Reading Section ACT Practice: Natural Science- 2 | Reading for ACT PDF Download

Passage 

If you’ve ever been in the central or eastern U.S. around dusk in late summer, you’ve probably witnessed a peculiar biological phenomenon—all around long, languorous insects begin to appear, with abdomens emitting a flickering, yellow-green glow. These insects, commonly known as “fireflies” or “lightning bugs”, produce their glow in a process called bioluminescence. Although the firefly is perhaps the most familiar species that emits its own light, bioluminescence has been discovered in varieties of fungi, fish, coral, squid, and even a bacterium that lives in elephant mucus. There are, in fact, more than thirty known, independent occurrences of bioluminescence in evolution, and while no two species seem to use their light in precisely the same way, they all share a few unique, fundamental processes that give them their special glow. 

Though there is considerable variation among the cell-signaling pathways of different bioluminescent life-forms, all involve an enzyme called luciferase, which catalyzes the reaction of its light-emitting substrate, luciferin, and adenosine triphosphate (ATP), which is an energy-storing compound used widely in both plant and animal cells. Molecular oxygen (O₂) is also peripherally involved in the reaction. This method of light-production is a naturally occurring type of chemiluminescence, where electromagnetic radiation is given off in a chemical reaction by the return of electrons from energetically excited states to their ground state in the form of visible light. 

What’s more, luminescence is fundamentally distinct from the way we create light in incandescent light bulbs. Incandescence uses the super-heating of a filament to generate light by thermal radiation, and a lot of energy is wasted in the process. In contrast, less than 20% of luminescent light is created by thermal radiation, which has earned it the nick-name “cold light”. This detail is of particular importance to bioluminescent species, as it guards against the generation of excess heat, which would cause an unnecessary expenditure of energy, and may interrupt the species’ homeostatic equilibrium. 

Perhaps equally remarkable to a species’ ability to luminesce is its ability to regulate when and for how long light will be emitted. Fireflies, for instance, which use their glow as a signal to attract a mate, emit light only for a brief duration of time, but alternate between “on” and “off” with a fairly high frequency. This is accomplished, biologists believe, by the quick molecular decay of nitric oxide. Near their luminescent organs, or “lanterns”, fireflies possess cells containing the enzyme nitric oxide synthase, which separates nitric oxide gas (NO) from the amino acid arginine. A nerve impulse stimulates the release of NO from these cells, which then passively diffuses into the adjacent lantern cells, where it temporarily blocks mitochondrial respiration. With the lantern cells’ respiration inhibited, its oxygen content begins to climb. This increase in oxygen triggers the light-producing reaction inside peroxisome organelles, which contain the key compounds luciferase and luciferin-ATP. When the nerve impulse stops, NO no longer enters the lantern cells, and the remaining NO is quickly degraded by enzymes. This allows cellular respiration to resume in the mitochondria, returning oxygen to its normal level, and in turn deactivating the peroxisome light-reaction. 

Although the signaling pathway for light production in fireflies is complex enough, the methods by which different species regulate their luminescence is as various as the uses for luminescence that those species employ. As we have seen, fireflies tend to use their light to attract a mate, but females of one large species of firefly have been known to mimic the glow pattern of another, smaller variety, in order to attract males of the smaller species and eat them. Similarly, the deep-sea dwelling anglerfish uses its glowing appendage to lure in unsuspecting prey. Other marine animals use bioluminescence as a defense mechanism. Certain crustaceans, for instance, expel a glowing liquid similar to the way many squid use ink, surprising and disorienting their potential predators. For many bioluminescent life-forms, such as fungi, the purpose of their light-emitting properties remains a mystery. However, it’s said that Benjamin Franklin suggested the use of foxfire (bioluminescent fungi) as a viable light source on an early variety of submarine, as it would consume considerably less oxygen than the combustion reactions of candle or lamplight. 

Despite the great diversity of uses for bioluminescence in the plant, animal and fungi kingdoms, and the wide evolutionary divides between the species that possess such properties, it is an unprecedented parallelism that all should have developed the use of the same fundamental compounds for their light-emitting processes, and one that garners continued fascination, both from the biologists working in their laboratories, and from the rest of us, watching the fireflies flicker in the late summer night.

Q1: What is the purpose of the passage?
(a) Explain a phenomenon
(b) Analyze an approach
(c) Discuss an argument
(d) Investigate a theory

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Q2: What is the function of the quotation marks around the words “fireflies” and “lightning bugs” in lines 6-7?
(a) To describe the light sources, such as fire and light, that bioluminescent creatures utilize
(b) To provide a quotation from a biologist about bioluminescent animals
(c) To analyze the origins of the phenomenon of bioluminescence
(d) To illustrate the common names of bioluminescent insects to the reader

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Q3: According to the passage, which of the following represents (an) environment(s) in which bioluminescent creatures have been located?
(a) Water only
(b) Air only
(c) Water and air only
(d) Land , sea and air

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Q4: We can infer from the passage that the author believes that bioluminescence evolved:
(a) As a result of a common ancestor
(b) Independently due to environmental needs
(c) Similarly due to requirements to combat nighttime darkness
(d) For the purposes of mating

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Q5: According to the third paragraph (lines 31-42), what is different in the outcomes of the processes of bioluminescence vs. incandescence?
(a) 20% more energy is utilized
(b) The use of thermal radiation
(c) Whether light is created
(d) Amount of heat wasted

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Q6: We can infer from the passage that a situation in which human utilization of bioluminescent technology would be most useful would most likely be which of the following?
(a) To minimize oxygen use and minimize heat generation
(c) To minimize oxygen use and maximize heat generation
(d) To maximize oxygen use and maximize heat generation

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Q7: According to the fourth paragraph (lines 43-66), the shift in levels of which of the following substances is most responsible for the chemical reaction that creates light?
(a) Nitric Oxide Gas
(b) ATP
(c) Oxygen
(d) Arginine

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Q8: Based on the information in the passage, which of the following is NOT a biological purpose of bioluminescence?
(a) Mating
(b) Catching food
(c) Protection
(d) Respiration

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Q9: Based on the information in the fourth paragraph (lines 43-66), the process by which nitric oxide goes through molecular decay is most important to the firefly mating process due to which of the following characteristics?
(a) Its speed
(b) Its frequency
(c) Its luminescence
(d) Its energy

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Q10: What may we infer is a reason the author states that “the firefly is perhaps the most familiar species that emits its own light” in lines 8-9?
(a) Its relatively large size
(b) The ease with which it is observed
(c) The number of species categorized as such
(d) Its unique manifestation of glowing light

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