Organismal Physiology Lecture No. 8: Ectothermic Cold Tolerance
Thursday October 4 , 2012
Wood Frogs & The Freezing Qualities Of Water:
-In the winter, wood frogs burrow into the soil (not far enough) and allow every tissue in their body to
freeze over; they are completely fine when they thaw out in the spring. As organisms are mostly water,
this has incredibly observable consequences on the wood frog.
-Water, as it freezes, expands and becomes a less dense solid. Not all water freezes at 0°C, in fact small
quantities of water can supercool to -20°C. This is why many insects tend to be smaller as water volume
is proportional to the likelihood that it will freeze. In order for ice to form, it requires an ordered
octagonal lattice structure (lower energy state). Certain proteins actually initiate the formation of ice
Insects & Cold Tolerance:
-In terms of their ability and strategy to tolerate cold environments, insects are classified into three
categories: Chill-susceptible (which die before they freeze), freeze-avoiding (which keep from freezing)
and freeze-tolerant (which survive freezing).
Observing Frozen Organisms:
-We can always tell for certain if an organism has frozen by looking at the supercooling point (SCP)
whereby all of the water molecules should be frozen and examining the exotherm point which is also
referred to as the latent heat of crystallization.
-Chill-susceptible insects die from physiological processes that have nothing to do with freezing (they die
before their bodies have frozen). This injury caused by the effects of cold (not ice) is known as chilling
injury. There are two types of chilling injury: acute chilling injury and chronic chilling injury. Acute chilling
injury is the result of quick exposure to low temperatures (not freezing) and is possibly caused by phase
changes in the cell membrane. Chronic chilling injury is caused by the loss of ion homeostasis which
eventually leads to the loss of water balance.
Chronic Chilling Injury:
-Ion balance is critical to life, in that 20-70% of cells require this to some degree. If cells cannot keep K
and Na on opposite sides of the cell membrane (due to plummeting body temperature), then they will
leak more ions than they are actually pumping in. The loss of ion homeostasis also means a loss of cell
potential (can’t make action potential anymore) and nerve cells and muscle cells cannot function
properly anymore. This loss of ion-balance causes a paralysis known as chill-coma. This coma is
reversible if the insect is brought back to warm temperatures. Chill-Coma & Temperature Variance:
-Chill-coma temperature varies among different species as the temperature at which insects enter chill-
coma depends on the environment they come from. Tropical insects tend to enter chill-coma at higher
temperatures than insects from greater latitude environments.
-Freeze-avoiding insects can keep their body fluids in the liquid state at temperatures below their
melting point. However, theoretically they cannot lower their freezing limit past -50°C. The probability
of freezing is pr