I have researched into osmosis and found that all living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases, usually CO2 and O2, taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell.

There are two ways that the molecules move through the membrane: passive transport and active transport. Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such an energy expenditure, and occurs spontaneously.

Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout an object.

An important concept in understanding diffusion is the concept of equilibrium. There are two types of equilibrium. Static equilibrium occurs when there is no action taking place. Dynamic equilibrium occurs when two opposing actions occur at the same rate. Diffusion occurs when a system is not at equilibrium.

As an example, one drop of ink is dropped into a beaker of water. At first, all of the ink molecules are in a small space and they are moving around in a random way. They move in straight lines and change direction only when they collide with each other or the surrounding water molecules. Some of the ink molecules near the edge of the drop move away from the centre of the drop. As a matter of fact, most of the molecules move away from the centre of the drop. Most of the molecules continue to move away from the original centre of the drop. They move in all different directions, and some may even move back toward the centre. Still, more are moving away from the drop than toward it until they find the wall of the beaker. Then they start moving back toward the centre again. More and more molecules bounce off of the glass until they start moving toward the centre, then they pass the centre and move toward the other side. Eventually the number of molecules moving away from the centre equals the number moving toward the centre, and equilibrium is established. At this point the molecules are evenly spread throughout the water, and diffusion stops.

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