Iowa Assessments Practice Test 9

The Frog That Freezes

¹On a cold morning in late autumn, a hiker in the northern woods might lift a pile of dead leaves and find what looks like a small brown stone. It is a wood frog, and it is frozen nearly solid. Its eyes are cloudy, its legs are stiff, and its heart is not beating. By most measures, it seems dead. Yet when the forest floor warms in spring, this same frog will thaw, take a breath, and hop away to find a pond.

²Most animals cannot survive being frozen. When water freezes, it forms sharp ice crystals that can tear the thin membranes around cells. Ice forming around a cell also pulls water out of it, leaving the cell shriveled and damaged. For a frog, a fish, or a person, ice inside the body is usually deadly.

³The wood frog survives by controlling where the ice forms. As the temperature drops below freezing, ice begins to form in the spaces between the frog's cells and in its body cavity, but not inside the cells themselves. At the first touch of ice on its skin, the frog's liver begins turning stored starch, called glycogen, into glucose, a kind of sugar, and sends it through the blood. Within hours, the frog's cells are packed with sugar. The sugar works somewhat like antifreeze in a car: it lowers the temperature at which the water inside the cells will freeze and keeps the cells from losing too much water.

⁴The result is a frog in which as much as two-thirds of the body water may be frozen while the cells themselves stay intact. Its heart stops. Its breathing stops. Its brain shows almost no activity. The frog can remain this way for weeks, and in some winters it thaws and refreezes several times.

⁵When the weather warms, the ice melts, and before long the heart begins to beat again. Within about a day, the frog is breathing, moving, and ready to head for the meltwater ponds where wood frogs breed in early spring.

⁶This unusual ability helps explain why wood frogs live farther north than any other North American amphibian, including parts of Alaska and Canada north of the Arctic Circle. Instead of burrowing below the frost line as some frogs do, a wood frog simply settles under the leaves on the forest floor and waits. Medical researchers are interested, too. If human organs could be frozen and thawed without damage the way wood frog tissues are, they could be stored much longer before a transplant.