8.1 Challenges Associated with Osmotic Stress

Maintaining proper ion balance and water balance is essential for all cells to ensure proper functioning. Ions are charged atoms that are formed when water dissolves solutes. The common ions that are transported across cellular membranes are: sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), chloride (Cl^–), and hydrogen (H⁺). Ion balance refers to maintaining adequate concentrations of these ions (for cell function) on both sides of the semipermeable cellular membrane. For example, most animal cells maintain a higher extracellular concentration of Na⁺ and Cl^– compared to the cytosol, and a higher cytosolic concentration of K⁺ relative to the extracellular space (Chapter 5.2). Ion balance is tightly linked to water balance, which refers to the cell regulating its water content. Recall that water will move from areas of high solute concentration (high osmolarity) to areas of low solute concentration (low osmolarity) (Chapter 5.3). Any change in ion balance can also impact water balance due to the impact of ions on osmolarity (total solute concentration). Water balance is important for maintaining appropriate cell size, solute concentrations, and pH. We use osmoregulation to refer to the process by which cells regulate their ion and water balance, for example by using active transport (Chapter 5.4).

The Impact of Osmotic Stress

Osmotic stress is cellular dysfunction caused by a change in the osmotic pressure (osmolarity) inside or outside a cell. Let’s consider a couple of ways in which changes in the cell’s extracellular environment can cause osmotic stress. An isosmotic (iso = same) environment is the ideal environment for a cell, because the external solute concentration is equal to the solute concentration in the cytosol, and there is not net movement of water across the plasma membrane (Figure 8.2). If the environment around a cell becomes a hypoosmotic (hypo = below) or hyperosmotic (hyper = above), this can cause osmotic stress. A hypoosmotic environment has a low osmolarity (solute concentration) relative to the cell’s cytosol, so water can move via osmosis into the cell in large quantities (Figure 8.2). In contrast to this, in a hyperosmotic environment (high osmolarity), water can move via osmosis out of the cell (Figure 8.2). You may refer to Figure 8.1 to see the cellular response of a human erythrocyte (red blood cell) to each of these environments. Note that the osmosis of water also depends on whether the membrane is permeable to solutes. However, for simplicity in this chapter, we assume that hypoosmotic environments are hypotonic (water enters the cell), and hyperosmotic environments are hypertonic (water leaves the cell); see Chapter 5.3 for a review of these terms.

Note that hypoosmotic and hyperosmotic environments do not always cause stress; many organisms are adapted to live in hypoosmotic environments (e.g., freshwater lakes and rivers) or hyperosmotic environments (e.g. sea water, hypersaline lakes) because they can osmoregulate effectively. Osmotic stress is usually brought about by a change in osmotic conditions. For example, an environment’s osmolarity may change daily, as in estuaries: the boundary between fresh and saltwater bodies. An environment’s osmolarity may change seasonally, as in some saltwater lakes that slowly lose water through evaporation in the summer, concentrating the remaining solutes.

Rapid changes in water content and ion concentrations during osmotic stress can result in different events such as cell membrane distortion, and protein aggregation. Rapid changes in cell volume can also cause permanent damage to the plasma membrane, either causing it to break due to an increase in cell volume (too much water in the cell), or causing parts of the plasma membrane to fuse together as a cell shrinks (too little water in the cell). During hyperosmotic shock, the amount of availability water within the cell decreases, causing macromolecules (e.g., proteins) to crowd together and potentially aggregate. High intracellular concentrations of ions (e.g., under hyperosmotic stress) can also cause damage, for example by causing proteins to denature. When ion concentrations in the cell become too low, this can also cause cellular dysfunction, for example by causing a change in pH (H⁺ concentration) that interferes with protein structure.