13.4 Osmotic Stress [in progress]

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 signaling pathways, and pH levels. Thus, the disruption in ion balance during osmotic stress can severely disrupt these essential cellular processes.

Mechanisms that protect cells from osmotic stress

To survive osmotic shock induced by freezing, a cell can decrease ice content, stabilize macromolecules, and coordinate repair and recovery (see Chapter 14 for more on recovery). One way a cell can reduce ice content is by accumulating compatible osmolytes, such as polyols, sugars, and amino acids. By increasing osmotic pressure inside the cell with these osmolytes, the cell should not lose water so rapidly when ice forms extracellularly, decreasing the extent of osmotic shock. In addition, multicellular organisms can accumulate these osmolytes in extracellular fluids (e.g., blood, hemolymph, interstitial fluid) to decrease the amount of ice that forms overall. If less ice forms, the change in osmotic pressure during freezing and thawing should be smaller, resulting in a less intense osmotic stress.

[figure showing the compatible osmolyte mechanism]

Because osmotic stress can cause molecular crowding (which can lead to protein aggregation) and general changes in ion concentrations (which can damage lots of macromolecules), it is important for freeze-tolerant organisms to combat osmotic stress by stabilizing macromolecules. For example, cells can use cryoprotectants (which are often also compatible osmolytes) to directly interact with and stabilize membranes and proteins during freezing. In addition, cells can protect macromolecules by utilizing molecular chaperones, which are proteins that help refold and stabilize other proteins. The prevention of macromolecule damage helps the cell remain functional, and as a result, the cell is more equipped to tolerate stress.