Concentration gradient. Sodium (Na) concentration gradient as a driving force for membrane transport. Differences between facilitated diffusion and simple diffusion
Subject area: polymers, synthetic fibers, rubber, rubber
It is quite difficult to visualize the formation of such a concentration gradient in a suspension due to the influence of solvent molecules. This phenomenon can be compared with the behavior of a mixture of two gases at constant temperature and pressure, but with a concentration gradient of both components. Let us consider a plane drawn through such a gas mixture perpendicular to the direction of the concentration gradient. Let us assume that the concentration of component A is higher on the left side of the plane and lower on the right; the distribution of component B should be reversed. Per unit time, the left side of the plane must collide larger number molecules A than in the right; for molecules B the opposite is true. Consequently, more molecules of A will move through the plane from left to right and, likewise, more molecules of B will move from right to left. As a result, the concentrations of the two components will equalize. This process is the diffusion of gases. If we now move on to a liquid suspension in which there is a similar concentration gradient of suspended particles, then it is clear that we can repeat the previous argument, applying it to the movement of solid particles and solvent molecules through a plane drawn at right angles to the concentration gradient. However total number particles per unit volume does not remain constant, and the reasoning should be modified accordingly. It is clear that the number of solvent molecules crossing the plane in the direction away from the place with a high concentration of suspended particles will be less than in the opposite direction due to the presence of particles blocking the path.
Fick's law for diffusion in one direction relates the positive flow of particles A with a negatively directed concentration gradient (constant density and low particle concentration):
As noted above, electroactive substances reach the electrode surface as a result of: 1) diffusion due to the concentration gradient between the electrode surface and the bulk solution, and 2) electrical migration of charged particles due to the potential gradient between the electrode and solution. This migration current must be eliminated or reduced as much as possible by adding a large excess of an inert electrolyte that does not participate in the reaction at the electrode. The resulting limiting current will only be a diffusion current. In order to exclude migration current, the concentration of the inert electrolyte must be at least 50 times greater than the concentration of the electroactive substance.
With an ideal diffusion current, the electroactive substance reaches the electrode only as a result of diffusion caused by the concentration gradient resulting from the loss of the substance at the electrode. This gradient exists throughout the diffusion layer, where the concentration changes from practically zero at the electrode surface to the concentration existing in the bulk of the solution. The diffusion current can be determined by the height of the wave on the current-voltage curve.
The basic laws of diffusion were, as is well known, formulated by Fick. Fick's first law establishes a connection between the diffusion flow rate / and the concentration gradient C over distance x from the
Since moisture can be removed from clay products only by evaporation from the surface, and from the internal parts moves outward only under the influence of a force associated with a concentration gradient *, it is impossible to completely eliminate shrinkage deformation during drying. It can, however, be minimized with sufficient drying time and with adequate temperature and humidity control to eliminate uneven distribution of moisture on the surface. This control, together with the thermal regime, is best achieved by using countercurrent dryers, mainly of the tunnel type. The more plastic the mixture and the more complex the shape, the more thorough the drying must be **.
When a polymer sample is extracted with a liquid with gradually increasing dissolving ability, the lower molecular parts are dissolved first, and then the rest. An improvement in the dissolving ability is achieved by changing the temperature or composition of the extracting liquid. Particularly good results are obtained when using a column with a concentration and temperature gradient, when multiple dissolution and polymer deposition
At a rotation speed of (4-6)-104 rpm, a centrifugal acceleration equal to ~106 g develops in the ultracentrifuge. When conducting an experiment like this - observing a nonequilibrium sedimentation process - it is called velocity sedimentation. Measurement of the position of boundary 16 and its displacement in time is carried out using optical circuits (see page 160), which makes it possible to calculate the sedimentation coefficient: „ _ \ Lt_ _ 1 d In r
Due to the thermal movement of macromolecules in a solution, the solute moves (diffusion) in the direction from higher to lower concentration. If you carefully “layer” a solvent (Co) on the surface of a polymer solution with a concentration of C, then gradually the boundary section A-A will be blurred (Fig. 1.11). Solvent molecules will diffuse in the x direction into the solution, and macromolecules will diffuse in the opposite direction into the solvent layer. The change in concentration along the segment dx is called the concentration gradient. The rate of change in concentration as a result of diffusion (diffusion rate) is described by the relation
When a cation exchanger of type (NM)l comes into contact with a dilute solution of a strong electrolyte M+A~, the value of [M+] in the ion exchanger will be significantly greater than [M+] in the solution, and [A~~] will be less than [A~]. Due to the fact that their concentration in the two phases is different, small mobile ions will tend to equalize it by diffusion, and this will lead to a violation of the electrical neutrality of the solution, to the appearance of a positive space charge in the solution and a negative one in the ion exchanger. As a result, Donnan equilibrium will be established between the concentration gradient caused by diffusion and the electrostatic potential that prevents it, and at the cation exchanger-solution boundary (Fig. 191) Fig. 191. Charge distribution diagram - a potential difference will arise - Donnan potential
Diffusion phenomena during the formation of the adhesive-substrate system are very diverse. These include surface diffusion of the adhesive, self-diffusion in the adhesive layer, and sometimes volumetric one- or two-way diffusion occurs across the adhesive-substrate interface. In addition, the listed processes have different mechanisms. For example, a distinction is made between activated, semi-activated and non-activated diffusion. These various processes will be discussed in more detail below. >> It is often assumed that the driving force for diffusion is the concentration gradient. However, the movement caused by the concentration gradient and leading to the gradual homogenization of the system does not exhaust all possible manifestations of this complex process. Very often, during diffusion, there is not an equalization of concentrations, but, on the contrary, a further separation of the components of the system. Therefore, it is more correct to assume that the driving force of diffusion is the difference in thermodynamic potentials, and the transfer of matter by diffusion is accompanied by a decrease in the free energy of the system. Alignment of thermodynamic potentials and approaching thermodynamic equilibrium achieved due to the thermal movement of atoms (molecules). The thermodynamic potential can be decomposed into energy and entropy components. The diffusion mechanism depends on the ratio of these components. In some cases, the internal energy of the system does not change during diffusion, and
Hello! According to the definition, the concentration gradient is directed from the side of lower concentration to the side of higher concentration. Therefore, diffusion is always said to be directed against the concentration gradient, i.e. from the side with higher concentration to the side with lower concentration.
However, when you read the literature about the life activity of a cell, photosynthesis, it always says that “along the concentration gradient” - this is in the direction of decreasing concentration, and “against the concentration gradient” - in the direction of increasing concentration and thus, for example, simple diffusion in cells (or, otherwise, ordinary diffusion) is directed along the concentration gradient.
But a contradiction arises. It turns out that the expression “along the concentration gradient” is actually a movement opposite to the direction of the concentration gradient. How can this be?
This persistent and widespread error is associated with differences in understanding the direction of the concentration gradient vector in physics and biology. Biologists prefer to talk about the direction of the concentration gradient vector from a larger to a smaller value, and physicists from a smaller to a larger value.
Table of contents of the topic "Transmission of information through electrical excitation.":1. Transmission of information through electrical excitation. Resting potential.
3. Changes in extracellular potassium concentration (K).
4. The influence of glia on the composition of the intercellular environment. Blood-brain barrier.
5. Action potential. Time course of action potential. Repolarization.
6. Trace potentials. The nature of the action potential. Threshold and excitability.
7. Membrane conductivity. Ionic currents during an action potential.
8. Kinetics of ion currents during excitation. Recording membrane currents.
9. Sodium (Na) and potassium (K) conductance during an action potential.
10. Inactivation of sodium (Na) current.
Diffusion potential. It was previously noted that the resting potential is diffusion potential ions, which passively move through channels in the membrane. In the resting state, the majority of open membrane channels are potassium (K) channels; therefore, the resting potential is determined to a first approximation by the transmembrane potassium concentration gradient (K). In Fig. Figure 2.2 shows the dependence of the measured potential on the extracellular potassium concentration (K).
Rice. 2.2. Dependence of resting potential in frog muscle fiber(ordinate) from extracellular potassium concentration (K) (abscissa, logarithmic scale). The circles indicate the values of the membrane potential measured at different concentrations of potassium ions [K+]0. The straight line reflects the relationship between the potassium equilibrium potential and [K+]0, calculated using the Nernst equation. Coefficient 58 takes into account the frog's reduced body temperature.After shifting the extracellular K+ concentrations the intracellular concentration initially remains at the same level, and during this short period of time the measured potassium (K) potential should, in accordance with the Nernst equation, change proportionally to the logarithm of [K+]0. This potassium (K) potential. E(k), indicated by the red line in Fig. 2.2. The recorded values of the resting potential in the upper range are very close to E(k), however, as [K+]0 decreases, they become less and less negative compared to E(k). This discrepancy should be attributed to the relatively larger contribution of sodium permeability PNa at low [K+]0 values. The deviation of the recorded resting potential values from E(k) disappears if the supply of sodium (Na) is stopped, for example, by replacing extracellular sodium (Na) with a cation incapable of diffusion such as choline. It follows that the normal resting potential is about 10 mV more positive than E(k).
Table of contents of the topic "Endocytosis. Exocytosis. Regulation of cellular functions.":1. Effect of the Na/K pump (sodium potassium pump) on membrane potential and cell volume. Constant cell volume.
3. Endocytosis. Exocytosis.
4. Diffusion in the transport of substances within the cell. The importance of diffusion in endocytosis and exocytosis.
5. Active transport in organelle membranes.
6. Transport in cell vesicles.
7. Transport through the formation and destruction of organelles. Microfilaments.
8. Microtubules. Active movements of the cytoskeleton.
9. Axon transport. Fast axon transport. Slow axon transport.
10. Regulation of cellular functions. Regulatory effects on the cell membrane. Membrane potential.
11. Extracellular regulatory substances. Synaptic mediators. Local chemical agents (histamine, growth factor, hormones, antigens).
12. Intracellular communication with the participation of second messengers. Calcium.
13. Cyclic adenosine monophosphate, cAMP. cAMP in the regulation of cell function.
14. Inositol phosphate "IF3". Inositol triphosphate. Diacylglycerol.
Meaning Na/K pump for cell is not limited to stabilizing normal K+ and Na+ gradients across the membrane. The energy stored in the membrane Na+ gradient is often used to facilitate membrane transport of other substances. For example, in Fig. Figure 1.10 shows the “symport” of Na+ and a sugar molecule into the cell. Membrane transport protein transports a sugar molecule into the cell even against a concentration gradient, at the same time Na+ moves along concentration and potential gradients, providing energy for the transport of sugars. Such transport of Sakharov completely depends on the existence high sodium gradient I; if the intracellular sodium concentration increases significantly, the transport of sugars stops.
Rice. 1.8. The relationship between the rate of transport of molecules and their concentration (at the entrance to the channel or at the binding site of the pump) during diffusion through the channel or during pumping transport. The latter becomes saturated at high concentrations (maximum speed, V max); the value on the x-axis corresponding to half the maximum pump speed (Vmax/2) is the equilibrium concentration of KtThere are different symport systems for different sugars. Amino acid transport into the cell is similar to the transport of sugars shown in Fig. 1.10; it is also provided by the Na+ gradient; There are at least five different symport systems, each specialized for one group of related amino acids.
Rice. 1.10. Proteins immersed in the lipid bilayer of the membrane mediate the symport of glucose and Na into the cell, as well as the Ca/Na antiport, in which the driving force is the Na gradient on the cell membrane
Besides simport systems there are also " anti-porters" One of them, for example, transfers one calcium ion out of the cell in one cycle in exchange for three incoming sodium ions (Fig. 1.10). The energy for Ca2+ transport is generated by the entry of three sodium ions along the concentration and potential gradient. This energy is sufficient (at resting potential) to maintain a high calcium ion gradient (from less than 10 -7 mol/L inside the cell to approximately 2 mmol/L outside the cell).
Characterizing the magnitude and direction of the greatest change in the concentration of a substance in the environment. For example, if we consider two areas with different concentrations of a substance, separated by a semi-permeable membrane, then the concentration gradient will be directed from the area of lower concentration of the substance to the area with higher concentration Lua error: callParserFunction: function "#property" was not found. )]][[K:Wikipedia:Articles without sources (country: Lua error: callParserFunction: function "#property" was not found. )]] .
Definition
The concentration gradient is directed along the path l, corresponding to the normal to the isoconcentration surface (semipermeable membrane). Concentration gradient value texvc not found; See math/README - help with setup.): \nabla C equal to the ratio of the elementary change in concentration dC to the elementary path length dl :
texvc not found; See math/README for setup help.): \nabla C = \frac(dC)(dl)
At a constant concentration gradient C along the way l :
Unable to parse expression (Executable filetexvc not found; See math/README - help with setup.): \nabla C = \frac(C_1 - C_2)(l)
Here C 1 And C 2- initial and final concentration value along the path length l(normal to the isoconcentration surface).
Concentration gradients can be responsible for the transport of substances, such as diffusion. Diffusion occurs against the concentration gradient vector [[K:Wikipedia:Articles without sources (country: Lua error: callParserFunction: function "#property" was not found. )]][[K:Wikipedia:Articles without sources (country: Lua error: callParserFunction: function "#property" was not found. )]][[K:Wikipedia:Articles without sources (country: Lua error: callParserFunction: function "#property" was not found. )]] .
The unit of measurement of the concentration gradient in the International System of Units (SI) is −4 (mol/m 4 or kg/m 4), as well as its fractional or multiple derivatives.
see also
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Literature
- Antonov V.F., Chernysh A.M., Pasechnik V.I. Biophysics - M.: VLADOS, 2000, p. 35. ISBN 5-691-00338-0
- Trifonov E. V.- St. Petersburg: 2011.
An excerpt characterizing the Concentration Gradient
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