Elements of Life
The creation of life requires a t of chemical elements for making the components of cells. Life on Earth us about 25 of the 92 naturally occurring chemical elements, although just 4 of the elements-oxygen, carbon, hydrogen, and nitrogen-make up about 96 percent of the mass of living organisms. Thus, a first requirement for life might be the prence of most or all of the elements ud by life.
The word “components” in the passage is clost in meaning to
∙ parts
∙ bodies
∙ combinations
∙ characteristics
Interestingly, this requirement can probably be met by almost any world. Scientists have determined that all chemical elements in the univer besides hydrogen and helium (and a trace amount of lithium) were produced by stars. The are known as heavy elements becau they are heavier than hydrogen and helium. Although all of the heavy elements are quite rare compared to hydrogen and helium, they are found just about everywhere.
According to paragraphs 1 and 2, living cells contain which of the following?
∙ All chemical elements in the univer except lithium
∙ About 25 different elements
∙ About 96 percent of all known elements
∙ Ninety-two naturally occurring elements
Heavy elements are continually being manufactured by stars and relead into space by stellar deaths , so their amount compared to hydrogen and helium gradually ris with time. Heavy elements make up about 2 percent of the chemical content (by mass) of our solar system; the other 98 percent is hydrogen and helium. In some very old star systems, which formed before many heavy elements were produced, the heavy-element share may be less than 0.1 percent. Nevertheless , every star system studied has at least some amount of all the elements ud by life. Moreover, when planetesimals-small, solid objects formed in the early solar system that may accumulate to become planets-conden within a forming star system, they are inevitably made from heavy elements becau the more common hydrogen and helium remain gaous. Thus, planetesimals everywhere should contain the elements needed for life, which means that objects built from planetesimals-planets, moons, asteroids, and comets-also contain the elements. The nature of solar-system formation explains why Earth contains all the elements needed for life, and it is why we expect the elements to be prent on other worlds throughout our solar system, galaxy, and univer.
Why does the author provide the information that _ stellar deaths _?
∙ To explain how it is that the elements required for life can be found everywhere
∙ To provide evidence that our solar system is relatively young
∙ To argue that some solar systems are more likely to support life than others
∙ To explain why heavy elements have greater mass than hydrogen and helium
∙
Paragraph 3 suggests that which of the following may be a difference between very old star systems and newer star systems?
∙ Older star systems are likely to have fewer planets, moons, asteroids, and comets than newer star systems.
∙ Newer star systems probably contain more hydrogen and helium than older star systems.
∙ Newer star systems probably contain more heavy elements than older star systems.
∙ The process of solar-system formation may have been fundamentally different in older star systems than in newer star systems.
∙
The word “Nevertheless” in the passage is clost in meaning to
∙ additionally
∙ however
∙ in particular
∙ on the contrary
∙
The word “inevitably” in the passage is clost in meaning to
∙ typically
∙ unsurprisingly
∙ necessarily
∙ naturally
Which of the ntences below best express the esntial information in the highlighted ntence in the passage? Incorrect choices change the meaning in important ways or leave out esntial information.
∙ Planetesimals may remain in star systems when hydrogen and helium combine with less common heavier elements.
∙ Planetesimals are compod of heavy elements becau hydrogen and helium stay in the form of gas.
∙ Planetesimals are small, solid objects that conden within a forming star system and m
ay become planets.
∙ When planetesimals accumulate to form planets, they inevitably contain gaous as well as heavy elements.
Note that this argument does not change, even if we allow for life very different from life on Earth. Life on Earth is carbon bad, and most biologists believe that life elwhere is likely to be carbon bad as well. However, we cannot absolutely rule out the possibility of life with another chemical basis, such as silicon or nitrogen. The t of elements (or their relative proportions) ud by life bad on some other element might be somewhat different from that ud by carbon-bad life on Earth. But the elements are still products of stars and would still be prent in planetesimals everywhere. No matter what kinds of life we are looking for, we are likely to find the necessary elements on almost every planet, moon, asteroid, and comet in the univer.
According to paragraph 4, which of the following is true about possible life on other planet
s?
