The simplest answer would be :
if any solution is not isotonic ,then concentration gradient will be presence of molecules in different concentrations on either side of semi permeable membrane.

Concentration Gradient
Concentration gradient refers to the difference in concentration of a substance between two regions. In the context of biology, this term is often used to describe the difference in the concentration of molecules or ions across a cell membrane.

Importance of Concentration Gradient
The concentration gradient is crucial for various biological processes such as osmosis and diffusion. It is essential for the movement of molecules and ions across cell membranes.

Osmosis and Diffusion
Osmosis is the movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. Diffusion, on the other hand, is the movement of molecules from an area of higher concentration to an area of lower concentration.

Role of Concentration Gradient in Osmosis and Diffusion
In both osmosis and diffusion, the concentration gradient plays a vital role. It is the driving force that allows molecules or ions to move from an area of higher concentration to an area of lower concentration. This movement occurs until equilibrium is reached, and the concentration is equal on both sides of the membrane.

Visual Representation
Imagine a glass of water with a drop of food coloring. Over time, the food coloring will spread evenly throughout the water due to diffusion, moving from an area of higher concentration (the drop) to lower concentration (the rest of the water). This visual representation helps to understand the concept of concentration gradient in action.
In conclusion, the concentration gradient is a fundamental concept in biology that drives the movement of molecules and ions across cell membranes through processes like osmosis and diffusion. Understanding this gradient is crucial for comprehending various biological phenomena.