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The cell membrane

All living organisms on Earth are composed of cells, and each cell is surrounded by a protective shell – membrane. However, the functions of the membrane are not limited to the protection of the organelles and the separation of one cell from another. The cell membrane is a complex mechanism, directly involved in reproduction, regeneration, nutrition, breathing and many other important functions of the cell.

The term "cell membrane"has been used for nearly a hundred years. The word"membrane"in Latin means"film". But in the case of the cell membrane would be correct to speak and together the two films connected in a certain way, and different sides of these films have different properties.

The cell membrane (cytolemma, plasma membrane) – a three-layer lipoprotein (fat-protein) envelope, separating each cell from neighboring cells and the environment, and performing the controlled exchange between the cells and the environment.

Crucial in this definition is not what the cell membrane separates one cell from another, and the fact that it provides its interaction with other cells and the environment. The membrane is very active, constantly working the cell structure, to which nature assigned many functions. In this article, you will learn all about the composition, structure, properties and functions of cell membranes, and also about the danger posed to human health disruption of cell membranes.

The content of the article:


History of the study of the cell membrane

In 1925, two German scientists, Gorter and Grendel were able to conduct a complex experiment on red blood cells of human blood erythrocytes. Using osmotic shock, researchers have found the so-called "shadow"– the empty membranes of red blood cells, then put them in one pile and measured the surface area. The next step was the calculation of the quantity of lipids in the cell membrane. With acetone, scientists have identified lipids from the "shadows" and determined that their just missing out on a double solid layer.

However, the experiment was allowed two gross errors:

  • The use of acetone is not possible to distinguish from membranes all lipids;
  • ploshad poverhnosti «tenej» byla vyschitana po suhomu vesu, chto tozhe nepravilno.

poskolku pervaya oshibka davala minus v raschetah, a vtoraya – plus, obshij rezultat okazalsya na udivlenie tochnym, i nemeckie uchenye prinesli v nauchnyj mir vazhnejshee otkrytie – lipidnyj bisloj kletochnoj membrany.

v 1935 godu drugaya para issledovatelej, danielli i douson, posle dolgih eksperimentov nad bilipidnymi plenkami prishli k vyvodu o prisutstvii v kletochnyh membranah belkov. inache nikak nelzya bylo obyasnit, pochemu eti plenki obladaut takim vysokim pokazatelem poverhnostnogo natyazheniya. uchenye predstavili vnimaniu obshestvennosti shematicheskuu model kletochnoj membrany, pohozhuu na sendvich, gde rol kusochkov hleba igraut odnorodnye lipidno-belkovye sloi, a mezhdu nimi vmesto masla – pustota.

v 1950 godu s pomoshu pervogo elektronnogo mikroskopa teoriu danielli-dousona udalos chastichno podtverdit – na mikrofotografiyah kletochnoj membrany byli otchetlivo vidny dva sloya, sostoyashih iz lipidnyh i belkovyh golovok, a mezhdu nimi prozrachnoe prostranstvo, zapolnennoe lish hvostikami lipidov i belkov.

v 1960 godu, rukovodstvuyas etimi dannymi, amerikanskij mikrobiolog dzh. robertson razrabotal teoriu o trehslojnom stroenii kletochnyh membran, kotoraya dolgoe vremya schitalas edinstvenno vernoj. odnako po mere razvitiya nauki rozhdalos vse bolshe somnenij otnositelno odnorodnosti etih sloev. s tochki zreniya termodinamiki takoe stroenie krajne nevygodno – kletkam bylo by ochen slozhno transportirovat veshestva vnutr i naruzhu cherez ves «buterbrod». krome togo, bylo dokazano, chto kletochnye membrany raznyh tkanej imeut raznuu tolshinu i sposob krepleniya, chto obuslovleno raznymi funkciyami organov.

v 1972 godu mikrobiologi s.d. singer i g.l. nikolson smogli obyasnit vse nestykovki teorii robertsona s pomoshu novoj, zhidkostno-mozaichnoj modeli kletochnoj membrany. uchenye ustanovili, chto membrana neodnorodna, assimetrichna, napolnena zhidkostu, i ee kletki prebyvaut v postoyannom dvizhenii. a belki, vhodyashie v ee sostav, imeut raznoe stroenie i naznachenie, krome togo, oni po-raznomu raspolagautsya otnositelno bilipidnogo sloya membrany.

v sostave kletochnyh membran prisutstvuut belki treh vidov:

  • perifericheskie – krepyatsya na poverhnosti plenki;
  • poluintegralnye – chastichno pronikaut vnutr bilipidnogo sloya;
  • integralnye – polnostu pronizyvaut membranu.

perifericheskie belki svyazany s golovkami membrannyh lipidov posredstvom elektrostaticheskogo vzaimodejstviya, i oni nikogda neform a continuous layer, as was considered earlier.And semi-integral and integral proteins serve to transport inside the cells of oxygen and nutrients, as well as for the withdrawal from it decay products, and even for several important functions, which you can find below.

Read more: Biological functions of lipids


Properties and functions of the cell membrane

The cell membrane performs the following functions:

  • Barrier – membrane permeability for different molecules are different.To pass the membrane of the cell, the molecule must have a certain size, chemical properties and electrical charge. Harmful or inappropriate molecules, due to the barrier function of the cell membrane, but cannot penetrate into the cell. For example, by reaction of peroxide membrane protects the cytoplasm from hazardous peroxides;
  • Transport through the membrane is passive, active, adjustable and selective sharing. A passive exchange is suitable for lipid-soluble substances and gases, consisting of very small molecules. These substances penetrate into and out of cells without the expenditure of energy, freely by diffusion. The active transport function of cell membranes is activated when in the cage or out of it you need to make the necessary but difficult the transported substance. For example, with the large size of the molecules, or are unable to cross silipigni layer due to hydrophobicity. Then start working proteins-pumps, including ATPase, which is responsible for the absorption in the cell of potassium ions and the ejection from it of sodium ions. Adjustable transport exchange necessary to carry out the functions of secretion and fermentation, for example, when cells produce and secrete hormones or gastric juice. All of these substances out of the cell through special channels in a given volume. A selective transport function is connected with those most integral proteins, which penetrate the membrane and provide a conduit for input and output strictly certain types of molecules;
  • Matrix – cell membrane determines and records the location of organelles relative to each other (the nucleus, mitochondria, chloroplasts) and regulates the interaction between them.
  • Mechanical – provides a limit of one cage from another, and, at the same time,— correct connection of cells inhomogenous tissue and the resistance of bodies to deformation;
  • Protective – as in plants and in animals, the cell membrane serves as a basis for building a protective frame. Examples include solid wood, thick rind, prickly thorns. In the animal world too, many examples of the protective function of cell membranes – a turtle shell, the chitinous shell, hooves and horns;
  • Energy — the processes of photosynthesis and cellular respiration would be impossible without the participation of proteins of the cell membrane, it is with the help of protein channels of cells exchange energy;
  • Receptorproteins embedded in the cell membrane, can have another important function. They serve as receptors by which the cell receives signals from hormones and neurotransmitters. And this, in turn, is necessary for nerve impulses and normal hormonal processes;
  • Enzyme is another important feature found in some proteins of cell membranes. For example, in the intestinal epithelium using such proteins are synthesized digestive enzymes;
  • Biopotentials – the concentration of potassium ions inside the cell is much higher than outside and the concentration of sodium ions, on the contrary, outside more than inside. This explains the difference in potentials inside the cell the charge is negative, outside is positive, which facilitates the movement of substances inside the cell and outside in any of the three types of exchange – phagocytosis, pinocytosis and exocytosis;
  • Marking on the surface of cell membranes are the so-called "shortcuts"- antigens composed of glycoproteins (proteins with attached thereto oligosacharide branched side chains). Because the side chain can have a huge variety of configurations, each cell type receives a unique label that allows other cells in the body to get to know them"in person" and correctly respond to them. That's why, for example, human immune cells, macrophages easily recognize a stranger penetrated into the body (infection, virus) and try to destroy it. The same thing happens with sick, mutated, and old cells – a shortcut on their cell membrane changes, and the body gets rid of them.

Cell metabolism occurs through a membrane, and can be accessed via three main reaction types:

  • Phagocytosis is the cellular process by which embedded in the cell membrane, phagocytes capture and digest solid particles of nutrients. In humans, phagocytosis is carried out by membranestwo types of cells: granulocytes (granular leukocytes) and macrophages (immune killer cells);
  • Pinocytosis – the process of capturing the surface of the cell membrane in contact with it molecules of the liquid. For the type of pinocytosis, the cell grows on the membrane a thin fluffy outgrowths in the form of the antennae, which surround a drop of liquid, and it turns out the bubble. First, the bubble protrudes above the surface of the membrane, and then "swallowed" - hiding inside the cell, and its walls have already merge with the inner surface of the cell membrane. Pinocytosis takes place in almost all living cells;
  • Exocytosis is the reverse process, in which cells are formed inside the secretory vesicles with the functional liquid (enzyme, hormone), and must find a way to get out of the cell into the environment. For this first bubble merges with the inner surface of the cell membrane then bulges outwards, bursts, churning the contents and again merges with the surface of the membrane, this time from the outside. Exocytosis takes place, for example, in cells of the intestinal epithelium and adrenal cortex.

The structure of the cell membrane

Cell membranes contain lipids three classes:

  • Phospholipids;
  • Glycolipids;
  • Cholesterol.

Phospholipids (combination of fat and phosphorus), and glycolipids (combination of fats and carbohydrates), in turn, consist of a hydrophilic head, from which depart two long hydrophobic tail. But cholesterol is a sometimes the space between these two tails and does not allow them to bend, which makes the membrane of some cells hard. In addition, the molecules of cholesterol regulate the structure of cell membranes and hinder the transition of polar molecules from one cell to another.

But the most important part, as can be seen from the previous section about the functions of cellular membranes are the proteins. Their composition, purpose and location are very diverse but have something in common that they all share: around the protein cell membrane is always located Annularly lipids. This special fats that are well structured, stable, are composed of more saturated fatty acids, and stand out from the membranes together with "related"proteins. It's kind of personal protective shell of protein, without which they would notworked.

The structure of the cell membrane three-layer. In the middle lies a relatively homogeneous liquid silipigni layer, and proteins, it is coated on both sides like a mosaic, partially penetrating into the interior. That is, it is wrong to think that the external protein layer of cell membranes unbroken. Proteins, in addition to the complex functions needed in the membrane to pass into cells and transport them outside of those substances which cannot penetrate through the fat layer. For example, ions of potassium and sodium. For them there is a special protein structure – ion channels, details of which we will describe below.

If you look at the cell membrane through a microscope, you can see the layer of lipids formed by tiny spherical molecules, which, like the sea, swim large protein chains of different shapes. Exactly the same membrane divide the inner space of each cell into compartments in which cozy are the nucleus, chloroplasts and mitochondria. Don't be inside the cell separate"rooms", the organelles would be stuck with each other and would be unable to perform its functions properly.

The cell is structured and separated using membranes a set of organelles that is involved in complex energy, the metabolic, information, and reproductive processes that provide the vital functions of the body.

As can be seen from this definition, the membrane is an important functional component of any cell. Its value is as great as the value of the nucleus, mitochondria and other cellular organelles. And the unique properties of the membrane due to its structure: it consists of two of the film, cobbled together with each other in a special way. The molecules of phospholipids are located in membrane hydrophilic heads outside and hydrophobic tails inside. So one side of the films moistened with water, and the other is not. So, these films are connected to each other with non-wetting sides inward, forming silipigni layer surrounded by protein molecules. This is the same "sandwich" structure of the cell membrane.

Ion channels of cell membranes

Consider in more detail the principle of operation of ion channels. What are they for? The fact that through the lipid membrane freely can penetrate only fat-soluble substances are gases, alcohols and fats. For example, in red blood cells constantly exchange oxygen and carbon dioxide, and our body did not have to resort to any additional tricks. But how can that be, when the need arises to transport through the cell membrane of aqueous solutions such as sodium salt and potassium?

Lay pilipinalayer way for such substances would be impossible because the holes would be immediately healed and stuck together back, such is the structure of any fat. But nature, as always, found a way out, and created a special transport protein structure.

There are two types of conductive proteins:

  • Conveyors – semi-integral protein pumps;
  • Cavalierly – integral proteins.

Proteins of the first type partially submerged in silipigni layer of the cell membrane, and head look outside and in the presence of the desired substance, they begin to behave like a pump: draw the molecule and absorb it into the cell. And proteins the second type of integral, have an elongated shape and are arranged perpendicular to pilipina layer of the cell membrane, penetrating her through and through. According to him, both the tunnels in the cage and out of cells substance move, unable to pass through the fat. It is through ion channels into the cell while potassium ions penetrate and accumulate in it, and the sodium ions, on the contrary, are ejected. There is a difference of electrical potential, as necessary for the proper functioning of all cells in our body.


The most important conclusions about the structure and function 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 cell membranes of the human body allowed to usenashvili a real breakthrough in science in General and medicine in particular. We do not randomly elaborated on ion channels, because it is here lies the answer to one of the most important issues of our time: why people are more likely to get cancer?

Cancer annually kills about 17 million lives worldwide and is the fourth most common cause of all deaths. According to who, the incidence of cancer is steadily increasing, and by the end of 2020 could reach 25 million per year.

What accounts for this epidemic of cancer, and where does the function of cell membranes? You say: the reason for the bad ecological situation, improper nutrition, bad habits and hard of heredity. And, of course, would be right, but if we talk about the problem more in detail, the reason is acidity of the human body. The above negative factors can lead to disruption of cellular membranes, depress breathing and nutrition.

Where there should be plus, minus is formed, and the cell cannot function normally. Butcancer cells need neither oxygen nor an alkaline environment – they are able to use anaerobic type of food. Therefore, under conditions of oxygen starvation and surpasses the level pH of a healthy cell is mutated, wanting to adapt to the environment and become cancer cells. So the man falls ill and Oncology. To avoid this, you only need to drink enough pure water daily, and refuse to carcinogens in food. But generally, people are well aware of the harmful products and the need for quality water, and do nothing – hope that the trouble will pass them by.

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

The antibiotics of the last generation when injected into the blood do not kill all cells in a row, and are looking for exactly the cells of the pathogen, focusing on markers in its cell membranes. Latest drugs against migraine, triptan, only constrict the inflamed blood vessels of the brain, while almost not affecting the heart and peripheral circulatory system. And learn they need the blood vessels in proteins in their cell membranes. There are many examples, so it's safe to say that knowledge about the structure and function of cell membranes is the basis for the development of modern medical science and save millions of lives every year.