How does a cell membrane affect water movement?
Excellent question. Keep in mind that the cell membrane is normally semi-permeable.
This means that it is selectively permeable. Permeable means some molecules can move through (across)it. This occurs in both directions.
So if the membrane is selective, that means only some types of molecules move into the cell and some types of molecules move out of the cell. This plasma (or cell)membrane can allow sodium, for example, to come into the cell.
Water generally gets across the cell membrane through something called passive transport. This is the same thing as diffusion.
No energy-no ATP-is needed for this. But, the water only moves across the membrane if there is less water on the side it moves to.
Water moves only from a high area of itself (one side of the membrane) to a lower area (the other side).
This depends on, also, the ion (like sodium, or Na+) concentration.
Let me know and I'd be glad to explain in more detail ;)