What is the concentration gradient?

Hint: The word ‘concentration’ means the amount of solute present in a solvent. Solute is the substance that is being dissolved whereas solvent is the substance in which something is dissolved and both of them together form a solution.

Complete answer:
Concentration gradient is the difference in the concentration of a particle in one region compared to the other, eg., when the concentration of particles is higher in one area than another. A solution is basically made up of two major components- a solute and a solvent. This change in the concentration of the solute particles causes imbalance in the solution and thus as a result, the movement of solute particles from highly dense areas to lower denser areas takes place.
In biology, the concentration gradient is caused due to the unequal distribution of particles for example ions between the two solutions i.e. the intracellular fluid, the solution which is inside the cell and extracellular fluid, the solution which is outside the cell.

Additional information:
During active transport, the particles show an uphill movement for transportation. This refers to the movement of solute particles against the concentration gradient i.e. from lower concentration to higher concentration. As this process takes place against the concentration gradient, it requires chemical energy i.e. either in the form of ATP or the electric gradient which occurs due to the difference in charge particles.

Note:
The transfer of solute particles from higher concentration to lower concentration of molecules in a solution takes place so that an equilibrium can be reached and the imbalance of solute particles in the solution can be eliminated.