在备考的过程中，考生一般都会拿TPO来进行托福考试的练习。因为其涵盖了大量托福考试的题型，能够帮助考生快速了解各类题型，从而更有针对性攻克自己的短板。今天小编为大家整理了托福阅读Elements of Life原文及题目，一起来了解一下吧!
Elements of Life
The creation of life requires a set of chemical elements for making the components of cells. Life on Earth uses about 25 of the 92 naturally occurring chemical elements, although just 4 of these 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 presence of most or all of the elements used by life.
Interestingly, this requirement can probably be met by almost any world. Scientists have determined that all chemical elements in the universe besides hydrogen and helium (and atrace amount of lithium) were produced by stars.These are known as heavy elements because they are heavier than hydrogen and helium. Although all of these heavy elements are quite rare compared to hydrogen and helium, they are found just about everywhere.
Heavy elements are continually being manufactured by stars and released into space by stellar deaths, so their amount compared to hydrogen and helium gradually rises 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. Insome 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 used by life.Moreover, when planetesimals—small, solidobjects formed in the early solar system that may accumulate to become planets—condense within a forming star system, they are inevitably made from heavy elements because the more commonhydrogen and helium remain gaseous.Thus, planetesimals everywhere should contain the elements needed for life, which means that objects built from planetesimals—planets, moons, asteroids, and comets-also contain these elements. The nature of solar-system formation explains why Earth contains all the elements needed for life, and it is why we expect these elements to be present on other worlds throughout our solar system, galaxy, and universe.
Note that this argument does not change, even if we allow for life very different from life on Earth. Life on Earth is carbon based, and most biologists believe that life elsewhere is likely to be carbonbased as well. However, we cannot absolutely rule out the possibility of life with another chemical basis, such as silicon or nitrogen. The set of elements (or their relative proportions) used by life based on some other element might be somewhat different from that used by carbon-based life on Earth. But the elements are still products of stars and would still be present 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 universe.
A somewhat stricter requirement is the presence of these elements in molecules that can be used as ready-made building blocks for life, just as early Earth probably had an organic soup of amino acids and other complex molecules. Earth's organicmolecules likely came from some combination of three sources: chemical reactions in the atmosphere, chemical reactions near deep-sea vents in the oceans, and molecules carried to Earth by asteroids and comets. The first two sources can occur only on worlds with atmospheres or oceans, respectively. But the third source should have brought similar molecules to nearly all worlds in our solar system.
Studies of meteorites and comets suggest that organic molecules are widespread among both asteroids and comets. Because each body in the solar system was repeatedly struck by asteroids and comets during the period known as the heavy bombardment (about 4 billion years ago), each body should have received at least some organicmolecules. However, these molecules tend to be destroyed by solar radiation on surfaces unprotected by atmospheres. Moreover, while these molecules might stay intact beneath the surface (as they evidently do on asteroids and comets), they probably cannot react with each other unless some kind of liquid or gas is available to move them about. Thus, if we limit our search to worlds on which organic molecules are likely to be involved inchemical reactions, we can probably rule out any world that lacks both an atmosphere and a surface or subsurface liquid medium, such as water.
考试题目：
1.The word "components" in the passage is closest in meaning to
A. parts B. bodies C. combinations D. characteristics
2.According to paragraphs 1 and 2, living cells contain which of the following?
A. All chemical elements in the universe except lithium B. About 25 different elements C. About 96 percent of all known elements D. Ninety-two naturally occurring elements

3.Why does the author provide the information that "Heavy elements are continually being manufactured by stars and released into space by stellar deaths"?
A. To explain how it is that the elements required for life can be found everywhere. B. To provide evidence that our solar system is relatively young. C. To argue that some solar systems are more likely to support life than others. D. To explain why heavy elements have greater mass than hydrogen and helium.
4.Paragraph 3 suggests that which of the following may be a difference between very old star systems and newer star systems?
A. Older star systems are likely to have fewer planets, moons, asteroids, and comets than newer star systems. B. Newer star systems probably contain more hydrogen and helium than older star systems. C. Newer star systems probably contain more heavy elements than older star systems. D. The process of solar-system formation may have been fundamentally different in older star systems than in newer star systems.
5.The word "Nevertheless" in the passage is closest in meaning to
A. additionally B. however C. in particular D. on the contrary
6.The word "inevitably" in the passage is closest in meaning to
A. typically B. unsurprisingly C. necessarily D. naturally
7.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
A. Planetesimals may remain in star systems when hydrogen and helium combine with less common heavier elements. B. Planetesimals are composed of heavy elements because hydrogen and helium stay in the form of gases. C. Planetesimals are small, solid objects that condense within a forming star system and may become planets. D. When planetesimals accumulate to form planets, they inevitably contain gaseous as well as heavy elements.
8.According to paragraph 4, which of the following is true about possible life on other planets?
A. It cannot be based on silicon or nitrogen. B. It could not survive on Earth. C. It probably would not be made of elements produced by stars. D. It is likely to have carbon as its chemical basis.
9.According to paragraph 5, all of the following are true of the organic molecules on early Earth EXCEPT:
A. Some of them were probably brought to Earth by asteroids or comets. B. Some of them probably formed in the atmosphere and oceans. C. They were probably significantly different from the organic molecules present on other planets in the solar system. D. They included complex molecules.
10.The phrase “widespread among” is closest in meaning to
A. frequently present in B. widely separated in C. a significant part of D. found throughout the bodies of