Electrochemical gradient

The electrochemical gradient is the gradient of the electrochemical potential and thus comes about through a chemical gradient and / or an electrical gradient.

${\displaystyle \nabla {\overline {\mu }}_{i}=\nabla \mu _{i}({\vec {r}})+\nabla z_{i}\mathrm {F} \varphi ({\vec {r}})}$

• ${\displaystyle z_{i}}$ Charge number of the ion
• F, Faraday's constant , F = 96485.33 C / mol
• ${\displaystyle \varphi }$, Local electrical potential

With this gradient, both differences in chemical concentration - a concentration gradient - and differences in electrical potential - an electrical voltage - are recorded together. Therefore it can also be used to describe changes in the distribution of charged particles such as ions in a liquid space in the presence of an electric field. With an electrochemical gradient = 0 for a certain ion, these charged particles are statistically distributed in such a way that electrical and chemical gradients are in equilibrium. In other words: the electrochemical gradient disappears in equilibrium.

Since ions carry a charge, both gradients occur in combination when they are distributed.

• Chemical gradient - a concentration gradient in the distribution space tends to equalize due to the temperature-dependent random movement of the particles ( Brownian molecular movement ). If there is an uneven distribution of an ion type, an electrical gradient is also associated with it.
• Electrical gradient - a potential difference in the electrical field tends to drop in voltage due to a balanced charge distribution. If there is an uneven distribution of charged particles of one type of ion, a chemical gradient is associated with it.

Electrochemical gradients in biological systems

In biological systems, the electrochemical gradient is relevant on membranes .

Examples for this are:

• the proton gradient (H ⁺ gradient) across mitochondria and chloroplast membranes, which contributes to the formation of ATP in the respiratory chain and during photosynthesis .
• the electrochemical gradient of K ⁺ across the membrane of nerve cells , which is important for the resting potential in the conduction of excitation .

Proton gradient across the mitochondrial membrane

→ Main article: respiratory chain

By far the most important systems of ATP regeneration in organisms are based on proton gradients , namely in the respiratory chain and during photosynthesis . Energy-rich food or sunlight provides the organism with electrons , the energy of which is first converted into a proton potential via the inner mitochondrial membrane. Responsible for this is the respiratory chain, in which H ⁺ pumps are used that are operated with high-energy electrons. The driving force of the protons caused by the proton gradient now drives the formation of ATP.

K ⁺ gradient on the membrane of nerve cells

→ Main article: Resting membrane potential of electrically excitable nerve cells

An example is used here to explain how the two gradients (electrical gradient and concentration gradient) work together.

In the nerve cell, K + is close to its electrochemical equilibrium and is mainly responsible for the creation of the resting electrical potential of −70 mV across the membrane.

There are negatively charged organic molecules in the cell, for example many proteins and enzymes. Let us assume that there are so many K ⁺ ions in a cell that they just compensate for this negative charge and the membrane potential is 0 mV. K ⁺ now follows the driving force of the concentration gradient and is therefore anxious to leave the cell. The more K ⁺ ions leave the cell, the more the electrical driving force of the negatively charged organic molecules in the cell acts on K ⁺ . This strives to pull K ⁺ back into the cell.

An equilibrium is soon established between the two opposing driving forces. The electrochemical gradient of K ⁺ is then equal to 0 and the net flow of K + across the membrane comes to a standstill. This results in a membrane potential of −70 mV and a higher concentration of K + in the cell than outside the cell. This example shows the difference between concentration gradient , electrical gradient (which equates to electrical voltage ) and the electrochemical gradient .

See also

• Electrochemical potential
• Electrochemical driving force

literature

• Jeremy M. Berg, John L. Tymoczko, Lubert Stryer : Biochemistry. 6 edition, Spektrum Akademischer Verlag, Heidelberg 2007. ISBN 978-3-8274-1800-5 .
• Donald Voet, Judith G. Voet: Biochemistry. 3rd edition, John Wiley & Sons, New York 2004. ISBN 0-471-19350-X .
• Bruce Alberts , Alexander Johnson, Peter Walter, Julian Lewis, Martin Raff, Keith Roberts: Molecular Biology of the Cell , 5th Edition, Taylor & Francis 2007, ISBN 978-0815341062 .