Osmosis

Water moves freely across the plasma membrane (remember: the plasma membrane is selectively permeable, which means it lets some materials into the cell and keeps others out). Water will move to reach equal concentration on both sides of the cell's membrane. This movement if called osmosis- the diffusion of water across a selectively permeable membrane. The concentration of the substance that is dissolved ub the water controls which direction across the membrane the water will move. The water wants the concentrations of the substances to be equal on both sides. So, the concentration gradient controls osmosis. The water will continue to move across the membrane until the concentrations on both sides are equal.