11月GMAT阅读机经：动物冬天冬眠.

　　十一月机经已经都放出来了，相信大家看了之后不难发现很多机经都不过是酒药换新汤罢了，所以大家要把握好这个规律。今天我们就来分享一下GMAT十一月阅读机经，希望对大家备考GMAT考试有所帮助。

　　【本月原始】

　　说是有些动物，蛙类等等，在冬天会让自己冻僵。第二段和第三段都是讲这些动物让自己冻僵的方法和注意的事。第二段提到的核心是antiprotein,第三段是save energy。最后一段说的是将这个“冬眠”应用到人类的什么技术里去。

　　【考古】

　　【主体结构】

　　某些冷血动物(会冬眠的乌龟，青蛙)体内的ice-nucleating(1段)帮助细胞crystalized，另外的anti-freezing帮助减少the cluster of crystalization。(2段)青蛙体内有种XX(忘了名字)帮助reduce energy needed for metabolism。(3段)医学应用。(4段)虽然这篇是比较长第三段，但很容易读懂。

　　【背景阅读】

　　Freeze tolerance The ability to withstand the long-term freezing of body fluids has developed in diverse groups of animals including some frogs and turtles, many types of insects, and a variety of intertidal marine molluscs and barnacles (Storey and Storey, 1989, 1996). Freeze tolerance occurs in several species of woodland frogs that hibernate in the leaf litter of the forest floor including the wood frog (Rana sylvatica) (Figure 3), the gray tree frog (Hyla versicolor), the spring peeper (Pseudacris crucifer) and the chorus frog (Pseudacris triseriata). The Siberian salamander (Salamandrella keyserlingii) and two turtle species, the terrestrial box turtle (Terrapene carolina) and the painted turtle (Chrysemys picta) also survive freezing. Freeze tolerance by painted turtles is limited to the newly hatched juveniles that stay in their underground nests for their first winter of life whereas the adults winter under water. The driving force for freeze tolerance was probably an inability to mount an fective dense against inoculative freezing by environmental ice. For example, the water-permeable skin of frogs is no barrier to ice propagation and although frogs chilled to -2°C may stay supercooled if they are sitting on a dry substrate, they begin to freeze in less than 30 seconds if they touch ice crystals. Since frogs need to hibernate in the humid the leaf litter to keep from desiccating, they have virtually no chance of avoiding freezing if ice penetrates into their microenvironment. Freezing can cause multiple types of damage to unprotected organisms (Figure 4). Ice formation inside of cells scrambles intracellular architecture and is lethal in virtually all instances so even freeze tolerant animals take precautions to limit ice formation to extracellular spaces. Extracellular ice can also do physical damage by squeezing or shearing cells, puncturing membranes or bursting microcapillaries so that upon thawing, the integrity of cells and organs is destroyed. Ice propagating through extracellular spaces such as the abdominal cavity, blood stream, gut lumen and bladder also causes severe dehydration of cells. This is because the formation of ice, which is a crystal of pure water, excludes the solutes that were dissolved in it and raises the concentration of the remaining unfrozen extracellular fluid. This highly concentrated fluid puts an osmotic stress on cells and draws water out of them so that they shrink in volume. If shrinkage exceeds a critical minimum cell volume, irreversible damage is done to the lipid membranes surrounding the cell and the cells are not viable after thawing. Freezing of blood also halts the delivery of oxygen and nutrients to organs which most organisms cannot tolerate for long. Freeze tolerant animals have developed denses against these possible injuries with adaptations that fall into several categories: (1) regulation of ice propagation through body tissues, (2) damage repair to deal with bleeding injuries caused by ice, (3) minimizing cell volume reduction during freezing, (4) membrane and protein stabilization, (5) resistance to oxygen deprivation, and (6) reactivation of vital signs (breathing, heart beat, nerve and muscle activity) after thawing (Storey and Storey, 1996). To control ice formation, freeze tolerant animals use specific nucleators (Figure 4). Instead of lowering their SCP in winter as freeze avoiding animals do, freeze tolerant animals raise their SCP by using nucleators so that freezing occurs begins just below the FP. Some species introduce special ice nucleating proteins into their blood whereas others use contact with environmental ice crystals or the presence of nucleating bacteria on the skin or in the gut to stimulate ice formation. The slow freeze initiated by nucleators allows the greatest possible time for organs to make metabolic adjustments bore blood circulation halts and permits a controlled dehydration of organs that sequesters most of the ice in extraorgan spaces (such as the abdominal cavity). This reduces the chance of internal damage to organs such as by ice expansion within the lumen of capillaries. Some freeze tolerant animals also appear to have AFPs in their body fluids which seems contradictory. However, it appears that the function of AFPs in freeze tolerant systems is to help regulate crystal growth and inhibit recrystallization, the process whereby small crystals regroup over time into larger crystals. In addition, freeze tolerant animals enhance their damage repair mechanisms so that bleeding injuries can be dealt with rapidly upon thawing. In wood frogs, for example, freezing stimulates the production of blood clotting proteins. Controlled dehydration of cells and organs can minimize ice damage but cell volume reduction can only go so far bore cell membranes collapse under compression stress. Generally, freeze tolerant animals can endure the conversion of up to ~65% of their total body water into extracellular ice but the remainder must remain liquid within cells. Water retention in cells is aided by the synthesis of high levels of glycerol or related carbohydrates which provide the same protection to the intracellular milieu of freeze tolerant animals that they do for all of the body water of freeze avoiding animals. Frogs use glucose as their cryoprotectant with levels of this blood sugar rising by 50-100 fold or more whenever body fluids begin to freeze (Storey and Storey, 1996) . Interestingly, frogs show no evidence of the debilitating fects of hyperglycemia that are evident at much lower sugar levels (2-10 fold above normal) in diabetics. Other cryoprotectants are also produced that stabilize the structure of cell membranes so that they can resist compression stress; the sugar, trehalose, and the amino acid, proline, are widely used for this function. They intercalate between the headgroups of membrane phospholipids to stabilize the bilayer structure that is key to biological function and prevent the lipids from collapsing into an amorphous gel. Freeze tolerant animals have also enhanced their ability to cope with oxygen deprivation for there is no breathing and no blood circulation while frozen. Again, high glycogen reserves are used to produce ATP energy via glycolysis with lactate build-up tolerated during the freeze. Freeze tolerant animals also show enhanced antioxidant denses that can minimize damage due to the production of oxygen free radicals when breathing resumes after thawing. The molecular mechanisms that reactivate vital signs during thawing are still largely unexplored. In frogs, a resumption of heart beat is the first detectable vital sign, followed soon thereafter by breathing and later by a return of coordinated muscle movements. Studies of the physiology and biochemistry of natural freezing survival by frogs are revealing numerous secrets that are being applied in the development of improved cryopreservation technology for the freezing storage of mammalian cells, tissues and organs.

　　动物的冬眠是一种奇妙的现象。人们观察了若干种动物冬眠，发现了许多意想不到的现象。

　　在加拿大，有些山鼠冬眠长达半年。冬天一来，它们便掘好地道，钻进穴内，将身体蜷缩一团。它们的呼吸，由逐渐缓慢到几乎停止，脉搏也相应变得极为微弱，体温更直线下降，可以达到5℃。这时，即使用脚踢它，也不会有任何反应，简直像死去一样，但事实上它却是活的。

　　松鼠睡得更死。有人曾把一只冬眠的松鼠从树洞中挖出，它的头好像折断一样，任人怎么摇撼都始终不会张开眼，更不要说走动了。把它摆在桌上，用针也刺不醒。只有用火炉把它烘热，它才悠悠而动，而且还要经过颇长的时间。

　　刺猬冬眠的时候，简直连呼吸也停止了。原来，它的喉头有一块软骨，可将口腔和咽喉隔开，并掩紧气管的入口。生物学家曾把冬眠中的刺猬提来，放人温水中，浸上半小时，才见它苏醒。

　　动物的冬眠真是各具特色，蜗牛是用自身的黏液把壳密封起来。绝大多数的昆虫，在冬季到来时不是“成虫”或“幼虫”，而是以“蛹”或“卵”的形式进行冬眠。熊在冬眠时呼吸正常，有时还到外面溜达几天再回来。雌熊在冬眠中，让雪覆盖着身体。一旦醒来，它身旁就会躺着1一2只天真活泼的小熊，显然这是冬眠时产生的仔。

　　动物冬眠的时间长短不一。西伯利亚东北部的东方旱獭和我国的刺猬，一次冬眠能睡上200多天，而苏联的黑貂每年却只有20天的冬眠。

　　动物的冬眠，完全是一项对付不利环境的保护性行动。引起动物冬眠的主要因素，一是环境温度的降低，二是食物的缺乏。科学家们通过实验证明，动物冬眠会引起甲状腺和肾上腺作用的降低。与此同时，生殖腺却发育正常，冬眠后的动物抗菌抗病能力反而比平时有所增加，显然冬眠对它们是有益的，使它们到翌年春天苏醒以后动作更加灵敏，食欲更加旺盛，而身体内的一切器官更会显出返老还童现象。

　　由此可见，动物在冬眠时期神经系统的肌肉仍然保持充分的活力，而新陈代谢却降低到最低限度。今天医学界所创造的低温麻醉、催眠疗法，便是因此而得到的启发。

　　和我们人类一样，动物中的鸟兽都是温血动物，那么冷血动物昆虫又是怎样熬过漫长的冬季呢?许多冬眠的昆虫会不会冻结呢?

　　昆虫学家进行了长期的观察和研究，终于查明了昆虫越冬的部分奥秘。冬天，为了防止汽车散热器结冰，人们要加入防冻液。昆虫竟然也会采用相似的办法，在严寒的冬季保护自己。

　　在冬天，昆虫要保持活动，不被冻僵是至关重要的。活的组织一旦被冻结，膨胀的冰晶体势必使细胞膜受到破坏，造成致命的创伤。当细胞里液体不足，不能保持维护生命所必需的酶活性时，即使没有完全被冻结，也会造成死亡。那么，昆虫是怎样解决这一难题的呢?它们主要是靠降低体内液体的冰点，从而提高抗寒能力，办法就是产生大量的“防冻液”。

　　昆虫是怎样制造防冻液的呢?天暖之后又怎样将防冻液除掉呢?为什么要除掉防冻液?这些问题直到现在仍找不到答案。

　　值得补充的是，科学家们又发现，蛙类也会自制防冻液。在实验室中，科学家们将许多青蛙冷冻起来，5～7天后，再慢慢地使之解冻，这些青蛙解冻后依然活着。经过认真分析和研究，科学家们发现了一种人们在防冻剂中常用的物质：丙三醇。与昆虫相似的是，到了春天，这些青蛙的液体中再也找不到这一物质了。

　　至今，人们尚未能完全揭开动物冬眠的奥秘。但是科学家们通过不断探索已经认识到，研究动物的冬眠不仅妙趣横生，而且颇有价值。

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