Active transport molecules. Active transport of ions and molecules through the membrane

Transport? Transmembrane movement of various high-molecular compounds, cellular components, supramolecular particles that are not able to penetrate through the channels in the membrane are carried out by means of special mechanisms, for example, using phagocytosis, pinocytosis, exocytosis, transferred through the intercellular space. That is, the movement of substances through the membrane can occur using various mechanisms that are divided into signs of specific carriers in them, as well as in energy consumption. Scientists subdivide transport substances to active and passive.

Main types of transport

Passive transport is a transfer of a substance through the biological membrane according to the gradient (osmotic, concentration, hydrodynamic and other), which does not require energy consumption.

It is the transfer of a substance through the biological membrane against the gradient. In this case, energy is consumed. Approximately 30 - 40% of the energy that is formed as a result of metabolic reactions in the human body is spent on the implementation of active vehicles of substances. If we consider the functioning of human kidney, then about 70% of consumed oxygen is spent on active transport.

Passive transport substances

it implies the transfer of various substances through biological membranes according to a variety of can be:

• electrochemical potential gradient;
• gradient concentration of substance;
• electric field gradient;
• osmotic pressure gradient and other.

The process of carrying out passive transport does not require any energy consumption. It can occur with the help of lightweight and simple diffusion. As we know, the diffusion is a chaotic movement of molecules of matter in a variety of media, which is due to the energy of thermal oscillations of the substance.

If the particle of the substance is an electronic system, the direction in which diffusion will occur is determined by the difference in the concentration of substances contained in environments that are separated by the membrane. For example, between cell compartments, inside the cell and outside it. If the particles of the substance, its ions have an electrical charge, then diffusion will depend not only on the difference in concentrations, but also on the charge size of this substance, the presence and signs of the charge on both sides of the membrane. The magnitude of an electrochemical gradient is determined by the algebraic amount of electrical and concentration gradients on the membrane.

What provides transport through the membrane?

Passive membrane transport is possible due to the presence of a substance, osmotic pressure arising between different sides of the cell membrane or an electric charge. For example, the average level of Na + ions contained in the blood plasma is about 140 mm / l, and its content in red blood cells is about 12 times more. A similar gradient, expressed in the difference in concentrations, is able to create a driving force that ensures the transfer of sodium molecules to blood plasma erythrocytes.

It should be noted that the speed of such a transition is very low due to the fact that the cell membrane is characterized by low permeability for the ions of this substance. A much greater permeability of this membrane has for potassium ions. The energy of cell metabolism is not used to perform the process of simple diffusion.

Diffusion speed

Active and passive transport of substances through the membrane is characterized by diffusion rate. It is possible to describe it using the fic equation: dm / dt \u003d -ksΔc / x.

In this case, the DM / DT is the amount of that substance that diffuses for one unit of time, and K is the diffusion process coefficient, which characterizes the permeability of the biomembrane for the diffusant substance. S is equal to the area at which diffusion occurs, and Δc expresses the difference in the concentration of substances from different sides of the biological membrane, while X characterizes the distance that is between diffusion points.

It is obvious that those substances that diffuse simultaneously on the gradients of concentrations and electric fields will be easily moved through the membrane. An important condition for the impact of the substance through the membrane is the physical properties of the membrane itself, its permeability for each particular substance.

Due to the fact that the bilayer membrane is formed by hydrocarbon radicals of phospholipids, possessing nature with ease diffuse through it. In particular, this refers to substances that are easily dissolved in lipids, for example, thyroid and steroid hormones, as well as some drugs of a narcotic nature.

Mineral ions and low molecular weight substances having a hydrophilic nature diffuse via passive ionic membrane channels, which are formed from channel-forming protein molecules, and sometimes through the defects of the membrane packaging of phospholipid molecules, which occur in the cell membrane as a result of thermal fluctuations.

Passive transport through the membrane - the process is very interesting. If the conditions are normal, then significant amounts of substances can penetrate through the bilayer membrane only if they are non-polar and have a small size. Otherwise, the transfer occurs by means of carrier proteins. Such processes involving protein-carriers are called non-diffusion, but by transport substances through the membrane.

Light diffusion

Light diffusion, like a simple diffusion, occurs according to the gradient of the concentration of the substance. The main difference is that in the process of transferring a substance, a special protein molecule is involved, called a carrier.

Light diffusion is a type of passive transfer of substance molecules through biomembranes, carried out by a concentration gradient using a carrier.

Self protein states

The protein carrier can be in two conformational states. For example, in a state, and this protein may have affinity with the substance that it transfers, its sections for binding to the substance are deployed inside, due to which it is time formed, open to one side of the membrane.

After the protein contacted the tolerated substance, its conformation changes and its transition to the state of B. With such a transformation of the carrier affecting the substance is lost. Due to the carrier, it is released and moved to the time already on the other side of the membrane. After the substance is transferred, the protein carrier changes its conformation again, returning to the state A. Similar transport of the substance through the membrane is called unport.

Light diffusion rate

Low molecular weight substances like glucose can be transported through the membrane by means of light diffusion. Such transport can occur from blood to the brain, in cells from interstitial spaces. The rate of substance transfer with this form of diffusion is capable of reaching up to 10 8 particles through the channel in one second.

As we already know, the speed of active and passive vehicles of substances with a simple diffusion is proportional to the difference in the concentration of the substance on both sides of the membrane. In the case of light diffusion, this speed increases in proportion to the increasing difference in the concentration of the substance to a certain maximum value. Above this value, the speed does not increase, even though the difference in concentrations from different sides of the membrane continues to increase. The achievement of such a maximum speed point in the process of implementing a light diffusion can be explained by the fact that the maximum speed involves engaging in the process of transferring all available carriers.

What concept still includes active and passive transport through membranes?

Exchange Diffusion

A similar type of transport of molecules of matter through the cell membrane is characterized by the fact that the molecules of the same substance are involved in the exchange, which are from different sides of the biological membrane. It is worth noting that with such transport substances on both sides of the membrane absolutely does not change.

A variety of exchange diffusion

One of the varieties of exchange diffusion is the exchange in which the molecule of one substance changes to two or more molecules of other substances. For example, one of the paths by which the removal of the positive calcium ions from the smooth muscle cells of the bronchi and vessels of the contracting myocytes of the heart is exchanged on sodium ions located outside the cell. One sodium ion in this case is exchanged for three calcium ions. Thus, sodium and calcium movement occurs through the membrane, which is mutually coherent. A similar type of passive transport Through the cell membrane is called an antiport. It is in this way a cell is able to free themselves from calcium ions, which are existing in excess. This process is necessary in order for smooth myocytes and cardiomyocytes relax.

This article covers the active and passive transport of substances through the membrane.

Passive transportationincludes simple and light diffusion - processes that do not require energy costs. Diffusion - transport of molecules and ions through a membrane from a high-to-area area with a low concentration, those. Substances come under a concentration gradient. Diffusion of water through semipermeable membranes is called osmosis. Water is capable of passing through the membrane pores formed by proteins, and tolerate molecules and ions of substances dissolved in it. Mixing of simple diffusion is carried out by transferring small molecules (for example, O2, H2O, CO2); This process is minority and proceeds at a rate proportional to the concentration gradient of the transported molecules on both sides of the membrane. Light diffusion is carried out through channels and (or) carriers proteins that have specificity in relation to the transported molecules. Transmembrane proteins, forming small aqueous pores, act as ion channels, which are transported by small water soluble molecules and ions by an electrochemical gradient. Proteins-carriers are also transmembrane proteins that undergo reversible changes in conformation, providing the transport of specific molecules through the plasmolm. They operate in the mechanisms of both passive and active transport.

Active transport It is an energy-intensive process, due to which the transfer of molecules is carried out with the help of carrier proteins against an electrochemical gradient. An example of a mechanism that ensures the oppositely directed active transport of ions is the sodium-potassium pump (presented by the Na +-C + -Tphaze protein), thanks to which Na + ions are output from the cytoplasm, and the ions to + are simultaneously transferred to it. The concentration of K + inside the cell is 10-20 times higher than outside, and the concentration of Na is the opposite. Such a difference in the concentrations of ions is provided by the work (Na * -k *\u003e pump. To maintain this concentration, the three Na ions from the cell per every two ions to * into the cell is carried out. In this process, the protein takes part in the membrane that performs the function of the enzyme splitting ATP, with the release of the energy required for the pump operation.
The participation of specific membrane proteins in passive and active transport indicates the high specificity of this process. This mechanism ensures maintenance of the constancy of the volume of the cell (by regulating the osmotic pressure), as well as the membrane potential. The active transport of glucose into the cell is carried out by a protein-carrier and combined with unidirectional transfer of the NA + ion.

Lightweight transport ions is mediated by special transmembrane proteins - ion channels providing selective transfer of certain ions. These channels consist of the actual transport system and the portable mechanism, which opens the channel for some time in response to (a) change in the membrane potential, (b) mechanical effects (for example, in the hair cells of the inner ear), (c) ligand binding (signal molecule or ion).

Transport through the membrane of small molecules.

Membrane transport may include a unidirectional transfer of some substance molecules or joint vehicles of two different molecules in one or opposite directions.

Through it, various molecules are passing through it and the larger the size of the molecules, the less the speed of passing them through the membrane. This property determines the plasma membrane as an osmotic barrier. The maximum penetrating ability is water and gas dissolved in it. One of the most important properties of the plasma membrane is associated with the ability to skip various substances into the cell or from it. This is necessary to maintain the constancy of its composition (i.e. homeostasis).

Transport ions.

In contrast to artificial bilayer lipid membranes, natural membranes, and primarily plasma membrane, they are still able to transport ions. The permeability for ions is small, and the speed of passing different ions is non-etinakov. Higher passing speed for cations (K +, Na +) and is significantly lower for anions (CL-). Transport of ions through Plasmalemmum is through the participation in this process of membrane transport proteins - permeasis. These proteins can carry out transport in one direction of the same substance (unite) or several substances at the same time (sympl), or together with the import of a single substance, output the other (antiport). For example, glucose can be in cells sympathetically together with the Na + ion. Transport ions can occur by gradient concentration- passivelyno extra energy costs. For example, the Na + ion from the external environment penetrates the cage, where its concentration is higher than in the cytoplasm.

The presence of protein transport channels and carriers should seem to lead to the equilibration of the concentrations of ions and low molecular weight substances on both sides of the membrane. In fact, this is not the case: the concentration of ions in the cytoplasm of cells is sharply different from only such in the external environment, but even from plasma of blood, washing cells in the animal body.

It turns out in the cytoplasm concentration K + almost 50 times higher, and Na + is lower than in blood plasma. Moreover, this distinction is supported only in a living cell: if the cell is killed or suppressed in it metabolic processes, after some time, ionic differences on both sides of the plasma membrane will disappear. You can simply cool the cells to + 20 ° C, and after some time the concentration of K + and Na + on both sides of the membrane will become the same. When cell heating, this difference is restored. This phenomenon is due to the fact that in cells there are membrane protein carriers that operate against a concentration gradient, spending energy at the expense of ATP hydrolysis. This type of work is called active transportand it is carried out with the help of protein ion pumps. The plasma membrane contains a two-buggy molecule (K + + Na +) - the pump, which is at the same time and atpasus. This pump, when working, pumps out one cycle 3 ion Na + and pumps 2 ion k + against the concentration gradient. At the same time, one ATP molecule is spent on the phosphorylation of the ATPase, with the result that Na + is transferred through the membrane from the cell, and K + is able to contact the protein molecule and then transferred to the cell. As a result of active transport using membrane pumps, regulation in the concentration cell and bivalent mg2 + and Ca2 + cations, also with the cost of ATP, occurs.

So the active transport of glucose, which is sympathetic (at the same time) penetrates the cell together with the flow of passively transported Na + ion, will depend on the activity (K + + Na +) - the pump. If this (k + -na +) is a pump to block, then the difference between the Na + concentration on both sides of the membrane will disappear, then the diffusion of Na + inside the cell, and at the same time the glucose intake in the cell will be stopped. As soon as the operation (K + -NA +) is the ATPase and the difference in the concentration of ions is created, the diffuse stream Na + immediately increases and the glucose transportation is at the same time. Likewise, it is carried out through the membrane and the flow of amino acids, which are transferred to special carrier proteins working as a system of sympathetic, transferring ions at the same time.

Active transport of sugars and amino acids in bacterial cells is due to a gradient of hydrogen ions. The participation of special membrane proteins involved in the passive or active transport of low molecular weight compounds, indicates the high specificity of this process. Even in the case of passive ion transport, the proteins "recognize" this ion, interact with it, are associated

specifically, they change their conformation and function. Therefore, already on the example of transporting simple substances membrane act as analyzers as receptors. Especially such a receptor role is manifested when absorbing by a cell of biopolymers.

Lipid bilayers are largely impermeable for the overwhelming majority of substances, and therefore transferring through the lipid phase requires significant energy costs.

Distinguish active transport and passive transportation (diffusion).

Passive transportation

Passive transport is the transfer of molecules at a concentration or electrochemical gradient, i.e. it is determined only by the difference in the concentration of the portable substance on the opposite sides of the membrane or the direction of the electric field and is carried out without the cost of ATP energy. Two types of diffusion are possible: simple and lightweight.

Simple diffusion happens without the participation of membrane protein. The speed of simple diffusion is well described by conventional diffusion laws for substances soluble in lipid bisal; It is directly proportional to the degree of hydrophobicity of the molecule, i.e. its fat-solventness, as well as a concentration gradient. The mechanism of diffusion of water-soluble substances is less studied. The transfer of a substance through lipid bilayer, for example, compounds such as ethanol, is possible through time pores in the membrane formed by the gaps in the lipid layer when the membrane lipids move. According to the mechanism of simple diffusion, a transmembrane transfer of gases is carried out (for example,

Fig. 22.5.

0 2 and C0 2), oxen, some simple organic ions and a number of low molecular weight fat-soluble compounds. It should be remembered that simple diffusion is carried out indiscriminately and is low.

Light diffusion, Unlike simple diffusion, specific membrane proteins in this process are facilitated. Consequently, light diffusion is a diffusion process associated with a chemical reaction of the interaction of the transported substance with the BSLC-PSRS. This process is specific and occur at higher speed than simple diffusion.

There are two types of membrane transport proteins: proteins-carriers called translocasa or permeazami and channel-forming proteins.Transport proteins associate specific substances and transfer them through the bilayer to the gradient of their concentration or electrochemical potential, and, therefore, for the implementation of this process, as with a simple diffusion, the energy costs of ATP are not required.

The specific mechanism for the functioning of translocase in lightweight diffusion is not sufficiently studied. It is believed that after binding a portable substance with a protein-carrier, a number of conformational changes of the latter occur, allowing the associated substance on one side of the membrane to transport to another according to the scheme (Fig. 22.5).

Another possible variant of the transfer mechanism - by the so-called relay type When the transport protein is generally not able to move through the bipper. In this case, the transportable substance may mostly transform itself from one protein to the other until it turns out to be on the opposite side of the membrane.

The channel-forming proteins (or proteins channels) form transmembrane hydrophilic channels through which the solutes of the appropriate sizes and charge molecules can be passed through lightweight diffusion. In contrast to the transport carried out by translocases, the transfer using channels does not have a high specificity, but can be carried out with a much larger speed, the NA of achieving saturation in a wide range of concentration of the transported substance (Fig. 22.6). Some channels are constantly open, while others are opened only in response to the binding of the transported substance. This leads to a change in the conformation of the transport protein, as a result of which the hydrophilic channel opens in the membrane and the substance is released on the other side of the membrane (see Fig. 22.6).

Fig. 22.6.

To date, the structure and mechanism of functioning of transport proteins is not sufficiently studied, which is largely due to the difficulty of their allocation in solubilized form. Apparently, the most common means of transmembrane transfer of substances according to the mechanism of lightweight diffusion is transport using channel-forming substances.

Fig. 22.7.

Proteins - carriers of all types resemble the enzymes associated with membranes, and the process of lightweight diffusion - an enzymatic reaction for a number of properties: 1) transport proteins have high specificity and have sections (sites) of binding for the transported molecule (by analogy - substrate); 2) When all binding sites are occupied (i.e., the protein is saturated), the speed of transport reaches the maximum value indicated U TLH (Fig. 22.7); 3) The carrier protein has a binding constant for it To m, an equal concentration of the transported substance, in which the speed of transport is half its maximum value (similarly To M. For the system, the enzyme substrate), transport proteins are sensitive to changing the pH value of the medium; 4) They are inhibited by competitive or non-competitive inhibitors. However, in contrast to the enzyme reaction, the molecule of the transported substance does not undergo covalent transformation when interacting with the transport protein (Fig. 22.7).

Light diffusion is usually characteristic of water-soluble substances: carbohydrates, amino acids, metabolically important organic acids, some ions. The lightweight diffusion is also carried out by transport of steroid hormones, a number of fat soluble vitamins and other molecules of this class. Practically directed flows of substances in a cell by simply and light diffusion are never stopped, since substances that entered the cell are involved in metabolic transformations, and their decrease is constantly replenished by transmembrane transfer by a concentration gradient.

Passive transport - transport of substances in a gradient of a concentration that does not require energy costs. Passively transport hydrophobic substances through lipid bilayer. Passively passes through themselves all proteins and channels and some carriers. Passive transport with the participation of membrane proteins is called light diffusion.

Other proteins-carriers (they are sometimes called proteins - pumps) are transferred through the membrane substance with energy costs, which is usually supplied during ATP hydrolysis. This type of transport is carried out against the gradient of the concentration of the portable substance and is called active transport.

Simport, Antiport and Unport

The membrane transport substances also differ in the direction of their movement and the number of substances portable to the data:

1) Unifier - transport of one substance in one direction depending on the gradient

2) Simport - transport of two substances in one direction through one carrier.

3) Antiport - moving two substances in different directions through one carrier.

Unport Conducts, for example, the potential-dependent sodium channel through which sodium ions are moved into the cell during the generation of the potential.

Simport It carries out a glucose carrier located on the outer (converted intestinal lumen) side of intestinal epithelium cells. This protein captures the glucose molecule and sodium ion and, changing the conformation, tolerates both substances inside the cell. At the same time, the energy of an electrochemical gradient is used, which is created in its queue due to hydrolysis of ATP sodium-potassium ATP-Aza.

Antiport Conducts, for example, sodium-potassium atpasis (or sodium-dependent ATPAZ). It transfers potassium ions into the cage. And from the cell - sodium ions.

The work of sodium-potassium atpase as an example of an antiport and active transport

Initially, this carrier attaches three ions from the inside of the membrane Na. +. These ions change the conformation of the active center of the ATPase. After such activation of the ATPAZ can hydrolyze one ATP molecule, and phosphate ion is fixed on the surface of the carrier from the inside of the membrane.

The separated energy is spent on the change in the conformation of the ATPase, after which three ions Na. + and ion (phosphate) turn out to be on the outside of the membrane. Here are ions Na. + split off, and replaced by two ions K. +. Then the conformation of the carrier varies to the original, and ions K. + It turns out to be on the inside of the membrane. Here are ions K. + Split, and the carrier is ready to work again.

More briefly actions of the ATPase can be described as follows:

1) she is from the inside the cells "takes" three ions Na. +, then splits the ATP molecule and joins phosphate to him

2) "throws" ions Na. + and joins two ions K. + From the external environment.

3) disconnects phosphate, two ions K. + throws inside the cell

As a result, a high concentration of ions is created in the extracellular medium Na. +, and inside the cell - high concentration K. +. Work Na. + , K. + - Atfaz creates not only the difference between concentrations, but also the difference of charges (it works as an electrical pump). On the outside of the membrane, a positive charge is created, on the inner - negative.

AND active transport. Passive transport occurs without energy costs on an electrochemical gradient. Passive includes diffusion (simple and light), osmosis, filtering. Active transport requires energy and occurs contrary to a concentration or electric gradient.
Active transport
This transport substances are contrary to a concentration or electric gradient, which is happening with energy costs. The primary active transport is distinguished, which requires the energy of ATP, and secondary (creation by the ATP of ionic concentration gradients on both sides of the membrane, and the energy of these gradients is used for transport).
Primary active transport is widely used in the body. It participates in the creation of the difference in electrical potentials between the inner and outer sides of the cell membrane. With the help of active transport, various concentrations of Na +, K +, N +, SI "and other ions in the middle of the cell and in extracellular fluid are created.
The transport of Na + and K + - Na +, - K + -Hacoc is better investigated. This transport occurs with the participation of a globular protein with a molecular weight of about 100,000. The protein has three areas for binding Na + on the inner surface and two parts for binding to + on the outer surface. There is a high activity of ATP-aza on the inner surface of the protein. The energy formed during hydrolysis of ATP leads conformational changes in the protein and at the same time the Na + ions are derived from the cell and the two ions are introduced to + using such a pump, a high concentration of Na + in extracellular fluid is created and a high concentration to + in cells.
Recently, the CA2 + -NCOS is intensively studied, thanks to which the Ca2 + concentration in the cell is tens of thousand times lower than outside it. The CA2 + -NCOS differ in the cell membrane and in the cells of the cell (sarcoplasmic network, mitochondria). CA2 + -NCSOs also function at the expense of a carrier protein in membranes. This protein has high ATP-nitrogen activity.
Secondary active transport. Thanks to the primary active transport, a high concentration of Na + Outside the cell is created, the conditions for diffusion Na + in the cell occur, but together with Na + other substances can enter it. This transport is directed in one direction, called Symport. Otherwise, the Na + entry stimulates the yield of another substance from the cell, these are two streams directed in different directions - antiport.
An example of a sympathetor may be glucose or amino acid transportation with Na +. The protein carrier has two areas for binding Na + and to bind glucose or amino acids. The five separate proteins are identified to bind five types of amino acids. Other types of sympathet are also known - transport N + along with in a cage, K + and SL cells and more.
Almost in all cells there is an antiport mechanism - Na + goes into a cell, and Ca2 + comes out of it, or Na + - into the cell, and H + from it.
Actively transported through the MG2 +, FE2 + membrane, NSO3, and many other substances.
Pinocytosis is one of the types of active transport. It lies in the fact that some macromolecules (mainly proteins, whose macromolecules have a diameter of 100-200 nm) are joined by the membrane receptors. These receptors are specific for different proteins. The addition of them is accompanied by the activation of the contracting proteins of the cell - actin and myosin, which form and close the cavity with this extracellular protein and a small amount of extracellular fluid. At the same time, pincitomic bubble is formed. He is distinguished by enzymes hydrolyzing this protein. Hydrolysis products are absorbed by cells. Pinocytosis requires the energy of ATP and the presence of Ca2 + in the extracellular medium.
Thus, there are many types of vehicles of substances through cell membranes. On different sides of the cell (in apical, basal, lateral membranes), various types of transport can occur. An example of this may be the processes occurring in