also a loose outer layer - the glycocalyx, which is formed by glycoproteins sticking out of the membrane. They are molecular receptors to which signaling controls bind.

2. Special protein structures are built into the membrane, ensuring its permeability to ions or other substances. We must not forget that in some places the sea of ​​fat is permeated through with integral proteins. And it is integral proteins that form special transport structures cell membrane (see section 1_2 Membrane transport mechanisms). Through them, substances enter the cell, and are also removed from the cell to the outside.

3. Enzyme proteins can be located on any side of the membrane (outer and inner), as well as inside the membrane, which affect both the state of the membrane itself and the life of the entire cell.

So the cell membrane is an active variable structure that actively works in the interests of the whole cell and connects it with the outside world, and is not just a "protective shell". This is the most important thing to know about the cell membrane.

In medicine, membrane proteins are often used as “targets” for medicines. Receptors, ion channels, enzymes, transport systems act as such targets. Recently, in addition to the membrane, genes hidden in the cell nucleus have also become targets for drugs.

Video:Introduction to Cell Membrane Biophysics: Structure of Membrane 1 (Vladimirov Yu.A.)

Video:History, structure and functions of the cell membrane: Structure of membranes 2 (Vladimirov Yu.A.)

© 2010-2018 Sazonov V.F., © 2010-2016 kineziolog.bodhy.

All living organisms on Earth are made up of cells, and each cell is surrounded by a protective shell - a membrane. However, the functions of the membrane are not limited to protecting organelles and separating one cell from another. The cell membrane is a complex mechanism that is directly involved in reproduction, regeneration, nutrition, respiration, and many other important cell functions.

The term "cell membrane" has been used for about a hundred years. The word "membrane" in translation from Latin means "film". But in the case of a cell membrane, it would be more correct to speak of a combination of two films interconnected in a certain way, moreover, different sides of these films have different properties.

The cell membrane (cytolemma, plasmalemma) is a three-layer lipoprotein (fat-protein) shell that separates each cell from neighboring cells and the environment, and carries out a controlled exchange between cells and the environment.

Of decisive importance in this definition is not that the cell membrane separates one cell from another, but that it ensures its interaction with other cells and the environment. The membrane is a very active, constantly working structure of the cell, on which many functions are assigned by nature. From our article, you will learn everything about the composition, structure, properties and functions of the cell membrane, as well as the danger posed to human health by disturbances in the functioning of cell membranes.

History of cell membrane research

In 1925, two German scientists, Gorter and Grendel, were able to conduct a complex experiment on human red blood cells, erythrocytes. Using osmotic shock, the researchers obtained the so-called "shadows" - empty shells of red blood cells, then stacked them in one pile and measured the surface area. The next step was to calculate the amount of lipids in the cell membrane. With the help of acetone, the scientists isolated lipids from the "shadows" and determined that they were just enough for a double continuous layer.

However, during the experiment, two gross errors were made:

The use of acetone does not allow all lipids to be isolated from the membranes;

The surface area of ​​the "shadows" was calculated by dry weight, which is also incorrect.

Since the first error gave a minus in the calculations, and the second - a plus, the overall result turned out to be surprisingly accurate, and the German scientists brought in scientific world The most important discovery is the lipid bilayer of the cell membrane.

In 1935, another pair of researchers, Danielly and Dawson, after long experiments on bilipid films, came to the conclusion that proteins are present in cell membranes. There was no other way to explain why these films have such a high surface tension. Scientists have presented to the attention of the public a schematic model of a cell membrane, similar to a sandwich, where the role of slices of bread is played by homogeneous lipid-protein layers, and between them instead of oil is emptiness.

In 1950, with the help of the first electron microscope, the Danielly-Dawson theory was partially confirmed - two layers consisting of lipid and protein heads were clearly visible on micrographs of the cell membrane, and between them there was a transparent space filled only with tails of lipids and proteins.

In 1960, guided by these data, the American microbiologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time was considered the only true one. However, as science developed, more and more doubts were born about the homogeneity of these layers. From the point of view of thermodynamics, such a structure is extremely unfavorable - it would be very difficult for cells to transport substances in and out through the entire “sandwich”. In addition, it has been proven that the cell membranes of different tissues have different thickness and method of attachment, which is due to different functions of organs.

In 1972, microbiologists S.D. Singer and G.L. Nicholson was able to explain all the inconsistencies of Robertson's theory with the help of a new, fluid-mosaic model of the cell membrane. Scientists have found that the membrane is heterogeneous, asymmetric, filled with fluid, and its cells are in constant motion. And the proteins that make up it have a different structure and purpose, in addition, they are located differently relative to the bilipid layer of the membrane.

Cell membranes contain three types of proteins:

Peripheral - attached to the surface of the film;

semi-integral- partially penetrate the bilipid layer;

Integral - completely penetrate the membrane.

Peripheral proteins are associated with the heads of membrane lipids through electrostatic interaction, and they never form a continuous layer, as was previously believed. And semi-integral and integral proteins serve to transport oxygen and nutrients into the cell, as well as to remove decay products from it and more for several important features, which you will learn about later.

The cell membrane performs the following functions:

Barrier - the permeability of the membrane for different types of molecules is not the same. To bypass the cell membrane, the molecule must have a certain size, chemical properties and electric charge. Harmful or inappropriate molecules, due to the barrier function of the cell membrane, simply cannot enter the cell. For example, with the help of the peroxide reaction, the membrane protects the cytoplasm from peroxides that are dangerous for it;

Transport - a passive, active, regulated and selective exchange passes through the membrane. Passive metabolism is suitable for fat-soluble substances and gases consisting of very small molecules. Such substances penetrate into and out of the cell without energy expenditure, freely, by diffusion. The active transport function of the cell membrane is activated when necessary, but difficult to transport substances need to be carried into or out of the cell. For example, those with a large molecular size, or unable to cross the bilipid layer due to hydrophobicity. Then protein pumps begin to work, including ATPase, which is responsible for the absorption of potassium ions into the cell and the ejection of sodium ions from it. Regulated transport is essential for secretion and fermentation functions, such as when cells produce and secrete hormones or gastric juice. All these substances leave the cells through special channels and in a given volume. And the selective transport function is associated with the very integral proteins that penetrate the membrane and serve as a channel for the entry and exit of strictly defined types of molecules;

Matrix - the cell membrane determines and fixes the location of organelles relative to each other (nucleus, mitochondria, chloroplasts) and regulates the interaction between them;

Mechanical - ensures the restriction of one cell from another, and, at the same time, the correct connection of cells into a homogeneous tissue and the resistance of organs to deformation;

Protective - both in plants and in animals, the cell membrane serves as the basis for building a protective frame. An example is hard wood, dense peel, prickly thorns. In the animal world, there are also many examples of the protective function of cell membranes - turtle shell, chitinous shell, hooves and horns;

Energy - the processes of photosynthesis and cellular respiration would be impossible without the participation of cell membrane proteins, because it is with the help of protein channels that cells exchange energy;

Receptor - proteins embedded in the cell membrane may have another important function. They serve as receptors through which the cell receives a signal from hormones and neurotransmitters. And this, in turn, is necessary for the conduction of nerve impulses and the normal course of hormonal processes;

Enzymatic - another important function inherent in some proteins of cell membranes. For example, in the intestinal epithelium, digestive enzymes are synthesized with the help of such proteins;

Biopotential- the concentration of potassium ions inside the cell is much higher than outside, and the concentration of sodium ions, on the contrary, is greater outside than inside. This explains the potential difference: the charge is negative inside the cell, positive outside, which contributes to the movement of substances into the cell and out in any of the three types of metabolism - phagocytosis, pinocytosis and exocytosis;

Marking - on the surface of cell membranes there are so-called "labels" - antigens consisting of glycoproteins (proteins with branched oligosaccharide side chains attached to them). Since side chains can have a huge variety of configurations, each type of cell receives its own unique label that allows other cells in the body to recognize them “by sight” and respond to them correctly. That is why, for example, human immune cells, macrophages, easily recognize a foreigner that has entered the body (infection, virus) and try to destroy it. The same thing happens with diseased, mutated and old cells - the label on their cell membrane changes and the body gets rid of them.

Cellular exchange occurs across membranes, and can be carried out through three main types of reactions:

Phagocytosis is a cellular process in which phagocytic cells embedded in the membrane capture and digest solid particles of nutrients. In the human body, phagocytosis is carried out by membranes of two types of cells: granulocytes (granular leukocytes) and macrophages (immune killer cells);

Pinocytosis is the process of capturing liquid molecules that come into contact with it by the surface of the cell membrane. For nutrition by the type of pinocytosis, the cell grows thin fluffy outgrowths in the form of antennae on its membrane, which, as it were, surround a drop of liquid, and a bubble is obtained. First, this bubble protrudes above the surface of the membrane, and then it is “swallowed” - it hides inside the cell, and its walls merge with inner surface cell membrane. Pinocytosis occurs in almost all living cells;

Exocytosis is a reverse process in which vesicles with a secretory functional fluid (enzyme, hormone) are formed inside the cell, and it must somehow be removed from the cell into the environment. To do this, the bubble first merges with the inner surface of the cell membrane, then bulges outward, bursts, expels the contents and again merges with the surface of the membrane, this time from the outside. Exocytosis takes place, for example, in the cells of the intestinal epithelium and the adrenal cortex.

Cell membranes contain three classes of lipids:

Phospholipids;

Glycolipids;

Cholesterol.

Phospholipids (a combination of fats and phosphorus) and glycolipids (a combination of fats and carbohydrates), in turn, consist of a hydrophilic head, from which two long hydrophobic tails extend. But cholesterol sometimes occupies the space between these two tails and does not allow them to bend, which makes the membranes of some cells rigid. In addition, cholesterol molecules streamline the structure of cell membranes and prevent the transition of polar molecules from one cell to another.

But the most important component, as can be seen from the previous section on the functions of cell membranes, are proteins. Their composition, purpose and location are very diverse, but there is something in common that unites them all: annular lipids are always located around the proteins of cell membranes. These are special fats that are clearly structured, stable, have more saturated fatty acids in their composition, and are released from membranes along with "sponsored" proteins. This is a kind of personal protective shell for proteins, without which they simply would not work.

The structure of the cell membrane is three-layered. A relatively homogeneous liquid bilipid layer lies in the middle, and proteins cover it on both sides with a kind of mosaic, partially penetrating into the thickness. That is, it would be wrong to think that the outer protein layers of cell membranes are continuous. Proteins, in addition to their complex functions, are needed in the membrane in order to pass inside the cells and transport out of them those substances that are not able to penetrate the fat layer. For example, potassium and sodium ions. For them, special protein structures are provided - ion channels, which we will discuss in more detail below.

If you look at the cell membrane through a microscope, you can see a layer of lipids formed by the smallest spherical molecules, along which, like the sea, large protein cells float. different shapes. Exactly the same membranes divide the internal space of each cell into compartments in which the nucleus, chloroplasts and mitochondria are comfortably located. If there were no separate “rooms” inside the cell, the organelles would stick together and would not be able to perform their functions correctly.

A cell is a set of organelles structured and delimited by membranes, which is involved in a complex of energy, metabolic, informational and reproductive processes that ensure the vital activity of the organism.

As can be seen from this definition, the membrane is the most important functional component of any cell. Its significance is as great as that of the nucleus, mitochondria and other cell organelles. And the unique properties of the membrane are due to its structure: it consists of two films stuck together in a special way. Molecules of phospholipids in the membrane are located with hydrophilic heads outward, and hydrophobic tails inward. Therefore, one side of the film is wetted by water, while the other is not. So, these films are connected to each other with non-wettable sides inward, forming a bilipid layer surrounded by protein molecules. This is the very “sandwich” structure of the cell membrane.

Ion channels of cell membranes

Let us consider in more detail the principle of operation of ion channels. What are they needed for? The fact is that only fat-soluble substances can freely penetrate through the lipid membrane - these are gases, alcohols and fats themselves. So, for example, in red blood cells there is a constant exchange of oxygen and carbon dioxide, and for this our body does not have to resort to any additional tricks. But what about when it becomes necessary to transport aqueous solutions, such as sodium and potassium salts, through the cell membrane?

It would be impossible to pave the way for such substances in the bilipid layer, since the holes would immediately tighten and stick together back, such is the structure of any adipose tissue. But nature, as always, found a way out of the situation and created special protein transport structures.

There are two types of conductive proteins:

Transporters are semi-integral protein pumps;

Channeloformers are integral proteins.

Proteins of the first type are partially immersed in the bilipid layer of the cell membrane, and look out with their heads, and in the presence of the desired substance, they begin to behave like a pump: they attract a molecule and suck it into the cell. And proteins of the second type, integral, have an elongated shape and are located perpendicular to the bilipid layer of the cell membrane, penetrating it through and through. Through them, as through tunnels, substances that are unable to pass through fat move into and out of the cell. It is through ion channels that potassium ions penetrate into the cell and accumulate in it, while sodium ions, on the contrary, are brought out. There is a difference in electrical potentials, so necessary for the proper functioning of all the cells of our body.

The most important conclusions about the structure and functions of cell membranes

Theory always looks interesting and promising if it can be usefully applied in practice. The discovery of the structure and functions of the cell membranes of the human body allowed scientists to make a real breakthrough in science in general, and in medicine in particular. It is no coincidence that we have dwelled on ion channels in such detail, because it is here that lies the answer to one of the most important questions of our time: why do people increasingly get sick with oncology?

Cancer claims about 17 million lives worldwide every year and is the fourth leading cause of all deaths. According to WHO, the incidence of cancer is steadily increasing, and by the end of 2020 it could reach 25 million per year.

What explains the real epidemic of cancer, and what does the function of cell membranes have to do with it? You will say: the reason is in poor environmental conditions, malnutrition, bad habits and heavy heredity. And, of course, you will be right, but if we talk about the problem in more detail, then the reason is the acidification of the human body. The negative factors listed above lead to disruption of the cell membranes, inhibit breathing and nutrition.

Where there should be a plus, a minus is formed, and the cell cannot function normally. But cancer cells do not need either oxygen or an alkaline environment - they are able to use an anaerobic type of nutrition. Therefore, in conditions of oxygen starvation and off-scale pH levels, healthy cells mutate, wanting to adapt to the environment, and become cancerous cells. This is how a person gets cancer. To avoid this, you just need to consume enough clean water daily, and discard carcinogens in food. But, as a rule, people are well aware of harmful products and the need for high-quality water, and do nothing - they hope that trouble will bypass them.

Knowing the features of the structure and functions of the cell membranes of different cells, doctors can use this information to provide targeted, targeted therapeutic effects on the body. Many modern drugs, getting into our body, are looking for the right "target", which can be ion channels, enzymes, receptors and biomarkers of cell membranes. This method of treatment allows you to achieve better results with minimal side effects.

Antibiotics of the latest generation, when released into the blood, do not kill all the cells in a row, but look for exactly the cells of the pathogen, focusing on markers in its cell membranes. The newest anti-migraine drugs, triptans, only constrict the inflamed vessels in the brain, with almost no effect on the heart and peripheral circulatory system. And they recognize the necessary vessels precisely by the proteins of their cell membranes. There are many such examples, so we can say with confidence that knowledge about the structure and functions of cell membranes underlies the development of modern medical science, and saves millions of lives every year.

Education: Moscow Medical Institute. I. M. Sechenov, specialty - "Medicine" in 1991, in 1993 "Occupational diseases", in 1996 "Therapy".

Outside, the cell is covered with a 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 the parts that are 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 vice versa.

Basic Functions of the Cell Membrane

One of the main properties of biological membranes is selective permeability (semipermeability)- some substances pass through them with difficulty, others easily and even towards a higher concentration. Thus, for most cells, the concentration of Na ions inside is much lower than in the environment. For K ions, the reverse ratio is characteristic: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to enter the cell, and K ions - to go outside. 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 that pumps Na ions out of the cell and simultaneously pumps K ions inside.

The desire 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, absorption of substances also occurs by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression where the captured particle enters. This recess increases, and, surrounded by a portion of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoeba and some other protozoa, as well as leukocytes (phagocytes). Similarly, the cells absorb liquids containing the substances necessary for the cell. This phenomenon has been called pinocytosis.

The outer membranes of various cells differ significantly both in the chemical composition of their proteins and lipids, and in 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.

The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, Various types intercellular contacts, i.e. communication between individual cells.

Many types of cells are characterized by the presence on their surface a large number protrusions, folds, microvilli. They contribute both to a significant increase in the surface area of ​​cells and improve metabolism, as well as to stronger bonds of individual cells with each other.

On the outside of the cell membrane, plant cells have thick membranes that are clearly visible in an optical microscope, consisting of cellulose (cellulose). They create a strong support for plant tissues (wood).

Some cells of animal origin also have a number of external structures that are located on top of the cell membrane and have a protective character. An example is the chitin of the integumentary cells of insects.

Functions of the cell membrane (briefly)

Function	Description
protective barrier	Separates the internal organelles of the cell from the external environment
Regulatory	It regulates the exchange of substances between the internal contents of the cell and the external environment.
Delimiting (compartmentalization)	Separation of the internal space of the cell into independent blocks (compartments)
Energy	- Accumulation and transformation of energy; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion.
Receptor (information)	Participates in the formation of excitation and its conduct.
Motor	Carries out the movement of the cell or its individual parts.

Cell— self-regulating structural and functional unit of tissues and organs. The cellular theory of the structure of organs and tissues was developed by Schleiden and Schwann in 1839. Subsequently, using electron microscopy and ultracentrifugation, it was possible to elucidate the structure of all the main organelles of animal and plant cells (Fig. 1).

Rice. 1. Scheme of the structure of the cell of animal organisms

The main parts of the cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.

The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows essential nutrients and chemical elements get inside the cell, and excess products come out of it. The plasma membrane consists of two layers of lipid molecules with the inclusion of specific proteins in it. The main membrane lipids are phospholipids. They contain phosphorus, a polar head, and two non-polar long-chain fatty acid tails. Membrane lipids include cholesterol and cholesterol esters. In accordance with the fluid mosaic model of the structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. Each type of membrane in any animal cell has its own relatively constant lipid composition.

Membrane proteins are divided into two types according to their structure: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types of membrane proteins: transport proteins, enzymes, receptors, and structural proteins. Some membrane proteins have enzymatic activity, while others bind certain substances and facilitate their transfer into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores consisting of several protein subunits that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. The cytoskeleton gives mechanical strength to the cell membrane. In various membranes, proteins account for 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.

Carbohydrates are also present in the membrane, which can covalently bind to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outer side of the membrane there are receptor sites that bind various hormones. Other specific sections of the membrane can> t recognize and bind some proteins alien to these cells and various biologically active compounds.

The inner space of the cell is filled with cytoplasm, in which most enzyme-catalyzed reactions of cellular metabolism take place. The cytoplasm consists of two layers: the inner, called the endoplasm, and the peripheral, the ectoplasm, which has a high viscosity and is devoid of granules. The cytoplasm contains all the components of a cell or organelle. The most important of the cell organelles are the endoplasmic reticulum, ribosomes, mitochondria, the Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.

Endoplasmic reticulum is a system of interconnected channels and cavities penetrating the entire cytoplasm. It provides transport of substances from the environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site for lipid synthesis in the cell.

Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to the outer surface of the membranes of the endoplasmic reticulum and the nuclear membrane. They interact with informational and transport RNA, and protein synthesis is carried out in them. They synthesize proteins that enter the cisternae or the Golgi apparatus and are then released to the outside. Ribosomes that are free in the cytoplasm synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Various functional proteins are synthesized in ribosomes: carrier proteins, enzymes, receptors, cytoskeletal proteins.

golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that have entered here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merge with it, and the substances contained in the vesicles are removed from the cell in the process of exocytosis.

Lysosomes - particles surrounded by a membrane with a size of 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.

Peroxisomes formed from a smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is cleaved under the influence of peroxidases and catalase.

Mitochondria contain outer and inner membranes and are the "energy station" of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP is synthesized in them, the substrates of the Krebs cycle are oxidized, and many biochemical reactions are carried out. ATP molecules formed in mitochondria diffuse into all parts of the cell. Mitochondria contain a small amount of DNA, RNA, ribosomes, and with their participation, renewal and synthesis of new mitochondria takes place.

Microfilaments are thin protein filaments, consisting of myosin and actin, and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures inside cells.

microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives the cells a characteristic appearance and shape, serves as a site for attachment of intracellular organelles and various bodies. In nerve cells, bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the functioning of the mitotic spindle during cell division is carried out. They play the role of motor elements in the villi and flagella in eukaryotes.

Core is the main structure of the cell, is involved in the transmission of hereditary traits and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. Inside it is the nucleolus. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The rest of the nucleus contains chromatin, consisting of DNA, RNA, and a number of specific proteins.

Functions of the cell membrane

Cell membranes play an important role in the regulation of intracellular and intercellular metabolism. They are selective. Their specific structure makes it possible to provide barrier, transport and regulatory functions.

barrier function It manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.

Regulatory function membrane is the regulation of intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in the activity of enzymes.

transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or with the help of active transport.

Diffusion - the movement of a gas or solute along a concentration and electrochemical gradient. The diffusion rate depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, electric and concentration gradients for charged particles. simple diffusion occurs through the lipid bilayer or through channels. Charged particles move along the electrochemical gradient, while uncharged particles follow the chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc. penetrate through the lipid layer of the membrane by simple diffusion. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and uncontrolled channels. Depending on the selectivity, there are ion-selective ropes that allow only one ion to pass through, and channels that do not have selectivity. Channels have a mouth and a selective filter, and controlled channels have a gate mechanism.

Facilitated diffusion - a process in which substances are transported across a membrane by special membrane carrier proteins. In this way, amino acids and monosugars enter the cell. This mode of transport is very fast.

Osmosis - movement of water across a membrane from a solution with a lower osmotic pressure to a solution with a higher osmotic pressure.

Active transport - transfer of substances against a concentration gradient using transport ATPases (ion pumps). This transfer occurs with the expenditure of energy.

Na + /K + -, Ca 2+ - and H + pumps have been studied to a greater extent. Pumps are located on cell membranes.

A type of active transport is endocytosis and exocytosis. With the help of these mechanisms, larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through the channels are transported. This transport is more common in the epithelial cells of the intestine, renal tubules, and vascular endothelium.

At In endocytosis, cell membranes form invaginations into the cell, which, when laced, turn into vesicles. During exocytosis, vesicles with contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.

The structure and functions of the cell membrane

To understand the processes that ensure the existence of electrical potentials in living cells, it is first of all necessary to understand the structure of the cell membrane and its properties.

At present, the fluid-mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972, enjoys the greatest recognition. The basis of the membrane is a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the surrounding aquatic environment (Fig. 2).

Membrane proteins are localized on the membrane surface or can be embedded at different depths in the hydrophobic zone. Some proteins penetrate the membrane through and through, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they participate in the formation of ion channels, play the role of membrane pumps and carriers of various substances, and can also perform a receptor function.

The main functions of the cell membrane: barrier, transport, regulatory, catalytic.

The barrier function is to limit the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and to maintain a relatively constant content of various substances inside the cells. So, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.

Rice. 2. Three-dimensional scheme of the fluid-mosaic model of the Singer-Nicolson membrane

Globular integral proteins embedded in a lipid bilayer are shown. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains involved in cell recognition of each other and in the intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent areas of the "tails". The inner regions of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)

There are channels in the membrane through which ions penetrate. Channels are potential dependent and potential independent. Potential-gated channels open when the potential difference changes, and potential-independent(hormone-regulated) open when the receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(in the depth of the channel) and inactivation(on the surface of the channel). The gate can be in one of three states:

• open state (both types of gate are open);
• closed state (activation gate closed);
• inactivation state (inactivation gates are closed).

Another characteristic feature membranes is the ability to selectively transfer inorganic ions, nutrients, and various metabolic products. There are systems of passive and active transfer (transport) of substances. Passive transport is carried out through ion channels with or without the help of carrier proteins, and its driving force is the difference in the electrochemical potentials of ions between the intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and the chemical nature of the groups lining the channel walls and mouth.

At present, channels with selective permeability for Na + , K + , Ca 2+ ions and also for water (the so-called aquaporins) are the most well studied. The diameter of ion channels, according to various studies, is 0.5-0.7 nm. The throughput of the channels can be changed; 10 7 - 10 8 ions per second can pass through one ion channel.

Active transport occurs with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures embedded in the membrane and carrying out the transfer of ions towards a higher electrochemical potential.

The operation of the pumps is carried out due to the energy of ATP hydrolysis. Currently, Na + / K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + / K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na +, K +, Ca 2+ ions, respectively , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). Molecular mechanism active transport has not been fully elucidated.