Concentration gradient

Concentration gradient of two solutions

A concentration gradient or concentration gradient (imprecisely also called substance gradient ) between two locations x ₁ and x ₂ exists if the respective concentrations of a substance there - c ₁ and c ₂ - differ from one another.

Only the word concentration gradient can also be used in the narrower sense for spatial decrease in concentration, as an antagonist of the word concentration increase .

The diffusion of substances occurs due to a concentration gradient. The electrochemical gradient is the concentration gradient of dissolved ions , which is of outstanding importance for biological systems, in particular cellular energy metabolism and nerve conduction .

In a narrower sense, the concentration gradient describes a continuous change in the local concentration. The concentration gradient is then the measure of the difference and thus the gradient of a concentration of a chemical substance . It is the quotient (more precisely: differential quotient ) from the concentration difference and the distance between two points in this space. In the one-dimensional case this can be written as

{\ displaystyle {\ frac {dc} {dx}}}

where the difference is the concentration of the substance and the distance. ${\ displaystyle dc}$ ${\ displaystyle dx}$

With the help of the Nabla operator , the concentration gradient can be represented in three dimensions:

{\ displaystyle {\ vec {\ nabla}} c = \ left ({\ frac {\ partial c} {\ partial x}}, {\ frac {\ partial c} {\ partial y}}, {\ frac { \ partial c} {\ partial z}} \ right) ^ {T}}

Here is the concentration; are the components of the position vector . ${\ displaystyle c}$ ${\ displaystyle x, y, z}$

Examples

simple gradient mixer for continuous gradients

Material gradients drive the directional spread of molecules by diffusion .

External forces such as gravity, magnetic fields and electric fields can create concentration gradients in originally homogeneous solutions and mixtures.

Crystallization from oversaturated solution, agglomeration of ferromagnetic particles in the magnetic field and migration in the direction of higher magnetic field strength, migration of ions in the electric field, temperature gradients, differences in exposure can cause concentration gradients - sometimes in 2-phase systems.

In the gas phase, in gas mixtures, concentration gradients are usually described by partial pressure gradients .

Mixing processes such as stirring, convection and wind in the troposphere and turbulence in flow processes reduce concentration gradients.

One application in biochemical separation is gradient electrophoresis . A material gradient is generated beforehand in a gel mixture. This can be a gel density gradient (variable pore size) or a pH gradient (usually with ampholytes ). During the subsequent electrophoretic separation, the substances are then concentrated in a corresponding sector.

Another application is the separation of substance mixtures in density gradients (e.g. from sucrose or cesium chloride ) by density gradient centrifugation .

In chromatography , mobile phases with compositions that change over time are often used in order to increase the selectivity during the elution of various adsorbed substances.

In biology , material gradients are a prerequisite for:

passive and secondary active membrane transport
in chemotaxis
in the energy metabolism of the cells
in morphogenesis and during embryonic development ( embryogenesis )
Excretory organs ( nephridia ) such as the kidney
Respiratory organs such as the skin , gills and lungs

Substance gradients are also important in geochemistry and ecology , for example gradients of the oxygen concentration or hydrogen sulfide concentration in water.

literature

Paul Reinhart Schimmel, Charles R. Cantor: Biophysical Chemistry: Part II: Techniques for the Study of Biological Structure and Function . HC Freeman Co., San Francisco, 1980, pp. 619-642. ISBN 0-7167-1190-7 .
Alfred Pingoud , Claus Urbanke: Working methods of biochemistry . DeGruyter, Berlin 1997, ISBN 3-11-016513-9 ( as a Google book ).
Richard Josiah Hinton, Miloslav Dobrota: Density Gradient Centrifugation , Volume 6 of Laboratory Techniques in Biochemistry and Molecular Biology , Elsevier, 1978. ISBN 9780080858753 .