Passive transport is the movement of molecules through the membrane in which no energy is required and molecules move in response to a concentration gradient. Osmosis is the movement of water from an area of high to low concentration of water, movement of water toward an area of high solute concentration. In the Cellular Fitness and Transport Across Membrane Lab, only water can cross the membrane while sodium and chlorine ions or dissolved salt cannot. Water will move across the membrane with the concentration gradient because there is more salt inside the tubing for the 5%, 10%, 20%, 30% solutions. The dialysis tubing was placed in a hypotonic solution (water). When the outside cell has a lower concentration of dissolved molecules than inside the cell, the solution is hypotonic, and water will move from the solution into the cell. A cell in any environment can be thought of as a plasma membrane separating two solutions: the cytoplasm and the extracellular fluid. In a hypertonic solution, water moves out of the cell causing the cell to shrivel. In an isotonic solution, water diffuses into and out of the cell at the same rate, with no change in the cell size. In a hypotonic solution, water moves into the cell causing the cell to swell and eventually burst. Osmotic pressure is measured as the force needed to stop osmosis. The strong cell wall of plant cells can withstand the hydrostatic pressure to keep the cell from rupturing, but this is not the same for animal cells.