The pressure needed to keep the volumes the same is the osmotic pressure. Water flows from lower to higher osmotic pressure. The movement of water across membranes and balance of water between the cell and its environment is crucial to organisms. Cells shrivel when in a hyperosmotic medium. Soaking your foot in epsom salts solution to reduce swelling.
Eye drops are isotonic, many marine animals are isotonic with sea water.
This is usually lethal. They are thickened with lignin. Fiber cells and sclereids are of this type. Water mainly moves through tracheids and vessel elements. Secondary walls and cells are actually dead at maturity.