Concentration gradients occur between areas of high and low concentration. Diffusion and osmosis move particles along or against gradients, while active transport mechanisms move particles against gradients.
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 either side of a cell border or membrane. If particles are allowed to freely pass through the membrane, a process called diffusion takes place, where particles move across the concentration gradient with a net flux towards the area of low concentration. This movement of molecules along the gradient eventually causes both areas to become 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 encounter 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 towards less concentrated regions. Different types of molecules also tend to mix with each other. Both of these processes are at work when a scent gradually wafts through a room, mixing with air molecules. Eventually, an even distribution of particles removes the concentration gradient.
In a process known as osmosis, molecules appear to move oppositely across gradients, from low to high particle concentrations. This can be seen in cases where a cell membrane blocks the passage of particles because they are too large. The water molecules in the solution may be small enough to pass through the membrane, even if the dissolved particles are not.
Water molecules move from the region outside the cell, where the particle concentration is relatively low, to the interior of the cell, where there is a high particle concentration. Like diffusion, this motion reduces the concentration gradient between the two areas. It can be described as a special form of diffusion, because water molecules are actually 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. So-called active transport mechanisms, such as the sodium-potassium pump, come into play. This pump transports sodium and potassium molecules across the cell membrane, ensuring a high concentration of potassium inside the cell and a high concentration of sodium outside. The process requires ATP, a special molecule that stores energy, which is found inside the cell.
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