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 rupturi