There are several ways that molecules move through the membrane to get into, or out of a cell. These include active and passive transport . Active transport requires that the cell  use energy to transport molecules (or even larger particles) through the cell membrane, using proteins as a sort of "gate". Passive transport also uses a protein gate, but does not require such an energy expenditure, and occurs spontaneously based on the laws of Thermodynamics. Very small molecules can pass through the membrane itself (aka osmosis).

The principle mechanism driving osmosis and passive transport is diffusion. Diffusion can be defined as the net movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. Diffusion is dependent on the motion of the molecules (therefore temperature) 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 the system.

Osmosis is a special case of diffusion. It is the phenomenon of water flow through a semi permeable membrane that blocks the movement of other molecules (e.g. salts or solutes). Osmosis is a fundamental concept found in all biological systems. It can also be applied to water purification and desalination, waste material treatment, and many other chemical and biochemical laboratory and industrial processes.

In a cell, molecules like proteins are too large to pass through the cell membrane, so they cannot diffuse from one side of the membrane to the other. But water molecules can, and do, pass through the membrane. Hence the cell membrane is said to be semipermeable, since it allows some molecules to pass through but not others. Maintaining proper "water balance" is very important for cells, they must contain enough, but not too much water. This can be a difficult proposition since the membrane is permeable to water.

In the experiment on the left, we will add red blood cells (rbc's) to three solutions containing different amounts of solute (NaCl) and watch for osmosis to occur. Depending on the relative solute concentration of the test tube solutions, compared to the rbc's, the following results may be observed: (mouseover to see results) Water could diffuse into the cells, the rbc's will swell and lyse. Water could diffuse out of the cells, the rbc's will shrink. There will be no net movement of water. The rbc's will not change shape.

There are terms which which describe the relative solute concentration of two or more entities. These are relative terms (like "bigger") which can only be used to compare and make no sense when used to describe a single item.

Isotonic - Two items have the same solute concentration (iso = same)

Hypotonic - A particular item has a solute concentration lower than that of another. e.g. solution A is hypotonic to solution B. (hypo = below)

Hypertonic - A particular item has a solute concentration higher than that of another. e.g. solution B is hypertonic to solution A. (hyper = above).

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