What is concentration? In a broad sense, this is the ratio of the volume of a substance and the number of particles dissolved in it. This definition is found in a wide variety of branches of science, from physics and mathematics to philosophy. In this case, we are talking about the use of the concept of “concentration” in biology and chemistry.

Gradient

Translated from Latin, this word means “growing” or “walking”, that is, it is a kind of “pointing finger” that shows the direction in which any value increases. As an example, we can use, for example, the height above sea level at different points on the Earth. Its (altitude) gradient at each individual point on the map will show a vector of increasing value until the steepest rise is reached.

In mathematics, this term appeared only at the end of the nineteenth century. It was introduced by Maxwell and proposed his own designations for this quantity. Physicists use this concept to describe the strength of an electric or gravitational field and a change in potential energy.

Not only physics, but also other sciences use the term “gradient”. This concept can reflect both qualitative and quantitative characteristics of a substance, for example, concentration or temperature.

Concentration gradient

Now we know what concentration is? This shows the proportion of a substance contained in a solution. It can be calculated as a percentage of the mass, the number of moles or atoms in a gas (solution), or a fraction of the whole. Such a wide choice makes it possible to express almost any ratio. And not only in physics or biology, but also in the metaphysical sciences.

In general, a concentration gradient is one that simultaneously characterizes the amount and direction of change of a substance in the environment.

Definition

Is it possible to calculate the concentration gradient? Its formula represents the difference between an elementary change in the concentration of a substance and the long path that the substance will have to overcome to achieve equilibrium between two solutions. Mathematically, this is expressed by the formula C = dC/dl.

The presence of a concentration gradient between two substances causes them to mix. If particles move from an area of ​​higher concentration to a lower one, then this is called diffusion, and if there is a semi-permeable obstacle between them, it is called osmosis.

Active transport

Active and passive transport reflects the movement of substances through the membranes or layers of cells of living beings: protozoa, plants, animals and humans. This process takes place using thermal energy, since the transition of substances occurs against a concentration gradient: from less to more. Most often, adenosine triphosphate or ATP is used to carry out this interaction, a molecule that is a universal source of energy of 38 Joules.

There are different forms of ATP that are located on cell membranes. The energy contained in them is released when molecules of substances are transferred through so-called pumps. These are pores in the cell wall that selectively absorb and pump out electrolyte ions. In addition, there is such a transport model as simport. In this case, two substances are simultaneously transported: one leaves the cell, and the other enters it. This saves energy.

Vesicular transport

Active and involves the transport of substances in the form of vesicles or vesicles, which is why the process is called, accordingly, vesicular transport. There are two types of it:

1. Endocytosis. In this case, bubbles are formed from the cell membrane as it absorbs solid or liquid substances. Vesicles may be smooth or have a border. Eggs, white blood cells, and kidney epithelium have this method of nutrition.
2. Exocytosis. Based on the name, this process is the opposite of the previous one. Inside the cell there are organelles (for example, the Golgi apparatus) that “package” substances into vesicles, and they subsequently exit through the membrane.

Passive transport: diffusion

Movement along a concentration gradient (from high to low) occurs without the use of energy. There are two options for passive transport - osmosis and diffusion. The latter can be simple and lightweight.

The main difference between osmosis is that the process of moving molecules occurs through a semi-permeable membrane. And diffusion along a concentration gradient occurs in cells that have a membrane with two layers of lipid molecules. The direction of transport depends only on the amount of substance on both sides of the membrane. In this way, polar molecules, urea, penetrate into cells, and proteins, sugars, ions and DNA cannot penetrate.

During the process of diffusion, molecules tend to fill the entire available volume, as well as equalize the concentration on both sides of the membrane. It happens that the membrane is impermeable or poorly permeable to the substance. In this case, it is affected by osmotic forces, which can both make the barrier denser and stretch it, increasing the size of the pumping channels.

Facilitated diffusion

When the concentration gradient is not a sufficient basis for the transport of a substance, specific proteins come to the rescue. They are located on the cell membrane in the same way as ATP molecules. Thanks to them, both active and passive transport can be carried out.

In this way, large molecules (proteins, DNA), polar substances, which include amino acids and sugars, and ions pass through the membrane. Thanks to the participation of proteins, the transport speed increases several times compared to conventional diffusion. But this acceleration depends on several reasons:

• gradient of matter inside and outside the cell;
• number of carrier molecules;
• rates of binding of the substance and the carrier;
• the rate of change in the inner surface of the cell membrane.

Despite this, transport is carried out thanks to the work of carrier proteins, and ATP energy is not used in this case.

The main features that characterize facilitated diffusion are:

1. Fast transfer of substances.
2. Selectivity of transport.
3. Satiety (when all proteins are occupied).
4. Competition between substances (due to affinity for protein).
5. Sensitivity to specific chemical agents - inhibitors.

Osmosis

As mentioned above, osmosis is the movement of substances along a concentration gradient through a semi-permeable membrane. The Lechatelier-Brown principle describes the process of osmosis most fully. It states that if a system in equilibrium is influenced from the outside, it will tend to return to its previous state. The phenomenon of osmosis was first encountered in the middle of the 18th century, but then it was not given much importance. Research into the phenomenon began only a hundred years later.

The most important element in the phenomenon of osmosis is a semi-permeable membrane, which allows only molecules of a certain diameter or properties to pass through. For example, in two solutions with different concentrations, only the solvent will pass through the barrier. This will continue until the concentration on both sides of the membrane is the same.

Osmosis plays a significant role in cellular life. This phenomenon allows only those substances that are necessary to maintain life to penetrate into them. The red blood cell has a membrane that allows only water, oxygen and nutrients to pass through, but proteins that are formed inside the red blood cell cannot get out.

The phenomenon of osmosis has found and practical application in everyday life. Without even knowing it, people in the process of salting food used precisely the principle of the movement of molecules along a concentration gradient. The saturated saline solution “pulled out” all the water from the products, thereby allowing them to be stored longer.

Equilibrium potential– is the value of the transmembrane difference electric charges, at which the current of ions into and out of the cell becomes the same, i.e. in fact, the ions do not move.

The concentration of potassium ions inside the cell is much greater than in the extracellular fluid, and the concentration of sodium and chlorine ions, on the contrary, is much greater in the extracellular fluid. Organic anions are large molecules that do not pass through the cell membrane.

This concentration difference or concentration gradient is the driving force for the diffusion of dissolved ions to a region of lower concentration or, in accordance with the second law of thermodynamics, to a lower energy level. Thus, sodium cations must diffuse into the cell, and potassium cations must diffuse out of it.

It is also necessary to take into account the permeability of the cell membrane for various ions, and it changes depending on the state of cell activity. At rest, only potassium ion channels are open at the plasma membrane, through which no other ions can pass.

Leaving the cell, potassium cations reduce the number of positive charges in it and at the same time increase their number on the outer surface of the membrane. The organic anions remaining in the cell begin to limit the further release of potassium cations, since an electric field arises between the anions of the inner surface of the membrane and the cations of its outer surface and electrostatic attraction. The cell membrane itself turns out to be polarized: positive charges are grouped on its outer surface, negative charges are grouped on the inner surface.

Thus, if the membrane is ready to pass any ions, then the direction of the ion current will be determined by two circumstances: the concentration gradient and the action of the electric field, and the concentration gradient can direct the ions in one direction, and the electric field in the other. When these two forces are balanced, the flow of ions practically stops, since the number of ions entering the cell becomes equal to the number leaving. This condition is called equilibrium potential.

Active transport T

Diffusion of ions should reduce the concentration gradient, but concentration equilibrium would mean death for the cell. It is no coincidence that it spends more than 1/3 of its energy resources on maintaining gradients and maintaining ion asymmetry. The transport of ions across the cell membrane against concentration gradients is active, i.e. energy-consuming mode of transport, it is provided by the sodium-potassium pump.

This is a large integral protein of the cell membrane, which continuously removes sodium ions from the cell and simultaneously pumps potassium ions into it. This protein has the properties of ATPase, an enzyme that breaks down ATP on the inner surface of the membrane, where the protein attaches three sodium ions. The energy released during the splitting of the ATP molecule is used to phosphorylate certain parts of the pump protein, after which the protein conformation changes and it takes three sodium ions out of the cell, but at the same time takes two potassium ions from the outside and brings them into the cell (Fig. 4.1).

Thus, in one pump cycle, three sodium ions are removed from the cell, two potassium ions are brought into it, and the energy of one ATP molecule is spent on this work. This is how a high concentration of potassium is maintained in the cell, and sodium in the extracellular space. If we consider that both sodium and potassium are cations, i.e. carry positive charges, then the net result of one cycle of operation of the pump to distribute electrical charges is the removal of one positive charge from the cell. As a result of this activity, the membrane becomes slightly more negative from the inside and therefore the sodium-potassium pump can be considered electrogenic.

In 1 second, the pump is capable of removing about 200 sodium ions from the cell and simultaneously transporting approximately 130 potassium ions into the cell, and 100-200 such pumps can be located on one square micrometer of the membrane surface. In addition to sodium and potassium, the pump transports glucose and amino acids into the cell against concentration gradients; This, as it were, passing transport, received the name: simport. The performance of the sodium-potassium pump depends on the concentration of sodium ions in the cell: the higher it is, the faster the pump works. If the concentration of sodium ions in the cell decreases, then the pump will reduce its activity.

Along with the sodium-potassium pump, there are special pumps for calcium ions in the cell membrane. They also use ATP energy to carry calcium ions out of the cell, resulting in a significant concentration gradient of calcium: there is much more of it outside the cell than in the cell. This causes calcium ions to constantly strive to enter the cell, but at rest the cell membrane almost does not allow these ions to pass through. However, sometimes the membrane opens channels for these ions and then they play very important role in the release of mediators or in the activation of certain enzymes.

Thus, active transport creates concentration and electrical gradients that play a prominent role in the entire life of the cell.

Concentration gradient(from lat. grady, gradu, gradus- progress, movement, flow, approach; con- with, together, jointly + centrum- center) or concentration gradient is vector physical quantity, characterizing the magnitude and direction of the greatest change concentrations any substance in the environment. For example, if we consider two regions with different concentrations of a substance, separated by a semi-permeable membrane, then the concentration gradient will be directed from the region of lower concentration of the substance to the region with higher concentration.

Active transport- transfer of matter through cellular or intracellular membrane(transmembrane A.t.) or through a layer of cells (transcellular A.t.), flowing against concentration gradient from an area of ​​low concentration to an area of ​​high, i.e., with the expenditure of free energy of the body. In most cases, but not always, the source of energy is the energy of high-energy bonds ATP.

Various transport ATPases, localized in cell membranes and involved in the mechanisms of substance transfer, are the main element of molecular devices - pumps that ensure the selective absorption and pumping out of certain substances (for example, electrolytes) by the cell. Active specific transport of non-electrolytes (molecular transport) is realized using several types of molecular machines - pumps and carriers. Transport of non-electrolytes (monosaccharides, amino acids and other monomers) can be coupled with simport- transport of another substance, the movement of which against the concentration gradient is a source of energy for the first process. Symport can be provided by ion gradients (for example, sodium) without the direct participation of ATP.

Passive transport- transfer of substances through concentration gradient from an area of ​​high concentration to an area of ​​low, without energy expenditure (for example, diffusion, osmosis). Diffusion is the passive movement of a substance from an area of ​​higher concentration to an area of ​​lower concentration. Osmosis is the passive movement of certain substances through a semi-permeable membrane (usually small molecules pass through, large molecules do not pass through).

There are three types of penetration of substances into cells through membranes: simple diffusion, facilitated diffusion, active transport.

Simple diffusion

In simple diffusion, particles of a substance move through the lipid bilayer. The direction of simple diffusion is determined only by the difference in the concentrations of the substance on both sides of the membrane. By simple diffusion they penetrate into the cell hydrophobic substances (O2, N2, benzene) and polar small molecules (CO 2, H 2 O, urea). Polar relatively large molecules (amino acids, monosaccharides), charged particles (ions) and macromolecules (DNA, proteins) do not penetrate.

Facilitated diffusion

Most substances are transported across the membrane using transport proteins (carrier proteins) immersed in it. All transport proteins form a continuous protein passage across the membrane. With the help of carrier proteins, both passive and active transport of substances is carried out. Polar substances (amino acids, monosaccharides), charged particles (ions) pass through membranes using facilitated diffusion, with the participation of channel proteins or carrier proteins. The participation of carrier proteins provides a higher rate of facilitated diffusion compared to simple passive diffusion. The rate of facilitated diffusion depends on a number of reasons: on the transmembrane concentration gradient of the transported substance, on the amount of the transporter that binds to the transported substance, on the rate of binding of the substance by the transporter on one surface of the membrane (for example, on the outer surface), on the rate of conformational changes in the transporter molecule, in as a result of which the substance is transferred through the membrane and released on the other side of the membrane. Facilitated diffusion does not require special energy costs due to ATP hydrolysis. This feature distinguishes facilitated diffusion from active transmembrane transport.

Gradient (in biology) Gradient in biology, a natural quantitative change in morphological or functional, including biochemical, properties along one of the axes of the body of an organism (or organ) at any stage of its development. Examples of G.: a decrease in the yolk content in amphibian eggs in the direction from the vegetative pole to the animal pole, unequal sensitivity to poisons and dyes of different parts of the body of coelenterates and worms. G., reflecting a decrease or increase in the intensity of metabolism or other physiological indicators, is called physiological, or metabolic. An example of physiological G.: a decrease in the ability to automatically contract parts of the heart in vertebrates from the venous end to the aortic end. The place of highest manifestation of the function is called highest level G., the area with the least manifestation of the function - level. According to the ideas of the American scientist Charles Child, physiological growth is the root cause of differentiation of the embryo and integration of the adult organism; however, often pregnancy is not a cause, but only a consequence of broader biological patterns of development. L. V. Belousov.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

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