13.4 Osmotic Stress [in progress]
Key Concepts
By the end of this section, you will be able to do the following:
- Explain when and why osmotic stress occurs during the process of freezing and thawing outlined in Chapter 13.1
- Describe the impact of osmotic stress on cells and their macromolecules
- Identify ways in which osmotic stress interacts with the other stressors that occur during freezing and thawing
- Explain which of the mechanisms described in Chapter 13.1 can help protect against osmotic stress, and how
Fluctuations in osmotic pressure are one of many physiological changes that occur during the freezing and thawing cycle. This rapid change in osmotic pressure causes water to suddenly move across the plasma membrane, resulting in osmotic stress (osmotic shock). This section will examine when osmotic stress arises, its impacts, and the defenses cells use to counteract it during freezing and thawing.
When osmotic stress occurs
Osmotic stress can occur both during freezing and thawing. Let’s consider the case where ice forms outside of cells (Figure 13.XX). As extracellular ice begins to form, the amount of liquid water decreases but the amount of solutes does not change, and those solutes remain in the water (ice contains only water molecules, no solutes). Therefore, the osmotic pressure in the extracellular fluid increases as ice forms, creating a hyperosmotic environment that causes water leaves the cell via osmosis, and the cytosolic osmotic pressure to increase. During thawing, as ice melts the amount of available liquid water increases. As a result, extracellular osmotic pressure decreases (there is more water for the solutes to be dissolved in), making the extracellular fluid hypoosmotic to the cell’s cytosol. Water then enters the cell via osmosis, as it typical under hypoosmotic stress.
Figure 13.XX: As ice forms outside the cell, external solute concertation increases due to the decline in available liquid water. This change in osmotic pressure results in water moving (blue arrows) out of the cell via osmosis until the osmotic pressures inside and outside the cell are equal. Once the extracellular ice melts, water dilutes the solutes within the surrounding environment. Consequently, water then moves (blue arrows) into the cell via osmosis until the osmotic pressures inside and outside the cell are equal.
Impact of osmotic stress on cells and their macromolecules
Osmotic stress impacts a myriad of cellular components. In the case of hyperosmotic shock (i.e., decrease in water availability), a cell’s volume decreases, its ion concentration rise, proteins can denature, and macromolecule crowding occurs. Conversely, when a cell endures hypoosmotic shock (i.e., an increase in water availability), the cell’s volume increases, its ion concentration decreases, and the cell may even lyse. Furthermore, many critical cellular processes rely heavily on ion balance, such as membrane potential, electrical signaling in excitable cells, some