about "seeeeee" 5/28jj 尋找資料

city

不知對不對   僅提供自己找的東西分享和參考 但尚未和"seeeeee"確認  

about"用放射性碳鉴别古物的年代。这个题只要有些那方面的常识就够了"

Radio-carbon dating is a method of obtaining age estimates on organic materials. It has been used to date samples as old as 50,000 years. The method was developed immediately following World War II by Willard F. Libby and coworkers, and has provided age determinations in archaeology, geology, geophysics and other branches of science. Radiocarbon determinations can be obtained on wood; charcoal; marine and fresh-water shell; bone and antler; peat and organic-bearing sediments, carbonate deposits such as tufa, caliche, and marl; and dissolved carbon dioxide and carbonates in ocean, lake and ground-water sources.

Each sample type has specific problems associated with its use for dating purposes, including contamination and special environmental effects. While the impact of radiocarbon dating has been most profound in archaeological research and particularly in prehistoric studies, extremely significant contributions have also been made in hydrology and oceanography. In addition, in the 1950s the testing of thermonuclear weapons injected large amounts of artificial radiocarbon ("Radiocarbon Bomb") into the atmosphere, permitting it to be used as a geochemical tracer.

Radioactive carbon, produced when nitrogen 14 is bombarded by cosmic rays in the atmosphere, drifts down to earth and is absorbed from the air by plants. Animals eat the plants and take C14 into their bodies. Humans in turn take carbon 14 into their bodies by eating both plants and animals. When a living organism dies, it stops absorbing C14 and the C14 that is already in the object begins to disintegrate. Scientists can use this fact to measure how much C14 has disintegrated and how much is left in the object. Carbon 14 decays at a slow but steady rate and reverts to nitrogen 14. The rate at which Carbon decays (Half-life) is known: C14 has a half-life of 5730 years. Basically this means that half of the original amount of C14 in organic matter will have disintegrated 5730 years after the organisms death; half of the remaining C14 will have disintegrated after another 5730 years and so forth. After about 50,000 years, the amount of C14 remaining will be so small that the fossil can't be dated reliably.

To discover how long an organism has been dead (to determine how much C14 is left in the organism and therefore how old it is), we count the number of beta radiations given off per minute per gram of material. Modern C14 emits about 15 beta radiations per minute per gram of material, but C14 that is 5730 years old will only emit half that amount, (the half-life of C14) per minute. So if a sample taken from an organism emits 7.5 radiations per minute in a gram of material, then the organism must be 5730 years old. The accuracy of radiocoarbon dating was tested on objects with dates that were already known through historical records such as parts of the dead sea scrolls and some wood from an Egyptian tomb. Based on the results of the Carbon 14 test the analysis showed that C14 agreed very closely with the historical information.

The natural radiocarbon activity in the geologically recent contemporary "pre-bomb" biosphere was approximately 13.5 disintegration's per minute per gram of carbon. A measurement of the radiocarbon content of an organic sample will provide an accurate determination of the sample's age if it is assumed that (1) the production of radiocarbon by cosmic rays has remained essentially constant long enough to establish a steady state in the 14C/12C ratio in the atmosphere, (2) there has been a complete and rapid mixing of radiocarbon throughout the various carbon reservoirs, (3) the carbon isotope ratio in the sample has not been altered except by radiocarbon decay, and (4) the total amount of carbon in any reservoir has not been altered. In addition, the half-life of radiocarbon must be known with sufficient accuracy, and it must be possible to measure natural levels of radiocarbon to appropriate levels of accuracy and precision.

It has long been recognized that if the radiocarbon atoms could be detected directly, rather than by waiting for their decay, smaller samples could be used for dating and older dates could be measured. A simple hypothetical example to illustrate this point is a sample containing only one atom of radiocarbon. To measure the age (that is, the abundance of radiocarbon), the sample can be placed into a mass spectrometer and that atom counted, or the sample can be placed into a Geiger counter and counted, requiring a wait on the average of 8000 years (the mean life of radiocarbon) for the decay. In practice, neither the atoms nor the decays can be counted with 100% efficiency, but the huge advantage for atom counting remains. Samples are also processed in a lab such as the Beta Analytic Inc.

city

about"讲米国的乡村音乐是怎样被引入市场的，细节忘了，但是题目不难"

American Country Music.
The music of America without doubt has influenced the whole World, and with its diversity it has appealed to every musical taste. If we look at the number of different cultures within America since the arrival of Columbus , it is easy to understand what makes American music special. The early settlers included many types of musical instruments in their belongings, and the blending of these instruments gave the Americans a unique sound. Some of those instruments included Guitar, Mandolin, violin, jaws harp, steel guitar, mouth organ, accordion, harpsichord, piano, to which we add the Dobro, drums, flute, electric guitar, bongo drums saxaphone, a wide range of wind instruments and we have todays country music. A far cry from the lonesome figure of Jimmie Rogers his guitar and a microphone.
----------------------------------------------------------
Country Music
Within Country Music there is a large number of styles, ranging from Hillbilly to Country Rock. There are still a large number of Music Fans who do not realize the inluence of Country Music and how those influences helped mould the pop music of today.
Early Folk Music:
When the early settlers arrived in America they not only brought musical instruments with them, they also brought the folksongs of their Homelands. Many of these songs and tunes were adapted with new lyrics to sing about this new Land and the events of the time. So when the settlers got together for a celebration they would hear the German and Austrian waltzes, The Swiss Polka, The French songs, The Irish dance tunes, The British Music Hall Tunes, The Spanish Flamenco, The Scottish Dance Music, The Welsh Ballads, and the traditional folk songs telling tales and stories. One of the earliest American songwriters was Stephen Foster [ see below ] who penned a lot of classic songs reflecting life during the early 1800's.
In the early history of recording artists, a legend was to appear around 1920, his name was JIMMIE RODGERS [ the singing brakeman ] Now Jimmie had worked on the railways, and also had rode the rails with the Hobos. His musical style was influenced by the Negro Blues and Jazz, but the main influence was Folk music. Jimmie did most of his recordings by himself with just his guitar. Though in a couple of his recordings we see Jimmie joined by another Legend of the time a certain LOUIS [ SATCHMO ] ARMSTRONG. Now and then he was also joined by THE CARTER FAMILY who also were to become as much legendary as Jimmie himself. Jimmie became ' The Father of Country Music' a well deserved title bestowed upon him by his peers in the music field. MOTHER MAYBELLE of the Carter Family became the Mother of Country music. Jimmie collected many of his songs from other sources, but wrote a large majority himself, here is a short listing of some of them.:
Frankie and Johnny---Muleskinner blues---'T' for Texas---Carolina Sunshine Girl---T. B. Blues.
The Latter song was written by Jimmie as a referance to the disease that struck him down, and some of his recordings were done between recovery breaks near the end of his life. The success of the recordings of Jimmie Rodgers and the Carter Family led to the signing of a never ending succession of folk based Artist and a new title was given to their music ; Country and Western. Round about the latter end of the 1960's the word Western was no longer used and it became Country Music.
Between 1930 and 1950 a host of popular Singers emerged but the Music was still not accepted in the cities , even though many popular songs were taken from the pens and recordings of country artist and given to POP ARTIST. Who then proceeded to record some very wishy washy versions of some very great recordings. Here is a short listing of some of those ' borrowed songs ':
Your Cheating Heart,-- Cold Cold Heart,-- Cool Water,-- Jambalaya,--Lovesick Blues,--Setting the Woods on Fire.
To make matters worse the Movie makers refused point blank to have ' Hillbilly's ' provide soundtracks for even their Western Movies. So we got stuck with people like FRANKIE LAINE and the like. Now Frankie Laine was a great singer and I liked a lot of his songs but he was not a country music singer. [ Fact: When Elvis made his first movie one of the first things he was informed was that there was no way his Hillbilly Band was going to appear or play on any of his movies,and they never did. ]
However in contradiction there was the advent of the 'singing cowboy ' movies which were very popular at the time, and three of the most popular of these stars were GENE AUTRY, TEX RITTER, and ROY ROGERS The latter becoming known as the 'King of the Cowboys'.
In 1947 a song called Move it on Over was the first hit recording of a singer called Hank Williams, the tune is better known as 'Rock around the Clock ' recorded by BILL HALEY which is credited as being the song that made ROCK N' ROLL World famous. Hank Williams went on to record many huge hits in the country field and as we can see in the list above many of his songs were recorded by Pop Artist. Hank was one of the first white artist to use the influence of the blues rythyms in country music. Hank died in 1953 but left behind a large number of classic songs, some of which were recorded under the name of ' LUKE THE DRIFTER '.

city

about"听力: 细胞生物学。关于细胞膜的扩散作用，"

Diffusion and Osmosis

Cell membranes are selectively permeable which means they are like the security gate at the school, some people can come and some cannot.  The cell membrane is the barrier that allows some things to enter and leave, and others cannot pass.  

Two ways of crossing the membrane:

1.     passive transport – takes no energy to pass the membrane

2.     active transport – takes energy to pass the membrane

Most common passive transport is called diffusion.

      

Diffusion – the movement of materials across a cell membrane from an area of high concentration to low

Diffusion of water molecules is called osmosis.

DIFFUSION THROUGH A CELL MEMBRANE

Introduction: Substances, such as water, ions, and molecules needed for cellular processes, can enter and leave cells by a passive process such as diffusion. Diffusion is random movement of molecules but has a net direction toward regions of lower concentration in order to reach an equillibrium.

Simple passive diffusion occurs when small molecules pass through the lipid bilayer of a cell membrane. Facilitated diffusion depends on carrier proteins imbedded in the membrane to allow specific substances to pass through, that might not be able to diffuse through the cell membrane.

Importance: The rate of diffusion is affected by properties of the cell, the diffusing molecule, and the surrounding solution. We can use simple equations and graphs to examine how particular molecules and their concentration affect the rate of diffusion. We can also compare simple and facilitated diffusion.

Question: How do rates of simple and facilitated diffusion differ in response to a concentration gradient?

Simple Diffusion

Method: The rate of simple diffusion can be expressed by a modification of Fick's Law for small, nonpolar molecules. The rate of diffusion, dn/dt, is the change in the number of diffusing molecules inside the cell over time.

Since the net movement of diffusing molecules depends on the concentration gradient, the rate of diffusion is directly proportional to the concentration gradient (dC/dx) across the membrane. The concentration gradient, dC/dx, is the difference in molecule concentration inside and outside of the cell across a cell membrane of width dx. This is equivalent to (C_out - C_in)/Dx where C_out and C_in are the substrate concentrations inside and outside the cell, and Dx is the width of the cell membrane. When the concentration outside the cell (C_out) is larger than inside the cell (C_in), the concentration gradient (dC/dx) will be positive, and net movement will be into the cell (positive value of dn/dt).

We can describe the rate of diffusion as directly proportional to the concentration gradient by the following equation:

where A is the membrane area and P is the permeability constant. P is a constant relating the ease of entry of a molecule into the cell depending on the molecule's size and lipid solubility.

Notice that when A and P are constants, this equation simply describes a line where dn/dt is a function of dC/dx. If we graph the rate of diffusion as a function of the concentration gradient, we get a simple linear function.

Interpretation: Notice the rate of diffusion increases as the concentration gradient increases. If the concentration of molecules outside the cell is very high relative to the internal cell concentration, the rate of diffusion will also be high. If the internal and external concentrations are similar (low concentration gradient) the rate of diffusion will be low.

Facilitated Diffusion

Method: Unlike simple diffusion, facilitated diffusion involves a limited number of carrier proteins. At low concentrations, molecules pass through the carrier proteins in a way similar to that of simple diffusion. At high solute concentrations, however, all the proteins are occupied with the diffusing molecules. Increasing the solute concentration further will not change the rate of diffusion. In other words, there is some maximum rate of diffusion (Vmax) when all the carrier pro teins are saturated. Therefore, we can not use a simple linear equation to describe the rate of diffusion. The rate of diffusion will increase with increasing solute concentration, but must asymptotically approach the saturation rate, V_max. How quick ly the carrier proteins become saturated can be described by the variable K, the concentration gradient at which the rate of diffusion is 1/2 Vmax. K and Vmax depend on properties of the diffusing molecule, such as its permeability (P), as well as the surface area (A) of the cell, but for simplification we give the equation as:

We can graph this equation, dn/dt as a function of dC/dx, to see how the rate of diffusion changes with increasing solute concentration outside the cell.

Interpretation: By graphing this equation, we see that at low concentrations of solute, the rate of diffusion into a cell occurs almost linearly, like simple diffusion. Notice that at low solute concentrations, the slope is much steeper than that of simple diffusion. Facilitated diffusion can increase the rate of diffusion of particular molecules at low concentrations. However, the rate of facilitated diffusion levels off with increasing solute concentration. Additional increases in external solute concentration cannot increase the rate of diffusion once carrier proteins are saturated.

Conclusion: Passive diffusion of solute into a cell is linearly related to the concentration of solute outside the cell. Carrier proteins increase the rate of diffusion by allowing more solute to enter the cell. Facilitated diffusion, however, approaches a maximum rate as the carrier proteins become saturated with solute.

Additional Questions:

Simple Diffusion

1. We graphed dn/dt as a function of dC/dx. What is the slope of this line? What do increases or decreases in the slope mean biologically?

2. Now assume the concentration gradient is a constant. How does the rate of diffusion (dn/dt) change with the surface area (A) of the cell and the permeability (P) of the diffusing molecule? Graph dn/dt as a function of A or P and describe the function.

Facilitated diffusion

1. Look at the equation for facilitated diffusion and find the horizontal asymptote. What happens to dn/dt as dC/dx approaches infinity?

2. Try graphing this equation with different values for K. How does this change the concentration at which the carrier proteins are saturated?

3. Compare simple and facilitated diffusion of glucose into erythrocytes by graphing rate of diffusion (micromoles per hour) as a function of external glucose concentration (mmol/cm³). For facilitated diffusion, V_max=500 micromoles per hour and K=1.5 mmol/cm³. For simple diffusion, A x P is 3 cm³/hour.

Membrane Transport Mechanisms

---

It is of seminal importance to the cell that it be able to transport molecules in and out of itself. Imagine that a protein having multiple transmembrane domains is structured so that these domains are arrayed in the plane of the membrane in a circle, thereby forming a cylinder, or, better yet, a barrel when viewed from the outside of the cell, with each of the staves of the barrel being one of the transmembrane domains. The center of the barrel could constitute a hole in the plasma membrane that is isolated from the lipid bilayer by an array of transmembrane domains around it. This hole could be used to transport substances into the cell or out from the cell. In fact, this hole can be a relatively hydrophilic environment if hydrophilic side chains from the membrane-spanning chains surrounding the hole protrude into the hole itself.

In practice, given the structure of known membrane proteins, these holes are only large enough to allow the passage of small molecules through the plasma membrane, almost always simple ions like hydrogen, potassium or sodium. The ions may pass through the hole or orifice by passive diffusion, in which case the protein that allows this transport is called an ion channel. Alternatively, the transmembrane protein may invest energy, usually derived from ATP, to actively force ions from one side of the plasma membrane to the other, in which case it will be an ion pump.

Given the importance of membrane transport, cells utilize a wide range of transport mechanisms. The mechanisms fall into one of three categories: simple diffusion, facilitated diffusion, and active transport.

Diffusion

Simple diffusion means that the molecules can pass directly through the membrane. Diffusion is always down a concentration gradient. This limits the maximum possible concentration of the molecule inside the cell (or outside the cell if it is a waste product). The effectiveness of diffusion is also limited by the diffusion rate of the molecule (see Purves box 5.B). Therefore, though diffusion is an effective enough transport mechanism for some substances (such as H2O), the cell must utilize other mechanisms for many of its transport needs.

Facilitated Diffusion

Facilitated diffusion utilizes membrane protein channels to allow charged molecules (which otherwise could not diffuse across the cell membrane) to freely diffuse in a nd out of the cell. These channels comes into greatest use with small ions like K+, Na+, and Cl-. The speed of facilitated transport is limited by the number of protein channels available, whereas the speed of diffusion is dependent only on the concentration gradient.

Active Transport

Active transport requires the expenditure of energy to transport the molecule from one side of the membrane to the other, but active transport is the only type of transport that can actually take molecules up their concentration gradient as well as down. Similarly to facilitated transport, active transport is limited by the number of protein transporters present.

We are interested in two general categories of active transport, primary and secondary. Primary active transport involves using energy (usually through ATP hydrolysis) at the membrane protein itself to cause a conformational change that results in the transport of the molecule through the protein. The most well-known example of this is the Na+-K+ pump. The Na+-K+ pump is an antiport, it transports K+ into the cell and Na+ out of the cell at the same time, with the expenditure of ATP.

Secondary active transport involves using energy to establish a gradient across the cell membrane, and then utilizing that gradient to transport a molecule of interest up its concentration gradient. An example of this mechanism is as follows: E. coli establishes a proton (H+) gradient across the cell membrane by using energy to pump protons out of the cell. Then those protons are coupled to lactose at the lactose permease transmembrane protein. The lactose permease uses the energy of the proton moving down its concentration gradient to transport lactose into the cell. This coupled transport in the same direction across the cell membrane is known as a symport. E. coli uses similar proton driven symports to transport ribose, arabinose, and several amino acids.

The Na+-glucose secondary transport mechanism

Another secondary active transport system uses the Na+-K+ pump as the first step, generating a strong Na+ gradient across the cell membrane. Then the glucose-Na+ symport protein uses that Na+ gradient to transport glucose into the cell.

This system is used in a novel way in human gut epithelial cells. These cells take in glucose and Na+ from the intestines and transport them through to the blood stream using the concerted actions of Na+-glucose symports, glucose permeases (a glucose facilitated diffusion protein), and Na+-K+ pumps. Note that the epithelial cells are joined together by tight junctions to prevent anything from leaking through from the intestines to the blood stream without first being filtered by the epithelial cells.

city

目前只有這些資料  

希望大家能一起幫忙找資料  至少對考試也不確

不知是否可請"seeeeee"能幫忙看一下大至資料符不符合

就降子   如果資料不合  請大家見諒囉

enjoylife517

谢谢city MM，祝顺利！

sony

狂晕, 这都被你找到了, 牛人~

但光看那么大篇的英文就想逃了...

6月4号考试, 真不知道该如何了...

谢谢楼主的分享, 一定仔细看完, 呵呵~

阿不

楼上的MM, 你终于要考了.我考了2次了,都不满意,这次想再战最后一次.我都没信心了.

能不能告诉我,你后来的复习方法么?

sony

以下是引用阿不在2004-5-29 9:20:00的发言：

楼上的MM, 你终于要考了.我考了2次了,都不满意,这次想再战最后一次.我都没信心了.

能不能告诉我,你后来的复习方法么?

呵呵， 都不好意思讲了，都说是“终于”了， 呵呵， 在CD都快算考试时间最久的家伙了。。。

我的复习在按着ENJOYLIFE517斑竹的建议复习着，但鉴于现在还在打工， 所以时间方面比较紧张， 而且我的单词很差，准备花点时间强攻一下， 呵呵， 一起加油哦！

http://forum.chasedream.com/dispbbs.asp?BoardID=19&ID=42680

看看这里， 也许有帮助哦~

seeeeee

hehe, I admire you so much,~~~ but your materials contains too many details and may result in some unnessessary frightenings. I suggest that you grasp those general points. Common sense is enouogh to answer those questions, as long as you really be relaxed and concentrated.(sorry, but this pc can't input chinese)

seeeeee

but it seems that your material about american country music is not that helpful, I suggest you do not read it, it's a little bit time-consuming

[此贴子已经被作者于2004-5-29 12:20:25编辑过]