The plasma membrane is permeable to water molecules, and allows the movement of water into and out of the cells, which is critical to life. Diffusion of water molecules across a selectively permeable membrane is a special kind of passive transport called osmosis. If a membrane is permeable to water, what happens when a solute cannot pass through the membrane? Let’s experiment by placing glucose molecules inside the cell, outside the cell, or both. We’ll try different concentrations of glucose on each side of the plasma membrane to see which way water will diffuse and what will happen to the cell. When a cell is in a solution that is more concentrated than its cytoplasm, it is said to be in a hypertonic solution. The solute molecules attract water molecules so that fewer water molecules are free to diffuse across the membrane. In a hypertonic solution, the concentration of free water molecules is higher on the inside of the cell than the outside. Osmosis occurs as water molecules move down their concentration gradient, leaving the cell. As a result, the cell shrinks. Here’s another scenario. The glucose molecules have been distributed evenly inside and outside the cell. The cell is in an isotonic solution, where the concentration of the external solution and the cytoplasm are the same. The solute molecules attract water molecules. Since the concentration of solute molecules is equal on both sides of the membrane, the concentration of free water molecules is also the same on both sides. Water flows back and forth across the membrane in equal amounts, and so the cell neither shrinks nor swells. Now there are more glucose molecules inside the cell than outside. This cell is in a hypotonic solution. The solute molecules attract water molecules, so fewer water molecules are free to diffuse across the membrane. In a hypotonic solution, the concentration of free water molecules is higher on the outside of the cell than the inside. Osmosis occurs as water molecules move down their concentration gradient, entering the cell. As a result, the cell swells. Now let’s see the effects of a hypotonic environment on animal and plant cells. Animal cells lack rigid cell walls. When they are exposed to a hypotonic environment, water rushes into the cell, and the cell swells. Eventually, if water is not removed from the cell, the pressure will exceed the tensile strength of the cell, and it will burst open, or lyse. Many single-celled protists living in freshwater environments have contractile vacuoles that pump water back out of the cell, which maintains osmotic equilibrium and avoids lysis. Plant cells are surrounded by rigid cell walls. When plant cells are exposed to a hypotonic environment, water rushes into the cell, and the cell swells, but it is kept from breaking by the rigid cell wall. The pressure of the cell pushing against the cell wall makes the cell turgid, which is the desired state for most plant tissues. For instance, placing a wilted celery stalk or lettuce leaf in a hypotonic environment of pure water will often revive the leaf by inducing turgor in the plant cells. Now let’s look at the effects of a hypertonic environment on plant and animal cells. Since animal cells lack rigid cell walls, when they are exposed to a hypertonic environment, water rushes out of the cell, and the cell shrinks. The resulting cells are dehydrated and lose most or all physiological functions while in the shriveled state. If the cells are returned to an isotonic or hypotonic environment, water reenters the cell and normal functioning may be restored. Since plant cells are surrounded by rigid cell walls, when plant cells are exposed to a hypertonic environment, water rushes out of the cell, and the plasma membrane pulls away from the cell wall at multiple places. This phenomenon, called plasmolysis, causes the plant to wilt and can lead to plant death. If plant cells are returned to an isotonic or hypotonic environment, water reenters the cell and normal functioning may be restored.
