13.3 Mechanical Stress [in progress]
Key Concepts
By the end of this section, you will be able to do the following:
- Explain when and why mechanical stress occurs during the process of freezing and thawing outlined in Chapter 13.1
- Describe the impact of mechanical stress on cells and their macromolecules
- Identify ways in which mechanical 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 mechanical stress, and how
When ice forms in or near cells, they are subject to mechanical stress. Mechanical stress refers to stress associated with force exerted on an object (e.g., on a cell). Because ice is harder than most cellular structures, ice can cause physical damage to cells that impairs function and can lead to cell death. This section will explore when freezing stress may induce mechanical stress, the impacts of mechanical stress on cells and their macromolecules, as well as some mechanisms that protect cells from freezing-induced mechanical stress.
When mechanical stress occurs
The process of freezing instigates mechanical stress. When an organism is cooled below its supercooling point, the bulk of ice formation occurs. As water transforms from liquid to solid, it increases in volume while decreasing in density. If ice forms in restricted spaces (e.g., in the cytosol), this will cause mechanical damage to surrounding structures (e.g., the plasma membrane). In addition, if ice propagation occurs rapidly, it is less controllable and is more likely to penetrate surrounding cells, increasing mechanical damage (Figure 13.XX). Mechanical damage may also occur during the transition from freezing to thawing due to changes in ice structure (recrystallization), although this damage is typically less severe than the mechanical damage experienced during initial freezing.
Figure 13.XX. Effects of ice propagation on the location of ice formation. If cells experience slow cooling, ice propagation is slower and more easily controlled, resulting in ice formation that does not affect cells internally. If cells experience rapid cooling, propagation is rapid and difficult to control, resulting in internal ice formation and subsequent physical damage to the cells. [remake figure to focus on slow vs. rapid cooling. Possibly also incorporate components from the right-hand side of the image below – i.e., injury from internal vs. external ice]
Impact of mechanical stress on cells and their macromolecules
Because water expands in volume upon freezing, ice formation inside closed spaces can cause damage to the boundaries of the space. The major impact of mechanical stress on cells is the damage it inflicts on membranes. Cellular membranes are comprised mainly of lipids and can be stretched, compressed, or torn upon the initiation of ice formation. Intracellular ice formation often causes a lot of damage, because ice formation inside a cell can cause rupturing of the cell membrane as the ice expands (Figure 13.XX). While extracellular ice formation usually causes less damage, ice outside the cell can also pose a threat. Ice crystals are hard and pointy, so ice in extracellular spaces can puncture cell walls and membranes (Figure 13.XX).
Mechanisms that protect cells from mechanical stress
Mechanisms organisms use to protect their cells from mechanical stress caused by freezing are controlling ice formation and propagation, decreasing ice content, and coordinating repair and recovery (see Chapter 14).
By controlling where ice formation occurs and how fast it occurs, organisms can reduce mechanical damage. To keep ice formation in extracellular spaces (e.g., the gut or hemolymph of animals), organisms will upregulate the transcription and translation of genes which code for ice nucleating agents (INA). INAs are proteins that promote heterogenous ice nucleation. By increasing the amount of INAs in extracellular spaces, organisms can promote extracellular ice formation, and avoid intracellular ice formation. INAs can also help stimulate freezing at higher subzero temperatures (increase the supercooling point), which allows for slower ice propagation and decreases the chances of intracellular ice formation.
[figure for how ice-nucleating agents can work?]
To decrease ice content, organisms can accumulate small solutes called compatible osmolytes, in and around their cells, to decrease or prevent ice formation. Sugars, amino acids, and polyols function as compatible osmolytes because they can accumulate in large amounts, without becoming toxic to the cell or organism. For example, trehalose, proline, and glycerol are common compatible osmolytes. Reducing the amount of ice will reduce the amount of mechanical damage that ice can cause cells.
[figure showing relationship between osmolyte concentration and ice content?]