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.