It's called osmosis. Water interacts chemically with ions and other things that it can dissolve, and so diffuses (osmoses) into areas where there are more ions or solutes. It's the same reason why sugar or, for that matter, salt, make such good preservatives - they suck the water out of and mummify any bacteria or other organism that tries to get into the food or other material that you are trying to preserve. Salt, of course, is sodium chloride, so it works in the exact same way.

It's related to how water moves into salt if you sprinkle a bit of salt [sodium chloride] into a little water. The most stable situation is when the salt and the water are all mixed. The sodium and chloride ions, and the water molecules continue to move around, but they're totally mixed already, and it would take energy to make the sodium and chloride ions separate themselves from the water molecules.

In cells, the process is called 'osmosis.' "Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides." From Wikipedia.

I don't think 'trans-cellular' is the word you mean. I think you're talking about stuff moving into and out of a cell.

"Transcellular transport involves the transportation of solutes by a cell through a cell. One classic example is the movement of glucose from the intestinal lumen to extracellular fluid by epithelial cells." Read more here.