When this occurs the system is said to be in equilibrium and while molecules still move, they remain evenly spread in the solution. A good example is using Condy's crystals (potassium permaganate) in water.
When the crystals settle on the bottom of the water and begin to dissolve, you can see that the bottom is concentrated with the coloured permaganate while the dilute solution is at the top.
This difference in concentration is called the concentration gradient and is required for diffusion to take place.
As time progresses, the random motion of the dissolved ions sees the colour begin to spread (30 min later). However, it can still be seen that the solution at the base of the beaker is still more concentrated than that at the top. Given more time, the remaining crystals dissolve, and the permaganate ions eventually move around in the fluid until they are evenly distributed and are at equilibrium. The permaganate ions have diffused from the bottom of the beaker until evenly spread.
In cells, differences are likely to occur between the inside of the cell and its external environment. If the membrane is permeable to the material it will cross the membrane and establish an equilibrium.
It should be pointed out that the concentration gradient does not always have to from the outside of a cell going in, it can operate in reverse, being more concentrated on the inside of the cell moving out. 2 way flow of substances in opposite directions can occur as shown above.
The movement of water which is driven by concentration gradients is called osmosis. The reason that water is dealt with separately is because its movement often happens in the opposite direction of other molecules.
Lets look at our Condy's crystal eaxmple again, but consider BOTH the permaganate ions and the water molecules in the beaker this time.
As the crystals dissolve, the permaganate ion concentration is high at the bottom of the beaker but low at the top. However, when we think about the water molecules, the concentration of these are high at the top and low at the bottom. So as the permaganate ions move upwards, the water molecules will be moving downwards to ensure everything is evenly distributed and an equilibrium is reached.
So while achieving the same result water needs to be considered separately because while it follows the same rules, it seems to act in a counter intuitive way.
In many cells substances may be unable to cross the membrane, and so to try and establish an equilibrium, water must move to do this. In the figure below, because the sucrose is unable to pass through the membrane, it is up to the water to move by osmosis in an effort to try and dilute it and establish an equilibrium.
Cells can behave in a number of ways to this changing amounts of water relative to the concentrations of water inside the cell membrane.
In (a), the cells have been placed in a hypertonic solution. This means that the amount of water inside the cell is greater than the outside environment. This happens when a high concentration of solutes are in the water surrounding the cells. As a result water leaves the cell causing shriveling or plasmolysis.
In (), the cells have been placed in a isotonic solution. This means that the amount of water inside the cell is equal to the outside environment. As a result water leaves and enters the cell at the same rate leaving it unchanged.
In (c), the cells have been placed in a hypotonic solution. This means that the amount of water inside the cell is less than than the outside environment. This happens when a very low concentrations of solutes are in the water surrounding the cells. As a result water enters the cell causing animal cells to burst, or plant cells to become turgid. The plant cell wall pushes back on the membrane, making it rigid, but stopping it from breaking. This is how lettuce and celery can be revived and made crisp again after storage.
Spectrophotometer cuvettes Wow, cool post. I'd like to write like this too - taking time and real hard work to make a great article... but I put things off too much and never seem to get started. Thanks though.
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