The tonicity of a solution determines if a cell gains or loses water in that solution. The tonicity depends on the permeability of the cell membrane for different solutes and the concentration of nonpenetrating solutes in the solution within and outside of the cell. If a semipermeable membrane hinders the passage of some solutes but allows water to follow its concentration gradient, water moves from the side with low osmolarity (i.e., less solute) to the side with higher osmolarity (i.e., higher solute concentration). Tonicity of the extracellular fluid determines the magnitude and direction of osmosis and results in three possible conditions: hypertonicity, hypotonicity, and isotonicity.

Isotonic Solutions

In biology, the prefix “iso” means equal or being of equal measurements. When extracellular and intracellular fluid have an equal concentration of nonpenetrating solute inside and outside, the solution is isotonic. Isotonic solutions have no net movement of water. Water will still move in and out, just in equal proportions. Therefore, no change in cell volume occurs.

Hypotonic Solutions

The prefix “hypo” means lower or below. Whenever there is a low concentration of nonpenetrating solute and a high concentration of water outside relative to inside, the environment is hypotonic. Water will move into the cell, causing it to swell. In animal cells, the swelling ultimately causes cells to burst and die. Freshwater is an example of a hypotonic environment. Freshwater organisms tend to have higher osmolarity (i.e., higher salt concentration) inside their cells than the surrounding body of water such as a lake or river.

Hypertonic Solutions

Conversely, the prefix “hyper” means more or above. During hypertonicity, the extracellular fluid contains more solute (i.e., high osmolarity) and less water than the inside of a cell. Thus, water moves out of the cell, causing animal cells to shrink. Saltwater is an example of hypertonic extracellular fluid because it has a higher osmolarity (i.e., higher salt concentration) in contrast to most intracellular fluids.

Osmoregulation

To avoid the shrinking and swelling that occurs in hypertonic and hypotonic solutions, animal cells must have strategies to maintain osmotic balance. The process by which osmotic balance is achieved is called osmoregulation. Osmoregulatory strategies can be grouped into two categories: regulating and conforming. Osmoregulators control and maintain their internal osmotic conditions independent of environmental conditions. Conversely, osmoconformers use active and passive internal processes to mimic the osmolarity of their environment.

Many animals, including humans, are osmoregulators. For instance, fish that live in saltwater, a hypertonic environment, are able to regulate water lost to the environment by taking in copious quantities of water and frequently excreting salt out. Fish that live in freshwater mitigate the constant osmosis of water into their cells by frequent urination that releases water out of the body.

Most marine invertebrates, such as lobsters and jellyfish, are osmoconformers. Osmoconformers maintain an internal solute concentration—or osmolarity—equal to that of their surroundings, and so they thrive in environments without frequent fluctuations.