Molecules move from areas of higher concentration to areas of lower concentration. This is known as moving down their concentration gradient.
In a simple example, if you have a drop of food coloring in a glass of water, the color will spread out over time. Initially, the concentration of the dye is high in the area where it was added, but as time passes, the molecules of the dye move into areas where there are fewer dye molecules, resulting in a more even distribution throughout the water. This movement continues until there is a uniform concentration of the dye in the water.
The process occurs naturally and does not require any energy input, which is why it’s often referred to as passive transport. In biological systems, this process is crucial for many functions, such as the exchange of gases in the lungs and the absorption of nutrients in the intestines.