5.4:
What is an Electrochemical Gradient?
The electrochemical gradient is the combination of both concentration and electrical gradients across a membrane.
In a cell, the plasma membrane functions as a barrier, keeping selective molecules and ions trapped inside while keeping others out.
This means that ions that are critical for cell function, such as sodium and potassium, cannot freely diffuse across the membrane.
Under normal conditions, there is generally more sodium outside of a cell than inside. This creates a chemical or concentration gradient where sodium would flow into the cell across the cell membrane, if given a path via channels or transporters.
The opposite is true for potassium, where there is a lower concentration of ions outside the cell than inside.
However, ion concentration is not the only factor creating a gradient across the cell membrane. The separation of charged ions and molecules also means an electrical gradient is present.
The prevalence of positively charged sodium ions outside of the cell and the abundance of negatively charged proteins inside are two major factors that contribute to the overall difference in charge across the membrane.
5.4:
What is an Electrochemical Gradient?
Adenosine triphosphate, or ATP, is considered the primary energy source in cells. However, energy can also be stored in the electrochemical gradient of an ion across the plasma membrane, which is determined by two factors: its chemical and electrical gradients.
The chemical gradient relies on differences in the abundance of a substance on the outside versus the inside of a cell and flows from areas of high to low ion concentration. In contrast, the electrical gradient revolves around an ion’s electrical charge and the overall charges of the intracellular and extracellular environments.
The electrical gradient of a positively-charged ion flows from positive to negative regions, while the reverse is true for negatively-charged ions. It is the combined action of these electrical and chemical factors that determine the ultimate direction of an electrochemical gradient. When an ion moves along this path, down its electrochemical gradient, energy is freed that can then power diverse biological processes.
Suggested Reading
Ianowski, Juan P., and Michael J. O’Donnell. “Electrochemical Gradients for Na+, K+, Cl– and H+ across the Apical Membrane in Malpighian (Renal) Tubule Cells of Rhodnius Prolixus.” Journal of Experimental Biology 209, no. 10 (May 15, 2006): 1964–75. [Source]