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Fig 8.9L shows molecules of 1 solute separated by a permeable membrane. the molecules will move randomly. Since there are more on the left to go through to the right, gradually molecules will move over to the right until there are the same number on each side. At this point there is no net movement; molecules are still moving across, but the same number in each direction. This can be diagramed as a U-tube with the 2 sides separated by a membrane. The difference in concentration between one side of the barrier and the other is called the concentration gradient.

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The rateof diffusion is dependent on: (1) the permeability of the barrier (how easy it is to get across) (2) the area and thickness of the barrier (larger area allows more transfer, greater thickness reduces transfer) (3) the magnitude of the concentration gradient Fig 8.9 R One of the important things about diffusion to remember is that the molecules of substance A don't care about the concentrations of substance B. They both move independently. Fig 8.10Osmosisis a special name given to the diffusion of water. At a superficial level, it appears that water violates the rule I just gave you. Here is an illustration of the movement of water across a membrane. Both sides initially have roughly the same number of molecules of water, yet water will move from a region of lower total solute concentration into a region with higher total solute concentration. If the solute can't pass through the membrane, more water will end up on one side.