Also the frog releases special proteins called Protein Ice Nucleases (or “PIN’s”) into their blood stream. These sugars are absorbed by the cells of the body causing the cytoplasm to become thick and syrupy and increasingly hypertonic to the surrouding interstitial fluids. Their livers start synthesizing and releasing large quantities of glucose (“sugar”) into their blood streams. “killed”) by either excessive dehydration or by the inevitable freezing of its cytoplasm.īut, let’s get back to our frogs: before the frogs are exposed to freezing temperatures they undergo many physiological changes in addition to the cell membrane changes I listed above. For a while this dehydration event may actually hold off cytoplasmic freezing, but eventually the cell will be irreversibly damaged (i.e. Usually the interstitial fluid around the cells freezes first and these ice crystals actually start pulling water out of the inside of the cells. Also, and maybe of a more immediate concern, the lack of warm blood entering the tissue means that the fluids in the tissue and in its cells may start to freeze. The lack of blood flow into the tissue means that oxygen is no longer being delivered and cell death from lack of oxygen may occur. Why do the cells die? First, the blood flow into the cold body part is curtailed in order to prevent excessive body heat loss (humans are endothermic organisms who use the heat from their metabolic activities to generate their body heat and there is only so much heat energy to go around!). When skin is exposed too long to freezing temperatures cells are destroyed and “frostbite” occurs. Let’s take a second and think about people. These changes help to keep our frog active at temperatures that are lower than optimal, but eventually temperatures start to approach the freezing point of water, and the frog is at risk of cell and tissue damage from the freezing of the water in its blood and cytoplasm.įrostbite on human toes (Photo by Dr. Also, amphibians add cholesterol to their cell membranes, and these steroids further keep the fatty acids from clumping together even at decreasing temperatures. This reduces the freezing point (which usually referred to as the “melting point” for some reason) of the cellular membrane and keeps the membrane “fluid” and functional at lower and lower temperatures. This “desaturase” enzyme makes the altered fatty acids more crooked and thus less able to stick together. An enzyme is stimulated that begins to add double bonds to the fatty acids of the cell membrane phospholipids. In my Cell Biology class I talk about changes that can be seen in cell membranes in both amphibians (like the green frog) and reptiles (like turtles and snakes) as seasonal temperatures begin to fall. How could they survive this extreme thermal trauma? Frogs are ectothermic (they rely on the heat of their environment for their body heat) and would seem quite vulnerable to freezing solid in the very cold winters of Western Pennsylvania even if they were underground or underwater. But where these frogs sit out the winter was just the beginning of this story. As we talked about her frogs’ appearance, behavior, and songs I decided that they must be Northern green frogs ( Lithobates clamitans melanota), and since her pond is quite shallow and expected to freeze solid over the winter, and also because it is surrounded by a very dense growth of myrtle, I speculated that the green frogs (which can either hibernate underwater or underground) would leave the soon-to-be-solid pond and dig a hibernaculum in the soil under the protective cover of the myrtle. Wikimedia Commons)Ī few weeks ago Jane Viti, one of my teaching colleagues, asked me what was going to happen to the two frogs that had been living in her small, backyard pond all summer. Northern green frog (photo by Contrbaroness.
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