When the concentration of molecules on both sides of a membrane is the SAME, the solution is

When the concentration of molecules of a substance is the same on both sides of a membrane the solution is isotonic.

Explanation:

A given substance, when it has the same concentration on both sides of the membrane, is in equilibrium and the solution is called isotonic, from the extracellular space with respect to the cytoplasm.

The cell membrane is semipermeable to the passage of water and some substances. Hypertonic and hypotonic solutions tend to establish the dynamics of entry and exit of molecules or water, due to the concentration or osmotic gradients that are established, respectively.

An isotonic solution does not generate gradients, because the concentration of molecules is equal in the extracellular space with respect to the intracellular space.