The concentration of a substance is the number of molecules present in a given volume of liquid or gas. A concentration gradient occurs between an area of high concentration and an area of low concentration. In nature, this commonly occurs on each side of a cell border, or membrane. If particles can freely cross the membrane, a process called diffusion takes place, where particles move across the concentration gradient with a net flow toward the area of low concentration. This movement of molecules down the gradient eventually leads to both areas becoming equal in concentration, a state known as dynamic equilibrium.
Thermal energy causes molecules in liquids and gases to move randomly in straight lines until they meet other particles or surfaces which cause them to change direction. The process known as diffusion means that molecules have a tendency to move away from areas where there are high concentrations and to move toward less concentrated regions. Different molecule types also tend to mix together. Both of these processes are at work when a scent gradually spreads across a room, mixing with air molecules. Eventually, an even distribution of particles removes the concentration gradient.
In a process known as osmosis, molecules seem to move the opposite way across gradients, from low to high concentrations of particles. This can be seen in cases when a cell membrane blocks particles from passing through because they are too large. The water molecules in the solution may be small enough to pass through the membrane, even though the dissolved particles are not.
Water molecules move from the region outside the cell, where the concentration of particles is relatively low, to the inside of the cell, where there is a high particle concentration. In the same way as diffusion, this movement reduces the concentration gradient between the two areas. It can be described as a special form of diffusion, because the water molecules are really moving from an area of high water concentration to an area of low water concentration.
Sometimes, cell biology works to actively move particles against a concentration gradient and against the influence of osmosis and diffusion. What are known as active transport mechanisms come into play, such as the sodium-potassium pump. This pump transports sodium and potassium molecules across the cell membrane, ensuring that there is a high potassium concentration inside the cell and a high sodium concentration outside. The process requires ATP, a special molecule in which energy is stored, which is found inside the cell.