托福阅读真题Official 54 Passage 3（四）

托福阅读真题Official 54 Passage 3（四）

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 a trace 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 stellardeaths,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.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 used by life.Moreover,when planetesimals—small,solid objects 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 common hydrogen 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 carbon based 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 organic molecules 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 organic molecules.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 in chemical reactions,we can probably rule out any world that lacks both an atmosphere and a surface or subsurface liquid medium,such as water.

Question 7 of 14

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.

正确答案：B

题目详解

题型分类：简化句子题

原文分析：简化题考察找主干和逻辑的能力，原句主干为“并且，当星子在一个正在形成的恒星系统中凝聚时，它们不可避免地由重元素构成，因为更常见的氢和氦仍然是气态的”。重点信息在主句部分，主句前后有因果关系。

选项分析：

B选项满足前后的因果关系，are composed of heavy elements对应原句主干made from heavy elements，hydrogen and helium stay in the form of gases对应原句主干the more common hydrogen and helium remain gaseous。B选项正确。

A选项，主句和从句反了，并且hydrogen and helium combine with less common heavier elements无中生有。

C选项，缺少重要的主句信息。

D选项，inevitably contain gaseous与原句inevitably made from heavy elements矛盾。

Question 8 of 14

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.

正确答案：D

题目详解

题型分类：事实信息题

原文定位：根据选项定位。

选项分析：

D选项出自第一句life elsewhere is likely to be carbon based。

A选项与第二句we cannot absolutely rule out the possibility of life with another chemical basis,such as silicon or nitrogen矛盾。

B选项survive on Earth无中生有。

C选项与倒数第二句the elements are still products of stars矛盾。