8.1 Challenges Associated with Osmotic Stress
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Evaluate the importance of maintaining ion and water balance within a cell.
- Compare how hyperosmotic and hypoosmotic stress impact cell structure and function by disrupting ion and water balance.
- Give examples of organisms that are well-adapted to hyperosmotic and hypoosmotic environments.
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 hy