Text Structure & Organization Practice Test

PASSAGE IV

NATURAL SCIENCE: This passage is adapted from the article The Shark’s Secret Armor.

For billions of years, nature has been conducting the ultimate research and development experiment. Through the ruthless process of natural selection, life has engineered solutions to problems that human designers still struggle to solve. This concept is the foundation of biomimicry, a discipline that seeks to emulate nature’s time-tested patterns and strategies. One of the most fruitful subjects of this study is a creature often feared but rarely understood as an engineering marvel: the shark.

To the naked eye, a shark looks sleek and smooth. However, if you were to run your hand along a shark's flank—from tail to head—it would feel like rough sandpaper. This roughness is caused by millions of microscopic scales called dermal denticles (literally "skin teeth"). Unlike the flat, overlapping scales of a goldfish, denticles are tiny, tooth-like structures made of dentin and enamel, featuring raised ridges aligned with the flow of water.

For decades, marine biologists were puzzled by these structures. Logic suggested that a perfectly smooth surface would create the least amount of friction, allowing the shark to slice through the water efficiently. Yet, the shark is one of the ocean's fastest predators. The secret lies in the physics of fluid dynamics. As a shark swims, water flows over its skin. On a smooth surface, this flow creates chaotic eddies and swirls known as turbulence, which suck the swimmer backward and increase drag.

The denticles disrupt this process. The microscopic ridges channel the water, organizing the flow into linear streams. This prevents the formation of turbulent eddies close to the skin, effectively reducing drag by up to 10 percent. In the 1980s, NASA engineers applied this principle to riblet-coated film on boat hulls, and later, swimsuit manufacturers created "sharkskin" suits. These suits were so effective at reducing drag that they were controversial in the 2008 Beijing Olympics, where swimmers shattered world records, leading to a ban on certain high-tech fabrics.

However, speed is not the only advantage denticles provide. In the ocean, any surface left stationary is quickly colonized by marine life—barnacles, algae, and bacteria—in a process known as biofouling. Large, slow-moving marine mammals like whales often host heavy colonies of barnacles. Sharks, however, remain remarkably clean.

Scientists discovered that the denticle pattern creates an inhospitable terrain for microscopic invaders. The ridges are spaced so precisely that bacteria cannot gain a foothold. The surface area available for attachment is minimized, and the physical stress on the bacterial cell walls prevents them from colonizing. This is a structural defense, not a chemical one. The shark does not secrete antibiotics or toxins; its skin simply makes it impossible for the bacteria to land.

This discovery has profound implications for human health. In hospitals, the battle against "superbugs"—bacteria resistant to antibiotics—is a constant crisis. Traditional cleaning relies on harsh chemicals, which bacteria can eventually evolve to resist. A biotechnology company, inspired by the shark, has developed a microscopic surface texture called Sharklet. When applied to medical devices, catheters, and hospital surfaces, this pattern inhibits bacterial growth by up to 94 percent without using a single drop of disinfectant.

The shark’s skin is a paradox: it is rough to go fast, and it is structured to stay clean. It challenges the human tendency to solve problems with brute force—more fuel for speed, stronger poisons for cleaning. Nature, by contrast, solves problems with geometry. As we face the challenges of the 21st century, from energy efficiency to antibiotic resistance, the solutions may already be swimming in the oceans, waiting for us to look close enough to see them.

PASSAGE IV

NATURAL SCIENCE: This passage is adapted from the article The Shark’s Secret Armor.

For billions of years, nature has been conducting the ultimate research and development experiment. Through the ruthless process of natural selection, life has engineered solutions to problems that human designers still struggle to solve. This concept is the foundation of biomimicry, a discipline that seeks to emulate nature’s time-tested patterns and strategies. One of the most fruitful subjects of this study is a creature often feared but rarely understood as an engineering marvel: the shark.

To the naked eye, a shark looks sleek and smooth. However, if you were to run your hand along a shark's flank—from tail to head—it would feel like rough sandpaper. This roughness is caused by millions of microscopic scales called dermal denticles (literally "skin teeth"). Unlike the flat, overlapping scales of a goldfish, denticles are tiny, tooth-like structures made of dentin and enamel, featuring raised ridges aligned with the flow of water.

For decades, marine biologists were puzzled by these structures. Logic suggested that a perfectly smooth surface would create the least amount of friction, allowing the shark to slice through the water efficiently. Yet, the shark is one of the ocean's fastest predators. The secret lies in the physics of fluid dynamics. As a shark swims, water flows over its skin. On a smooth surface, this flow creates chaotic eddies and swirls known as turbulence, which suck the swimmer backward and increase drag.

The denticles disrupt this process. The microscopic ridges channel the water, organizing the flow into linear streams. This prevents the formation of turbulent eddies close to the skin, effectively reducing drag by up to 10 percent. In the 1980s, NASA engineers applied this principle to riblet-coated film on boat hulls, and later, swimsuit manufacturers created "sharkskin" suits. These suits were so effective at reducing drag that they were controversial in the 2008 Beijing Olympics, where swimmers shattered world records, leading to a ban on certain high-tech fabrics.

However, speed is not the only advantage denticles provide. In the ocean, any surface left stationary is quickly colonized by marine life—barnacles, algae, and bacteria—in a process known as biofouling. Large, slow-moving marine mammals like whales often host heavy colonies of barnacles. Sharks, however, remain remarkably clean.

Scientists discovered that the denticle pattern creates an inhospitable terrain for microscopic invaders. The ridges are spaced so precisely that bacteria cannot gain a foothold. The surface area available for attachment is minimized, and the physical stress on the bacterial cell walls prevents them from colonizing. This is a structural defense, not a chemical one. The shark does not secrete antibiotics or toxins; its skin simply makes it impossible for the bacteria to land.

This discovery has profound implications for human health. In hospitals, the battle against "superbugs"—bacteria resistant to antibiotics—is a constant crisis. Traditional cleaning relies on harsh chemicals, which bacteria can eventually evolve to resist. A biotechnology company, inspired by the shark, has developed a microscopic surface texture called Sharklet. When applied to medical devices, catheters, and hospital surfaces, this pattern inhibits bacterial growth by up to 94 percent without using a single drop of disinfectant.

The shark’s skin is a paradox: it is rough to go fast, and it is structured to stay clean. It challenges the human tendency to solve problems with brute force—more fuel for speed, stronger poisons for cleaning. Nature, by contrast, solves problems with geometry. As we face the challenges of the 21st century, from energy efficiency to antibiotic resistance, the solutions may already be swimming in the oceans, waiting for us to look close enough to see them.