Osmosis

The masses come from a teaching model of plant cells with walls. Water moves into or out of the cells by osmosis until no more net movement happens; in 30 minutes the cells get most of the way there. The model is set so that there is no net movement of water near 0.3 mol/dm³: real potatoes vary, so a class finds its own value.

Cells take in water until their contents press on the wall, which stretches only a little, so the gain is limited. They lose water until their contents no longer press on the wall, and only after more loss do the contents pull away from the wall. The cell drawings are schematic, not micrographs.

Water keeps crossing the membranes both ways in every tube: the mass change shows the net movement.