托福閱讀背景材料匯總
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托福閱讀背景材料:距離土星第七遠(yuǎn)的土星衛(wèi)星
Excitement Builds for the Possibility of Life on Enceladus
Scientists tackle the question of how to search for life on Saturn’s sixth-largest moon
By Annie Sneed on June 28, 2016
Saturn's icy moon Enceladus is thought to host a liquid ocean beneath its frozen surface that could be hospitable to life. Credit: NASA
Saturn’s frozen moon Enceladus is a tantalizing world—many scientists are increasingly convinced it may be the best place in our solar system to search for life. NASA’s Cassini spacecraft, currently orbiting Saturn, has made intriguing observations of icy jets spewing from a suspected underground liquid ocean on the mysterious world that might be hospitable to alien life.
Cassini’s tour is due to wind down in 2017, and scientists badly want to send a dedicated mission to Enceladus to look for signs of life. In fact, some have already started seriously thinking about exactly how they might do this—including planetary scientist Carolyn Porco, who is the imaging team leader for Cassini. Earlier this month, she gathered a group of researchers including oceanographers, organic chemists and astrobiologists at the University of California, Berkeley, to strategize how to search for extraterrestrials on Enceladus—which, according to Porco, “is a total bitch of a problem to solve.”
Although Enceladus is small in size and shrouded in a thick shell of ice, it appears to be a habitable world: It has a source of energy from friction created by its orbit around Saturn, organic compounds that are building blocks for life and a liquid water ocean underneath all that ice. But just because Enceladus may be hospitable to life does not mean life exists there; it will take much more work to definitively prove it. At the Berkeley meeting, scientists laid out the data Cassini has collected for Enceladus—they discussed analyses of its geysers, measurements of its ice shell, ideas on what its ocean chemistry might be like, and more. Yet even with all the newest data and models scientists have, they are not even close to detecting organisms on Enceladus—hence the need for a space mission.
Finding life there would be a profound revelation that we are not alone in the cosmos. Furthermore, the discovery of organisms—or the lack thereof—could answer the subtler mystery of how life started on Earth. Researchers at the meeting presented two major opposing theories about how life here originated (in the ocean versus on land), and the group discussed how exploring Enceladus would inform this debate. “It would be a test of one of the ideas about the origin of life,” Porco says—specifically, the proposition that Earth’s species sprang in the sea. For example, if organisms exist in Enceladus’s ocean and presumably arose there, it would support the theory that life began on Earth in hydrothermal vents (hot, nutrient-rich, deep-sea vents on the ocean floor) rather than in patches of water on land.
托福閱讀背景材料：甲烷水合物
Gas Hydrate
Gas Hydrate又稱甲烷水合物(Methane Hydrate)系水分子與甲烷于低溫高壓(0℃，26大氣壓或10℃，76大氣壓下)形成類似冰狀物質(zhì)，在常溫常壓下即分解成水與甲烷，Gas Hydrate在水深數(shù)百公尺的大陸邊緣地區(qū)存在如太平洋海域之大陸邊緣，大西洋的大陸斜坡，南極大陸周邊海域，Kvenvoden 1996年估計(jì)Gas Hydrate總儲(chǔ)量為1 x 1016(m)3，如能夠開采將成為重要之能源，因?yàn)榧淄闉闇厥倚?yīng)氣體，故其與全球氣候之變化將為海突候之變化將為海洋鉆探研究之重點(diǎn)。Gas Hydrate之存在與海底之穩(wěn)定性亦有密切之關(guān)系。
什么是甲烷水合物
「可燃燒的冰塊」，這是許多人在介紹甲烷水合物時(shí)很喜歡引用的標(biāo)題。在實(shí)驗(yàn)中人工合成的甲烷水合物就像一塊不透明的冰塊般純白、潔凈，在室溫下一點(diǎn)火，它就自我燃燒起來(lái)(圖一)。嚴(yán)格說(shuō)來(lái)，甲烷水合物指的是甲烷氣體分子在高壓及低溫的狀態(tài)下，被呈籠狀晶結(jié)架構(gòu)的水分子所包合，而形成一種類似冰晶的化合物。這種氣體分子和水分子的結(jié)合并不依靠化學(xué)的鍵結(jié)，純粹是氣體分子被包裹在水分子的籠狀架構(gòu)空隙中，與一般化學(xué)反應(yīng)所產(chǎn)生的水合物性質(zhì)并不相同。有的學(xué)者因而認(rèn)為應(yīng)稱其為「甲烷氣水包合物」(methane clathrate)，以別于一般以化學(xué)鍵方式結(jié)合的水合物。另外，大自然中與水分子結(jié)成籠狀包合物的氣體分子并不限于甲烷，乙烷、丙烷等烷氫類氣體，甚至二氧化碳、氮?dú)獾瘸Ｒ姎怏w，在適當(dāng)?shù)母邏旱蜏貤l件下，均會(huì)與水分子結(jié)合成氣水包合物。事實(shí)上，這類氣水包合物的英文名稱為「gas hydrate」，直譯應(yīng)為「天然氣水合物」或「瓦斯水合物」。由于自然界中的天然氣水合物其氣體成份以甲烷為主(超過(guò)90%)，因此一般人常把氣水包合物稱為「甲烷水合物」。本文隨俗，用「甲烷水合物」代表學(xué)理上較嚴(yán)謹(jǐn)?shù)摹柑烊粴鈿馑衔铩挂辉~。
托福閱讀背景材料：納米材料及其應(yīng)用
About 納米材料及其應(yīng)用
納米技術(shù)在生物工程上的應(yīng)用
眾所周知，分子是保持物質(zhì)化學(xué)性質(zhì)不變的最小單位。生物分子是很好的信息處理材料，每一個(gè)生物大分子本身就是一個(gè)微型處理器，分子在運(yùn)動(dòng)過(guò)程中以可預(yù)測(cè)方式進(jìn)行狀態(tài)變化，其原理類似于計(jì)算機(jī)的邏輯開關(guān)，利用該特性并結(jié)合納米技術(shù)，可以此來(lái)設(shè)計(jì)量子計(jì)算機(jī)。美國(guó)南加州大學(xué)的Adelman博士等應(yīng)用基于DNA分子計(jì)算技術(shù)的生物實(shí)驗(yàn)方法，有效地解決了目前計(jì)算機(jī)無(wú)法解決的問(wèn)題—“哈密頓路徑問(wèn)題”，使人們對(duì)生物材料的信息處理功能和生物分子的計(jì)算技術(shù)有了進(jìn)一步的認(rèn)識(shí)。
雖然分子計(jì)算機(jī)目前只是處于理想階段，但科學(xué)家已經(jīng)考慮應(yīng)用幾種生物分子制造計(jì)算機(jī)的組件，其中細(xì)菌視紫紅質(zhì)最具前景。該生物材料具有特異的熱、光、化學(xué)物理特性和很好的穩(wěn)定性，并且，其奇特的光學(xué)循環(huán)特性可用于儲(chǔ)存信息，從而起到代替當(dāng)今計(jì)算機(jī)信息處理和信息存儲(chǔ)的作用。在整個(gè)光循環(huán)過(guò)程中，細(xì)菌視紫紅質(zhì)經(jīng)歷幾種不同的中間體過(guò)程，伴隨相應(yīng)的物質(zhì)結(jié)構(gòu)變化。Birge等研究了細(xì)菌視紫紅質(zhì)分子潛在的并行處理機(jī)制和用作三維存儲(chǔ)器的潛能。通過(guò)調(diào)諧激光束，將信息并行地寫入細(xì)菌視紫紅質(zhì)立方體，并從立方體中讀取信息，并且細(xì)菌視紫紅質(zhì)的三維存儲(chǔ)器可提供比二維光學(xué)存儲(chǔ)器大得多的存儲(chǔ)空間。
到目前為止，還沒(méi)有出現(xiàn)商品化的分子計(jì)算機(jī)組件。科學(xué)家們認(rèn)為：要想提高集成度，制造微型計(jì)算機(jī)，關(guān)鍵在于尋找具有開關(guān)功能的微型器件。美國(guó)錫拉丘茲大學(xué)已經(jīng)利用細(xì)菌視紫紅質(zhì)蛋白質(zhì)制作出了光導(dǎo)“與”門，利用發(fā)光門制成蛋白質(zhì)存儲(chǔ)器。此外，他們還利用細(xì)菌視紫紅質(zhì)蛋白質(zhì)研制模擬人腦聯(lián)想能力的中心網(wǎng)絡(luò)和聯(lián)想式存儲(chǔ)裝置。
納米計(jì)算機(jī)的問(wèn)世，將會(huì)使當(dāng)今的信息時(shí)代發(fā)生質(zhì)的飛躍。它將突破傳統(tǒng)極限，使單位體積物質(zhì)的儲(chǔ)存和信息處理的能力提高上百萬(wàn)倍，從而實(shí)現(xiàn)電子學(xué)上的又一次革命。
有關(guān)納米技術(shù)
華人科學(xué)家：美國(guó)納米技術(shù)應(yīng)用研究四大熱點(diǎn)
正在美國(guó)從事納米技術(shù)研究的華人青年科學(xué)家崔屹博士17日接受新華社記者采訪時(shí)表示，美國(guó)納米技術(shù)的應(yīng)用研究目前正在半導(dǎo)體芯片、癌癥診斷、光學(xué)新材料和生物分子追蹤等四大熱點(diǎn)領(lǐng)域快速發(fā)展，其中在芯片和癌癥診斷領(lǐng)域的應(yīng)用可望在10年內(nèi)出現(xiàn)劃時(shí)代的突破。
崔屹說(shuō)，在癌癥研究領(lǐng)域，利用納米技術(shù)制成的傳感器可望使各種癌癥的早期診斷成為現(xiàn)實(shí)。目前，崔屹和他的同事已經(jīng)在實(shí)驗(yàn)室環(huán)境下實(shí)現(xiàn)了對(duì)前列腺癌、直腸癌等多種癌癥的早期診斷。納米傳感器靈敏度很高，在進(jìn)行血液檢測(cè)時(shí)，當(dāng)傳感器中預(yù)置的某種癌細(xì)胞抗體遇到相應(yīng)的抗原時(shí)，傳感器中的電流會(huì)發(fā)生變化，通過(guò)這種電流變化可以判斷血液中癌細(xì)胞的種類和濃度。這一研究成果可望于近期發(fā)表在美國(guó)《科學(xué)》雜志上。崔屹指出，目前越來(lái)越多的風(fēng)險(xiǎn)投資正在涌入這一領(lǐng)域，但這一技術(shù)在實(shí)用中還有一些技術(shù)難題需要解決。他估計(jì)，今后可能會(huì)有多種納米傳感器集成在一起被置入人體，以用來(lái)早期檢測(cè)各種疾病。
在半導(dǎo)體芯片領(lǐng)域，如何讓芯片體積更小、速度更快是科學(xué)界一直研究的課題。目前用于芯片制造的光刻技術(shù)已經(jīng)接近于發(fā)展極限，要想把更多的晶體管集成到一塊芯片上已經(jīng)越來(lái)越難。目前，美國(guó)納米技術(shù)專家們?cè)噲D把納米級(jí)的半導(dǎo)體材料做成晶體管，從而可以讓一塊芯片上容納更多的晶體管。這種芯片的運(yùn)算速度可望比傳統(tǒng)的硅芯片提高上千倍。這一研究方向在2001年取得基礎(chǔ)性研究突破后，目前在應(yīng)用研究中越來(lái)越熱。據(jù)崔屹估計(jì)，這一技術(shù)可望在10年后達(dá)到實(shí)用化。
此外，納米技術(shù)在光學(xué)材料和生物分子追蹤兩個(gè)領(lǐng)域的應(yīng)用也是研究熱門。在光學(xué)材料研究領(lǐng)域，科學(xué)家們?cè)噲D改變某些半導(dǎo)體材料的分子結(jié)構(gòu)，用來(lái)生產(chǎn)特定的光學(xué)器件。比如，一些科學(xué)家試圖讓某種半導(dǎo)體材料內(nèi)部具有納米級(jí)的線狀結(jié)構(gòu)，這種材料用于顯示器制造領(lǐng)域可以大大提高顯示器的清晰度和顏色逼真度。而在生物分子追蹤領(lǐng)域，科學(xué)家把某種納米顆?！罢场痹谏锓肿由?，然后利用納米顆粒的發(fā)光特性研究生物分子的行蹤。這對(duì)研究艾滋病病毒等在人體內(nèi)的活動(dòng)過(guò)程十分有益。
崔屹說(shuō)，美國(guó)在納米應(yīng)用研究領(lǐng)域中享有資金和人才優(yōu)勢(shì)，一直走在世界前列，但距離納米技術(shù)實(shí)用化仍有一段路要走。與美國(guó)相比，其他國(guó)家則主要處于納米技術(shù)的基礎(chǔ)研究階段。
現(xiàn)年27歲的崔屹畢業(yè)于中國(guó)科技大學(xué)，后在哈佛大學(xué)獲納米應(yīng)用專業(yè)博士，目前在加州大學(xué)伯克利分校從事研究工作。過(guò)去幾年，崔屹在《自然》和《科學(xué)》等權(quán)威雜志上發(fā)表多篇研究論文，同時(shí)還是2003年美國(guó)“米勒”杰出青年科學(xué)家獎(jiǎng)和2001年美國(guó)材料研究學(xué)會(huì)金獎(jiǎng)得主。
托福閱讀背景材料：p53的抗癌作用
What is p53 ?
After the identification of the p53 protein and the subsequent cloning of p53 genes from several species, early observations suggested that p53 may function as an oncogene, because overexpression of p53 appeared to cause oncogenic transformation of cells. In the late 1980s, however, several critical discoveries defined the normal function of p53 to be anti-oncogenic. Wild-type p53 genes, when introduced into cells, were found to be growth suppressive. The screening of DNA from colon cancer patients revealed that p53 mutations occur with unusually high frequency in tumor tissue, an observation that was extended to most of the other major forms of human cancer. Indeed, members of Li-Fraumeni cancer-prone families were shown to carry germ-line p53 mutations. The importance of these observations was underscored by the finding that mice that are homozygous null for p53, although developmentally competent, are highly predisposed to tumors.
The functional character of the p53 protein was determined by experiments showing that p53 contains a strong transcriptional activation domain within its amino terminus and that it is a tetrameric, sequence-specific DNA-biding protein with a defined cognate binding site containing two copies of the 10-mer (5'-RRRCA/TT/AGYYY-3'). Although the p53 protein acts as a transcriptional activator of genes containing p53-binding sites, it is also capable of strongly inhibiting transcription from many genes lacking p53-binding sites. Several oncogenic DNA viruses express viral gene products that associate with and inhibit the trans-activation function of p53, notably SV40 large T antigen, the adenovirus E1B 55-kD protein, and the E6 protein of oncogenic forms of human papillomavirus (HPV E6). In cells, p53 can associate with a 90-kD protein, identified as the product of the mdm-2 oncogene, which is amplified in some types of tumors. When bound to mdm-2, p53 can no longer function as an activator of transcription.
p53 plays multiple roles in cells. Expression of high levels of wild-type (but not mutant) p53 has two outcomes: cell cycle arrest or apoptosis. The observation that DNA-damaging agents induce levels of p53 in cells led to the definition of p53 as a checkpoint factor, akin, perhaps, to the product of the fad9 gene in yeast. While dispensable for viability, in response to genotoxic stress, p53 acts as an "emergency brake" inducing either arrest or apoptosis, protecting the genome from accumulating excess mutations. Consistent with this notion, cells lacking p53 were shown to be genetically unstable and thus more prone to tumors.
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