CLT Practice Test 9

Science

¹This passage is an original essay written for PrepBanq practice.

²In 1773 the German pastor Johann August Ephraim Goeze peered through a microscope and described a tiny creature he called a "little water bear." Three years later the Italian scientist Lazzaro Spallanzani gave the group the name it still [1] bears: tardigrades, or "slow steppers." The name fits: under the microscope, the animals lumber along slowly on their stubby legs, rather like tiny, clumsy bears.

³Biologists have now described more than 1,300 species of these animals. [2] Most is less than a millimeter long, with a plump body and eight stubby legs tipped with claws. They live almost everywhere: in moss and lichens, in sand and ocean sediments, in hot springs, and in the soils of Antarctica. What has made them [3] famous, however, is not where they live but what they can survive.

⁴Many species that live in moss face a recurring crisis. When the moss dries out, so [4] does the animals inside it. Instead of dying, a drying tardigrade pulls in its head and legs and curls into a shriveled form called a tun. In this state its body may lose nearly all of its water, and its metabolism slows to a tiny fraction of the normal rate. Scientists call this condition cryptobiosis, from Greek words meaning "hidden life," because the animal's signs of life are so [5] feint that they can hardly be detected. [6]

⁵The tun can endure conditions that would kill most [7] animals. Extreme cold, intense radiation, and even the vacuum of space. In 2007, European researchers sent dried tardigrades into orbit aboard a satellite and exposed them directly to space; some survived and later produced healthy offspring.

⁶How does a dried tardigrade protect its cells? [8] Researchers have found that some species produce a sugar called trehalose, and this sugar, trehalose, may help to hold cell parts in place and keep them steady as the water leaves the cells. Others make unusual proteins found in no other animals. As the cells dry, some of these proteins appear to form a glassy gel that surrounds and shields delicate molecules. One protein, nicknamed Dsup for "damage suppressor," even reduced radiation damage to DNA when scientists inserted it into human cells grown in the laboratory. The protein appears to bind to DNA and act as a shield, although researchers are still studying exactly how it works. [9] Pretty wild, right? Scientists are totally hoping these molecules can someday keep medicines good without a fridge.

⁷Still, tardigrades are not invincible. Many species cannot form a tun at all if they dry out too quickly, because they need time to build up their protective molecules. Active, fully hydrated tardigrades are much more vulnerable than tuns, and the toughness that has made these tiny [10] animal's famous belongs mainly to the dormant form.