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We began with the observation that something caused the cell membrane to pull away from the cell wall when salt was added to a sample of onion cells. The center of the cell, in other words, shrank in the presence of salt water.

We had to think for a while about what could have caused this. It is our opinion that distilled water may pass freely through a cell, but salt cannot pass through a cell membrane. The water outside the cell is constantly in motion, in turn causing the salt particles to jitter. They pass through the cell wall, but are unable to permeate the cell membrane. They displace some of the space taken up by the cell membrane, causing the interior of the cell to shrink and expell some of its water. This explains why the size of the cell wall was not effected, but the interior of the cell was. Basically, the underlying concept seems to be that water can pass through a cell membrane but salt cannot.