Many molecules in and around cells exist in concentration gradients across the cell membrane, meaning that the molecules are not always evenly distributed inside and outside of the cell. Hypertonic solutions have higher concentrations of dissolved molecules outside the cell, hypotonic solutions have lower concentrations outside the cell, and isotonic solutions have the same molecular concentrations inside and outside of the cell. Diffusion drives molecules to move from areas where they are in high concentration to areas where they are in a lower concentration. The diffusion of water is referred to as osmosis.
TL;DR (Too Long; Didn't Read)
When placed into an hypertonic solution, animal cells will shrivel up, while plant cells will stay firm thanks to their air-filled vacuole. In a hypotonic solution, the cells will take on water and appear more plump. In an isotonic solution, they'll stay the same.
A solution is hypertonic to a cell if it has a higher solute (dissolved substance) concentration than the cell does. As a result, it also has a lower concentration of water than the cell does. Cell membranes and plant cell walls are semipermeable barriers, which means that certain molecules can diffuse through them, while other molecules can not. Many solutes are too large or charged to cross the cell membrane but water can diffuse freely. In a hypertonic environment, osmosis forces water out of the cells.
Responses to Hypertonic Solutions
Plant cells have large sacs of fluid called vacuoles. When full, vacuoles push outward onto the plant's cell walls, keeping them stiff. When plants are placed in hypertonic solutions, their vacuoles shrink and no longer provide enough pressure to keep the plant from wilting. Because of their rigidity, the cell walls keep their rectangular shape but are less plump. In contrast, animal cells lack a cell wall, and so they shrivel up like raisins.
A solution is hypotonic to a cell if it has a lower solute concentration than the cell does. As a result, it also has a higher concentration of water than the cell does. Osmosis draws water out of the solution and into the cells. As a result, plant and animal cells both appear more plump when placed in a hypotonic solution. When viewed under a microscope, the vacuoles of plant cells appear noticeably larger.
If the solution has the same solute concentration, and thus the same water concentration, as the cells do, it is isotonic to the cells. As a result, there would not be a concentration gradient since a gradient by definition involves a difference. There would thus be no net flow of water between the cell and the solution. This does not mean that water would not move between them, just that the rate of exiting and entering the cell are equal. There is no net change in the cell's appearance.