The other branch of chemistry is organicchemistry, the chemistry of the substances which are associated with livingthings. For a time it was believed that the substances which are associated withliving things were so marvelous that they could not be made by hand, frominorganic materials. This is not at all true—they are just the same as thesubstances made in inorganic chemistry, but more complicated arrangements ofatoms are involved. Organic chemistry obviously has a very close relationshipto the biology which supplies its substances, and to industry, and furthermore,much physical chemistry and quantum mechanics can be applied to organic as wellas to inorganic compounds. However, the main problems of organic chemistry arenot in these aspects, but rather in the analysis and synthesis of thesubstances which are formed in biological systems, in living things. This leadsimperceptibly, in steps, toward biochemistry, and then into biology itself, ormolecular biology.
化学的另一分支，是有机化学，即与活着的东西有关联的实质体的化学。我们曾经相信，与活着的东西有关联的实质体，非常奇妙，以至于我们不可能用无机材料来制造它们。这完全不是真的，它们与无机化学中所做成的实质体一样，只是原子的排列，更复杂一些。很明显，有机化学与生物学有着密切的关系，生物学提供了有机化学{中所用到}的实质体，也与工业有关系，更进一步，很多物理化学和量子力学，可被应用于有机化合物，正如它们被应用于无机化合物一样。然而，有机化学的主要问题并不在这些方面，而勿宁说，是在那些实质体的分析和综合中，这些实质体，本来是在生物系统中、即活着的东西中，形成的。这一点，在不知不觉中，一步一步地把我们引向了生物化学，然后，引向了生物本身，或者分子生物学。

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3–3Biology 生物学
Thus we come to the science of biology,which is the study of living things. In the early days of biology, the biologistshad to deal with the purely descriptive problem of finding out whatliving things there were, and so they just had to count such things as the hairsof the limbs of fleas. After these matters were worked out with a great deal ofinterest, the biologists went into the machinery inside the living bodies,first from a gross standpoint, naturally, because it takes some effort to getinto the finer details.
这样，我们就来到了生物学，它研究活着的东西。在生物学的早期，生物学家们，主要是要处理描述性的问题，比如活着的东西都有些什么，所以，他们就会做这样的事情：去数跳蚤腿上的毛有多少根。在兴致勃勃地完成了这些事情之后，生物学家们进入到这些生物体的机械研究，首先，从一个粗糙的观点出发，这很自然，因为要得到进一步的细节，是需要一些努力的。

There was an interesting early relationshipbetween physics and biology in which biology helped physics in the discovery ofthe conservation of energy, which was first demonstrated by Mayer in connectionwith the amount of heat taken in and given out by a living creature.
在早期，物理学与生物学之间，有一个有趣的关系，即生物学曾帮助物理学，找到了能量守恒定律，该定律，首先是由迈尔（Mayer）演证的，即通过与下面现象相联系：一个活的生物所摄入的热量与排出的热量。

The opposite effect is that, from somewherein the brain, a message is sent out along a nerve. What happens at the end ofthe nerve? There the nerve branches out into fine little things, connected to astructure near a muscle, called an endplate. For reasons which are not exactlyunderstood, when the impulse reaches the end of the nerve, little packets of achemical called acetylcholine are shot off (five or ten molecules at a time) andthey affect the muscle fiber and make it contract—how simple! What makes amuscle contract? A muscle is a very large number of fibers close together,containing two different substances, myosin and actomyosin, but the machineryby which the chemical reaction induced by acetylcholine can modify the dimensionsof the muscle is not yet known. Thus the fundamental processes in the musclethat make mechanical motions are not known.
相反的情况就是，从大脑中某处，发出一个消息，沿着神经走。那么，在神经的末端会发生什么呢？在那里，神经的分支，深入到非常精细的事物中，与接近肌肉的一个结构相连，该结构被称为端板{endplate}。由于一些尚未被准确理解的原因，当脉冲到达神经末端时，一种被称为乙酰胆碱的化学小包被射出（每次5到10个分子），它们会影响肌肉纤维，让它收缩--多么简单！那么，什么使肌肉收缩呢？肌肉就是大量的纤维紧紧地聚在一起，包含两种不同的实质体，肌凝蛋白和肌纤凝蛋白；由乙酰胆碱，引起化学反应，由此反应，形成一种机制，该机制可以修改肌肉尺寸，但现在并不了解这个机制。这样，肌肉中使得机械运动得以成立的基础过程，并不知道。

Fig. 3–1.The Krebs cycle. 克雷布斯循环
In the cells of living systems there aremany elaborate chemical reactions, in which one compound is changed intoanother and another. To give some impression of the enormous efforts that havegone into the study of biochemistry, the chart in Fig. 3–1 summarizesour knowledge to date on just one small part of the many series of reactions whichoccur in cells, perhaps a percent or so of it.
在活着的系统的细胞中，有很多精心完成的化学反应，在其中，一个化合物，被转换成另外一个，然后再转换成另一个。生物化学的研究，现在做了大量努力，在这方面，要提供一些印象，见图3-1，细胞中发生了很多系列的反应，图中只是小结了目前我们所知道的知识中的一小部分，或许只有其中的百分之一。

Here we see a whole series of molecules whichchange from one to another in a sequence or cycle of rather small steps. It iscalled the Krebs cycle, the respiratory cycle. Each of the chemicals and each ofthe steps is fairly simple, in terms of what change is made in the molecule,but—and this is a centrally important discovery in biochemistry—these changes arerelatively difficult to accomplish in a laboratory. If we have onesubstance and another very similar substance, the one does not just turn intothe other, because the two forms are usually separated by an energy barrier or“hill.” Consider this analogy: If we wanted to take an object from one place toanother, at the same level but on the other side of a hill, we could push itover the top, but to do so requires the addition of some energy. Thus most chemicalreactions do not occur, because there is what is called an activation energyin the way. In order to add an extra atom to our chemical requires that we getit close enough that some rearrangement can occur; then it will stick.But if we cannot give it enough energy to get it close enough, it will not goto completion, it will just go part way up the “hill” and back down again. However,if we could literally take the molecules in our hands and push and pull theatoms around in such a way as to open a hole to let the new atom in, and then letit snap back, we would have found another way, around the hill, which wouldnot require extra energy, and the reaction would go easily. Now there actually are,in the cells, very large molecules, much larger than the ones whosechanges we have been describing, which in some complicated way hold the smallermolecules just right, so that the reaction can occur easily. These very largeand complicated things are called enzymes. (They were first called ferments,because they were originally discovered in the fermentation of sugar. In fact,some of the first reactions in the cycle were discovered there.) In the presenceof an enzyme the reaction will go.
这里我们看到的，是一个分子的完整系列，在其中，分子以很小的步骤，从一个变成另一个，构成一个系列或循环。它被称为克雷布斯循环，或呼吸循环。其中的每种化合物和每一步，就分子中发生的变化而言，都相当简单，但是，在生物化学中的一个重要的发现就是：要在实验室中实现这些变化，相对比较困难。如果我们有一种实质体，还有另外一种与它类似的实质体，那么，第一个并不会变成第二个，因为，这两种形式，通常会被一种能量障碍或“小山丘”分开。考虑这么一个类比：如果我们想把一个对象，从小山丘的这一面，移到那一面，两点在一个水平面上，那么，我们就需要把它推到山顶，但是，这样做的话，我们就需要额外的能量。所以，大多数化学反应，并不会发生，因为，缺乏一个我们称为激活能量的东西。为了把一个原子，附加给为我们的化工品，我们就必须让它靠的足够近，这样的话，重新排列就会发生，然后，它们就会紧连在一起。但是，如果我们不能给它提供足够的能量，让它们变得足够近，它就不会完成，它只会上到“小山丘”上，然后又返回原地。然而，如果我们可以真正地把分子拿在手上，通过推拉旧原子，撕开一个洞，就可以让新的原子进去，然后，又让旧原子弹回来，这样，我们就可以找到另外一条路，即绕着山走，那么就不需要额外的能量了，反应的就会很容易地进行。现在，在细胞中有非常大的分子，比我们前面所描述过的东西要大得多，这些大的分子，以某种复杂的方式，正确地控制着小的分子，这样，反应就可以很容易地进行。这些非常大的和复杂的分子，被称为酶。（它们最初被称为发酵素，因为最初是在糖的发酵物中发现的。）有酶在场，反应就会进行。

An enzyme is made of another substance calledprotein. Enzymes are very big and complicated, and each one is different,each being built to control a certain special reaction. The names of theenzymes are written in Fig. 3–1 at eachreaction. (Sometimes the same enzyme may control two reactions.) We emphasizethat the enzymes themselves are not involved in the reaction directly. They donot change; they merely let an atom go from one place to another. Having doneso, the enzyme is ready to do it to the next molecule, like a machine in a factory.Of course, there must be a supply of certain atoms and a way of disposing ofother atoms. Take hydrogen, for example: there are enzymes which have specialunits on them which carry the hydrogen for all chemical reactions. For example,there are three or four hydrogen-reducing enzymes which are used all over ourcycle in different places. It is interesting that the machinery which liberatessome hydrogen at one place will take that hydrogen and use it somewhere else.
酶是由另外一种实质体，即蛋白质所构成。酶很大，而且很复杂，每种酶都不相同，每一种酶都是用来控制某种特定反应的。在图3-1的每个反应中，都写有酶的名字。（有时候呢，同一种酶，可以控制两种反应。）我们要强调，酶本身并没有直接卷入反应，它们并不变化，它们只是让一个原子从一个地方去到另一个地方，这样做了之后，就就准备对下一个分子做此事，就像工厂里的机器一样。当然，应该有提供确定原子的方法，和放置其他原子的方式。以氢为例，有一些酶的上面，有特殊的单位，用来为所有的化学反应，承载氢。例如，有3到4种氢还原的酶，它们被用于我们循环中的不同地方。这个机制很有趣，就是它在一个地方的释放氢，然后，把这个氢带到另外一个地方去使用。

The most important feature of the cycle of Fig. 3–1 is thetransformation from GDP to GTP (guanosine-di-phosphate to guanosine-tri-phosphate)because the one substance has much more energy in it than the other. Just as thereis a “box” in certain enzymes for carrying hydrogen atoms around, there are specialenergy-carrying “boxes” which involve the triphosphate group. So, GTP hasmore energy than GDP and if the cycle is going one way, we are producing moleculeswhich have extra energy and which can go drive some other cycle which requiresenergy, for example the contraction of muscle. The muscle will not contractunless there is GTP. We can take muscle fiber, put it in water, and add GTP,and the fibers contract, changing GTP to GDP if the right enzymes are present.So the real system is in the GDP-GTP transformation; in the dark the GTP which hasbeen stored up during the day is used to run the whole cycle around the otherway. An enzyme, you see, does not care in which direction the reaction goes,for if it did it would violate one of the laws of physics.
在图3–1中循环的最重要的特点，就是从GDP到GTP的转换（鸟苷二磷酸到鸟苷三磷酸），因为，一种具体材料，比另外一种，有更多的能量。在某种酶中，有一个“箱子”，用来装载氢原子，与此类似，也有特殊的装载能量的“箱子”，与三磷酸有牵扯。于是，GTP就比GDP，含有更多的能量，如果这个循环，只是往一个方向走，那么，我们就是在产生有额外能量的分子，并且，这些分子会驱动其他需要能量的循环，例如，肌肉的收缩。肌肉不会收缩，除非有GTP。我们可以把肌肉纤维拿来，放入水中，加入GTP，肌肉就会收缩，如果正确的酶在场的话，就会把GTP转换为GDP。所以，真正的系统，就在GDP到GDP的转换中；白天GTP被存储起来的，在黑暗中，GTP被反过来使用，以运行整个循环。你看，酶并不在乎反应往哪个方向走，如果它在乎的话，它就会违背一条物理学定律。

Physics is of great importance in biologyand other sciences for still another reason, that has to do with experimentaltechniques. In fact, if it were not for the great development of experimentalphysics, these biochemistry charts would not be known today. The reason is thatthe most useful tool of all for analyzing this fantastically complex system isto label the atoms which are used in the reactions. Thus, if we could introduceinto the cycle some carbon dioxide which has a “green mark” on it, and thenmeasure after three seconds where the green mark is, and again measure after tenseconds, etc., we could trace out the course of the reactions. What are the “greenmarks”? They are different isotopes. We recall that the chemical propertiesof atoms are determined by the number of electrons, not by the mass of thenucleus. But there can be, for example in carbon, six neutrons or sevenneutrons, together with the six protons which all carbon nuclei have.Chemically, the two atoms C12 and C13 are the same, but they differ in weight and they have different nuclearproperties, and so they are distingui****le. By using these isotopes ofdifferent weights, or even radioactive isotopes like C14 , which provide a more sensitive means for tracing very small quantities,it is possible to trace the reactions.
在生物学和其他科学中，物理学之所以有巨大作用，还有另一个原因，即与实验技术有关。事实上，如果不是实验物理学的巨大发展，这些生物化学的图表，今天可能还不会知道。理由如下，在分析这个巨复杂的系统时，最有用的工具，就是给那些被使用到的原子，贴上标签。这样如果我们给这个循环，引入二氧化碳，它们身上，有一种“绿色标志”，然后，过三秒钟，测量一下这些绿色标志在什么地方，过十秒，再测一下，这样，我们就可以跟踪到这个反应的过程。这些“绿色标志”是什么呢？它们就是各种不同的同位素。我们可以回想一下，原子的化学属性，是由电子的数量决定的，而不是由原子核的质量决定的。例如，所有碳原子的核中都有六个质子，但是，可以有六个中子或七个中子与这些质子在一起。从化学上讲，原子C12与原子C13是同样的，但是，它们的重量不同，且有不同的原子核属性，所以，它们是可以区分的。通过使用这些不同重量的同位素，甚至包括有辐射性的同位素，如C14，跟踪反应是可能的；C14为跟踪非常小的量，提供了一种更敏感的方式。

Now, we return to the description of enzymesand proteins. Not all proteins are enzymes, but all enzymes are proteins. Thereare many proteins, such as the proteins in muscle, the structural proteinswhich are, for example, in cartilage and hair, skin, etc., that are not themselvesenzymes. However, proteins are a very characteristic substance of life: first ofall they make up all the enzymes, and second, they make up much of the rest of livingmaterial. Proteins have a very interesting and simple structure. They are a series,or chain, of different amino acids. There are twenty different amino acids,and they all can combine with each other to form chains in which the backboneis CO-NH, etc. Proteins are nothing but chains of various ones of these twentyamino acids. Each of the amino acids probably serves some special purpose. Some,for example, have a sulfur atom at a certain place; when two sulfur atoms arein the same protein, they form a bond, that is, they tie the chain together attwo points and form a loop. Another has extra oxygen atoms which make it an acidicsubstance, another has a basic characteristic. Some of them have big groups hangingout to one side, so that they take up a lot of space. One of the amino acids, calledproline, is not really an amino acid, but imino acid. There is a slight difference,with the result that when proline is in the chain, there is a kink in the chain.If we wished to manufacture a particular protein, we would give these instructions:put one of those sulfur hooks here; next, add something to take up space; thenattach something to put a kink in the chain. In this way, we will get a complicated-lookingchain, hooked together and having some complex structure; this is presumablyjust the manner in which all the various enzymes are made. One of the greattriumphs in recent times (since 1960), was at last to discover the exact spatialatomic arrangement of certain proteins, which involve some fifty-six or sixtyamino acids in a row. Over a thousand atoms (more nearly two thousand, if wecount the hydrogen atoms) have been located in a complex pattern in two proteins.The first was hemoglobin. One of the sad aspects of this discovery is that wecannot see anything from the pattern; we do not understand why it works the wayit does. Of course, that is the next problem to be attacked.
现在，我们回头再讲酶和蛋白质。不是所有的蛋白质，都是酶，但是，所有的酶都是蛋白质。有很多种蛋白质，例如肌肉中的蛋白质，结构性的蛋白质，例如在软骨、头发、和皮肤中的，它们本身不是酶。然而，蛋白质是生命中非常有特点的具体物质：首先，它们构成了所有的酶，其次，它们构成了大多数其他的生命材料。蛋白质有一个非常有趣的和简单的结构。它们是一条不同蛋白质的系列或链。有20种不同的氨基酸，它们都可以相互结合起来，形成链条，链中的主干是，CO-NH,等等。蛋白质无非是这20种氨基酸的各种组合所构成的链条。每种氨基酸大约都有某种特殊目的。例如，有些蛋白质，在某些位置，有一个硫原子，当两个硫原子在同一个蛋白质中时，它们就形成一个键，也就是说，它们在两个点，把这个链绑住，形成了一个环。另外一种氨基酸，有一个氧原子，这使得它成为一种酸性的具体物质，还有一种氨基酸，有一种基本特点。有些氨基酸，有一大团，挂在外面，这样，它们就占据了很大空间。其中一种氨基酸，被称为脯氨酸{ proline }，它不是真正的氨基酸，而是亚氨酸。有一点不同，就是当脯氨酸在链中时，链中就会有一个结{kink}。如果我们希望制造某种蛋白质，我们就要提供这些结构：把那些硫钩子中的一种，放在这里，然后，增加某些东西，以占据空间，然后，再附加一些东西，以把那个结放在链中。以这种方式，我们将得到一个看上去很复杂的链，相互钩在一起，还有一些复杂的结构；所有不同的酶，都被假定是这样被制造出来的。近来（自1960年）最大的胜利，就是最终发现了，某些蛋白质的原子的准确的空间排列，该蛋白质在一行中，牵扯到56或60个氨基酸。在两种原子中，都有超过1000个原子（如果我们把氢原子算上的话呢，就接近2000），在一个复杂的模型中被定位。第一个蛋白质，是血红蛋白，这个发现的一个让人悲观的方面，就是我们从这个模型出发，看不到任何东西。我们不能理解，它为什么是这样工作的。当然，这是下一个需要攻击的问题。

Another problem is how do the enzymes know whatto be? A red-eyed fly makes a red-eyed fly baby, and so the information for thewhole pattern of enzymes to make red pigment must be passed from one fly to thenext. This is done by a substance in the nucleus of the cell, not a protein, calledDNA (short for desoxyribose nucleic acid). This is the key substance which ispassed from one cell to another (for instance sperm cells consist mostly ofDNA) and carries the information as to how to make the enzymes. DNA is the “blueprint.”What does the blueprint look like and how does it work? First, the blueprintmust be able to reproduce itself. Secondly, it must be able to instruct the protein.Concerning the reproduction, we might think that this proceeds like cellreproduction. Cells simply grow bigger and then divide in half. Must it be thuswith DNA molecules, then, that they too grow bigger and divide in half? Every atomcertainly does not grow bigger and divide in half! No, it is impossible to reproducea molecule except by some more clever way.
另外一个课题就是：酶是如何知道，它该做个什么样的酶的？一个红眼苍蝇，制造一个红眼的苍蝇的婴儿，于是，制造红色模型所需的信息，就应该从一个苍蝇，传递到其下一代。这是由细胞核中的一个具体物质来做的，它被称为DNA（脱氧核糖核酸的缩写），而不是由蛋白质。这是从一个细胞传递到另一个细胞的关键性的具体物质（例如，精子细胞的大部分，都是DNA），包含着诸如：如何制造酶所需的信息。DNA就是“蓝图”。蓝图看上去什么样，它又是如何工作的呢？首先，蓝图应该能够复制自己。其次，它应该可以指导蛋白质。说到复制，我们可能会想，它会像细胞复制那样去进行。细胞只是简单地长大，然后，一分为二。DNA分子也应该是这样吗，那么，它们也会长大，然后一分为二？每个原子当然不会越长越大，然后一分为二！不，要复制一个分子，除非是通过一些聪明的方法，否则不可能。

Fig. 3–2.Schematic diagram of DNA. DNA的原理示意图
The structure of the substance DNA wasstudied for a long time, first chemically to find the composition, and then withx-rays to find the pattern in space. The result was the following remarkablediscovery: The DNA molecule is a pair of chains, twisted upon each other. The backboneof each of these chains, which are analogous to the chains of proteins butchemically quite different, is a series of sugar and phosphate groups, as shownin Fig. 3–2.Now we see how the chain can contain instructions, for if we could split this chaindown the middle, we would have a series BAADC… and every living thing could have a different series. Thus perhaps, insome way, the specific instructions for the manufacture of proteins arecontained in the specific series of the DNA.
具体物质DNA的结构，已经被研究了很长时间，首先是化学性的，以找到其成分，然后，使用X光，以找到其在空间的样子。结果就是下面很值得注意的发现：DNA分子，是一对链，互相拧在一起。这个链可以与蛋白质的链相比，但化学上完全不同；每个链的主干，是一系列由糖和磷酸盐所形成的组，如图3-2。现在，我们可看到，链是如何包含指示的，因为，如果我们可以把这个链从中间分开，我们就可以得到一系列的BAADC…，这样，每个活着的事物，就可以有一个不同的系列。这样，或许以某种方式，那些用来制造蛋白质的特殊指示，就包含在DNA的特殊序列中。

Attached to each sugar along the line, andlinking the two chains together, are certain pairs of cross-links. However,they are not all of the same kind; there are four kinds, called adenine, thymine,cytosine, and guanine, but let us call them A , B , C , and D . The interesting thing is that only certain pairs can sit oppositeeach other, for example A with B and C with D . These pairs are put on the two chains in such a way that they “fittogether,” and have a strong energy of interaction. However, C will not fit with A , and B will not fit with C ; they will only fit in pairs, A against B and C against D . Therefore if one is C , the other must be D , etc. Whatever the letters may be in one chain, each one must haveits specific complementary letter on the other chain.
沿着线、附加在每一个糖上、并且把两个链连在一起的，是一对交联键。然而，它们并不全是同一类，共有四种，被称为腺嘌呤，胸腺嘧啶，胞核嘧啶，鸟嘌呤，但是，让我们把他们称为A , B , C , 和D。有趣的是，只有某些对，可以对面而坐，例如A与B，C与D。这些对子被放在两个链上的方式，就是它们相互适应，且有很强的交互能量。然而，C与A不适合，B与C不适合；它们只是成对的：A与B，C与D。因此，如果一个是C，那么另外一个就一定是D，等等。无论一个链上的字母是什么，在另一个链上，一定有一个与它匹配的字母。

What then about reproduction? Suppose wesplit this chain in two. How can we make another one just like it? If, in thesubstances of the cells, there is a manufacturing department which brings upphosphate, sugar, and A , B , C , D units not connected in a chain, the only ones which will attach to oursplit chain will be the correct ones, the complements of BAADC… , namely, ABBCD… Thus what happens is that the chain splits down the middle during celldivision, one half ultimately to go with one cell, the other half to end up in theother cell; when separated, a new complementary chain is made by each half-chain.
复制又是怎么回事呢？假设我们把这个链，分成两个。我们如何制造一个与原来一样的另一半呢？如果在细胞的具体物质中，有一个制造部门，它把磷酸盐、糖和A、B、C、D等单元培养出，这些单元在一个的链中也没有连接，那么，能附加到我们的分裂出的半链上的单元，只能是那些是正确的单元，即BAADC的匹配，只能是ABBCD。这样，所发生的事情就是，在细胞分裂的时候，链从中间向下，分成两个半链，其中一个，最终跟着一个细胞走，另外一个，进入另外一个细胞；当分开的时候，一个新的匹配，被每一个半链制造出来。