PreACT Practice Test 15

INFORMATIONAL: This passage is adapted from an original article about how wood frogs survive freezing.

¹On a cold morning in early spring, a hiker in the forests of the northern United States or Canada might turn over a layer of dead leaves and find what looks like a dead frog. Its body is hard. Its eyes are cloudy. It does not breathe, and its heart does not beat. Yet if the hiker set the frog gently back under the leaves and returned a few days later, after a thaw, the animal might well be gone, hopping toward a pond to breed. The wood frog is one of a small number of animals that can survive having much of its body frozen solid.

²For most animals, freezing is deadly, and the main danger is not the cold itself but the ice. When water inside a cell freezes, the ice crystals can grow large enough to tear the cell's delicate membranes. Even ice that forms outside cells can cause harm: as the water around the cells freezes, it draws water out of the cells, which can shrink until they are damaged beyond repair.

³The wood frog's strategy is not to prevent freezing but to control it. When ice first touches the frog's skin, often from frost on wet leaves, freezing begins at the surface and spreads inward. Within minutes, the frog's liver begins converting a stored carbohydrate called glycogen into glucose, a sugar, and releasing it into the blood. Over the next several hours, glucose levels in the frog's body rise to many times their normal level, and the sugar is carried into cells throughout the body.

⁴The glucose works something like antifreeze in a car. Dissolved sugar lowers the temperature at which the water inside cells will freeze, and it helps cells hold on to their water instead of losing it to the ice forming around them. As a result, ice gathers mainly in the spaces between cells and in the body cavity, while the cells themselves stay unfrozen: shrunken, but intact. In a fully frozen wood frog, roughly two-thirds of the body's water may be ice.

⁵The process carries risks of its own. A frozen frog's cells receive no fresh oxygen, because its blood is not moving. Researchers have found that wood frog cells can tolerate low oxygen for far longer than the cells of many other animals, slowing their energy use until circulation returns. Thawing brings its own danger. In many animals, a sudden return of oxygen-rich blood can itself damage tissues; the wood frog appears to have defenses that limit this harm.

⁶The frog's tolerance has limits, and those limits vary. Frogs from the far north, in places such as interior Alaska, can survive lower temperatures and longer periods frozen than frogs from more southern populations. Scientists studying the northern frogs have found that they build up larger stores of glycogen in the liver before winter, giving them more raw material for producing glucose when the freeze comes.

⁷The wood frog's abilities have drawn interest from scientists far outside the study of frogs. Doctors who transplant organs face a problem the frog solves every winter: human organs can be kept outside the body for only a limited time before they are damaged, and freezing them destroys them. Some researchers hope that studying how the frog protects its cells (with sugars, with tolerance for low oxygen, with careful control over where ice forms) will suggest new ways to preserve tissues for longer.

⁸Such applications remain distant, and a frog is not a kidney. But the wood frog is a reminder that what looks like a single, simple event, a body freezing, can be broken into many smaller problems, each with its own solution. The frog does not avoid winter. It has found a way to let winter in, carefully, and to survive it.