13.1 The Processes and Challenges of Freezing and Thawing [in progress]
13.1 The Processes and Challenges of Freezing and Thawing [in progress]
Key Concepts
By the end of this section, you will be able to do the following:
- Describe the process of freezing and thawing using appropriate terminology.
- Explain what happens to ice content, water content, and osmotic pressure inside and outside of cells during extracellular ice formation and thawing.
- List the main stressors that occur during the process of freezing and thawing.
- List the main mechanisms that are hypothesized to protect freeze-tolerant animals from these stressors.
Freezing is lethal to most organisms, but remarkably there are some organisms that are freeze-tolerant and survive internal ice formation. To understand freeze tolerance, it is important to think through all of the changes that can occur when a cell is cooled, frozen, and then thawed (Figure 13.2). Many types of organisms can survive freezing, including some plants and animals. In this chapter section, we will focus on what happens to animals and their cells during cooling, freezing, and thawing. Each of these processes has its own associated stresses, which we will cover in the subsequent sections. A review of Chapters 8 – 12 will provide a good basis for understanding these stresses in the context of freezing.
Figure 13.2 Summary of the processes and challenges associated with cooling, freezing, and thawing over time in a freeze-tolerant cricket under laboratory-controlled temperature exposure. Cell temperature generally follows the environmental temperature, which was manipulated by the researcher. A single cell is shown, along with the impacts of extracellular ice (blue hexagons), although ice can also form intracellularly (not shown). Stressors are highlighted in yellow shapes, and the explanation of each stressor will be expanded on in the chapter sections that follow.
Cooling (and supercooling)
As environmental temperatures drop, the organism and its cell begin to cool. At low temperatures, proteins become less flexible, chemical reaction rates decrease, membranes become less fluid, and the viscosity of liquids (e.g., the cytosol, circulatory fluids) increases. These changes associated with low temperature stress usually impair organismal function and cause energy stress, even before freezing occurs. Animals such as insects lose the ability to move due to the impact of low temperatures on their neurons and muscles, and they enter chill coma – a (usually reversible) paralysis induced by chilling. Oxidative stress can also occur at low temperatures due to the reduced efficacy of antioxidant enzymes at low temperatures.
The fluids in and around cells will remain liquid until ice begins to freeze in or around the organism. The temperature at which this happens differs among species and among individuals within a species. Every solution (including the fluids in an organism’s body) has a melting point – the temperature at which a solid changes phase into a liquid. For example, pure water h