A cell placed in a hypotonic solution will swell up or gain water, and it can possibly rupture if the excess water inside the cell is not removed. Although this can occur in cells without walls, this does not happen in cells that do have cell walls like plant cells. Similarly, freshwater organisms like protozoans contain vacuoles that remove water from inside their cells.
A hypotonic solution is one in which the concentration of water molecules are greater on the outside of a cell than it is inside the cell. When water molecules move from an area of greater concentration to one of lower concentration, it is called osmosis.
Besides hypotonic solutions, hypertonic and isotonic solution are two other types of solutions that can also affect cells. In a hypertonic solution, cells tend to shrink, while in an isotonic solution, cells are in homeostasis.