首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
医学
Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials t
Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials t
admin
2013-12-06
77
问题
Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials to pass while constraining the movement of other molecules was a major step in the evolution of the cell. Cell membranes are differentially permeable barriers separating the inner cellular environment from the outer cellular(or external)environment.
Water potential is the tendency of water to move from an area of higher concentration to one of lower concentration. Energy exists in two forms: potential and kinetic. Water molecules move according to differences in potential energy between where they are and where they are going. Gravity and pressure are two enabling forces for this movement. These forces also operate in the hydrologic(water)cycle. Remember in the hydrologic cycle that water runs downhill(likewise it falls from the sky, to get into the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle).
Diffusion is the net movement of a substance(liquid or gas)from an area of higher concentration to one of lower concentration. You are on a large(10ft×10ft×10ft)elevator. An obnoxious individual with a lit cigar gets on at the third floor with the cigar still burning. You are also unfortunate enough to be in a very tall building and the person says "Hey we’re both going to the 62nd floor!" Disliking smoke you move to the farthest corner you can. Eventually you are unable to escape the smoke! An example of diffusion in action. Nearer the source the concentration of a given substance increases. You probably experience this in class when someone arrives freshly doused in perfume or cologne, especially the cheap stuff.
Since the molecules of any substance(solid, liquid, or gas)are in motion when that substance is above absolute zero(0 degrees Kelvin or -273 degrees C), energy is available for movement of the molecules from a higher potential state to a lower potential state, just as in the case of the water discussed above. The majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall(or net)movement is thus from high to low concentration. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.
The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell. Electron microscopic examinations of cell membranes have led to the development of the lipid bilayer model(also referred to as the fluid-mosaic model). The most common molecule in the model is the phospholipid, which has a polar(hydrophilic)head and two nonpolar(hydrophobic)tails. These phospholipids are aligned tail to tail so the nonpolar areas form a hydrophobic region between the hydrophilic heads on the inner and outer surfaces of the membrane. This layering is termed a bilayer since an electron microscopic technique known as freeze-fracturing is able to split the bilayer.
Molecules of a system will reach a state of equilibrium______.
选项
A、when no energy is input into the system
B、when the molecules move from a higher to a lower potential state
C、when the substance is above absolute zero
D、when molecules move from a lower to a higher concentration
答案
A
解析
根据第四段,绝对零度以上的物质的分子都处于运动状态;如果分子均匀地分布于一个系统,就不会有能量输入这个系统,因而系统内的分子就会处于一种平衡状态。
转载请注明原文地址:https://kaotiyun.com/show/y5U3777K
本试题收录于:
医学博士外语题库考研分类
0
医学博士外语
考研
相关试题推荐
A、Thesizeofafamily.B、Thesourceofincome.C、Thebasichumanneeds.D、Thedeathofthebreadwinner.C录音讲到寿险数量的计量方法是用一家人的the
Atthesametime,thecommercialvalueofEnglishtuitionasa______hasinitiatedcompetitionformoreeffectivemethodsandmat
Biotechnologyisexpectedtobringimportantadvancesinmedicaldiagnosisandtherapy,insolvingfoodproblems,inenergysavi
Dry-cleaningmachinesthatuseliquidcarbondioxideasasolventwillgoonsaleintheUSnextyear—thankstochemistsinNort
Appliedresearch,undertakentosolvespecificpracticalproblems,hasanimmediateattractivenessbecausetheresultscanbese
AdolfHitlersurvivedanassassinationattemptin1944withthelampofpenicillinmadebytheAllies,amicrobiologistintheU
Untilafewyearsagomostexpertsbelievedthatyoungchildrencouldn’tlie.ThelatedevelopmentalpsychologistJeanPiagetbe
A、Togiveanexampleofabadreactiontopenicillin.B、Toshowhowpenicillinhaschangedovertheyears.C、Toemphasizetheim
A、Visithermoreoften.B、Stoparguinginfrontofotherpeople.C、Callheriftheyneedherhelp.D、Givehertheirnewaddress.
A、Gotobedearlier.B、Gotothegymlessoften.C、Gotothegymlaterintheday.D、Gotothegymwithafriend.B由“butwhosay
随机试题
Ahobbycanbealmostanythingapersonlikestodoinhissparetime.Hobbyists【C1】______pets,buildmodelshipsandperform【C2】
下列关于DQE的描述不正确的是
A、气血运行不畅、迟滞而痛B、气血不足,冲任、胞脉失于濡养,不荣则痛C、瘀血内停,阻滞冲任、子宫,不通则痛D、感染邪毒,邪毒与血搏结,阻滞而痛E、产后血虚,经脉失养产后腹痛气血两虚证的病机是
()是决定信息系统实施成败的关键因素。
拱架种类按材料分为()。
在复利计算时,一定时期内,复利计算的周期越短,频率越高,那么( )。
阅读下列材料:材料一:贞观四年,(唐太宗)诏发卒修洛阳之乾元殿(按:隋炀帝营建的宫殿,毁于战火)。……张玄素上书谏曰:“……臣问阿房成,秦人散;……乾元毕工,隋人解体。且以陛下今时功力何如隋日?承凋残之后,役疮痍之人,费亿万之功,袭百王之弊,以此
网络操作系统的基本任务是:屏蔽本地资源与网络资源的差异性,为用户提供各种基本网络服务功能,完成网络【 】的管理,并提供网络系统的安全性服务。
【S1】【S8】
(1)Afteralongdayattheoffice,manyofusfindourselvestakingoutourstressonfriends,children,orsignificantothe
最新回复
(
0
)