While studying the movement of particles in a medium, you may come across various chemistry terms. To mention a few, such terms include concentration gradient, diffusion and osmosis. They are related to each other, either directly or indirectly. Each of the three factors play a crucial role in even distribution of molecules in a medium. In fact, the overall functioning of plant and animal cells is highly dependent upon these phenomena. The following is a brief explanation regarding this gradient phenomenon. Concentration Gradient Definition Briefly speaking, concentration gradient refers to the change in concentration of a solute through a distance in a solution. With reference to this, molecular diffusion occurs from a region of high concentration to another of lower concentration. And osmosis is the term used for diffusion of water molecules across a semipermeable membrane. In osmosis, movement of water takes place from a region of high water concentration (low solute content) to low water concentration (high solute content). Determining Concentration Gradient So, what is concentration gradient in biology? It is the gradual alteration in the solute concentration between two places in a particular solution. In order to determine this gradient, total number of solute particles in a small volume from a region is compared with the total number of particles in the same quantity from another region. Over here, distance is used as a function to determine the gradient of concentration in a solution. To be more precise, one can determine the amount of change in concentration from one place to another by using this gradient. For better understanding about this topic, let's take an example. When you put a few drops of food dye (say red color) in a transparent glass filled with water, you will notice the color differences. The region where the color is bright red represents higher concentration of the dye, while that of clear water at the bottom indicates lower concentration. Within a few seconds, the dye molecules disperse from a region of high concentration (dark red) to that of low concentration. This phenomenon is called molecular diffusion. Another example is the diffusion of oxygen from the lungs into the blood circulatory system along the concentration gradient. In a laboratory study, the behavior of bacteria is analyzed in relation to the concentration gradient of a chemical. Over here, the distance traveled by bacteria is proportional to the concentration of the chemical; meaning lower the concentration, the shorter is the distance traveled, while higher the concentration, the longer is the traveled distance. This is because, as the bacterium moves in response to the chemical signal, it tends to move forward as long as there are higher number of molecules. The opposite behavior is observed in case of bacteria moving towards low concentration area. Biologically active cells regulate an optimal concentration gradient across a cell membrane. This balance is crucial for transportation of nutrients, waste products, or any other substances from the interior to the exterior of the cell and vice versa. A gradient is created when there is an imbalance in the distribution of ions. When this occurs, molecular diffusion or movement of solutes takes place from higher concentration area to lower concentration region. It is to be borne in mind that diffusion may still continue, even if there is no difference in the gradient of concentration. Based on whether the molecules are moving along the gradient of concentration or against it, there are two types of diffusion. When it travels freely from high to low gradient, it is known as passive diffusion. In contrary to this, active diffusion occurs against the gradient. And energy is required for this forceful movement. Thus, concentration gradient and molecular diffusion are related to each other.