So my understanding of water flow is as follows:

Basically, water moves about randomly because it has thermal energy. It will occasionally hit other water molecules and "bounce" back. As such, in areas of high pressure, water bumps into other water molecules a lot. However, if a high pressure water tank were connected to a low pressure empty tank, some water would be hit in the direction of the empty tank, but it wouldn't get hit back. Over time, more and more water is hit over until the water pressure equalizes. A vacuum doesn't suck so much as not push back, if that makes sense. So my trouble with hypotonic/hypertonic solutions is as follows. In a hypotonic situation, the solute concentration outside the cell is lower than inside the cell. A hypertonic solution is the reverse. So a hypotonic solution has fewer water molecules, so you would expect water to flow out of the cell because it's not getting hit back as much. But it flows in. The reverse is true for hypertonic solutions. Why is this? Thanks!

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    $\begingroup$ Why do you think a hypotonic solution has fewer water molecules? $\endgroup$
    – Bryan Krause
    Sep 11, 2020 at 20:28

1 Answer 1


A hypotonic solution has lower solute concentration outside the cell than inside the cell. In other words, the ratio of solute to water is higher inside the cell than outside. In order to equalize the concentrations, the solution inside the cell must be diluted, by drawing in water from outside the cell. A hypotonic solution has more water molecules per solute molecule than inside the cell, so water moves into the cell.

Your mistake is in thinking that a hypotonic solution has fewer water molecules than the cell. In a relative sense, it's the opposite - the hypotonic solution has a lower concentration than inside the cell, and therefore more water per solute than inside.


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