端木·宇 2008-6-19 22:23
The Molecules of Life
The elements involved in life processes can, and do, form millions ofdifferent compounds. Thankfully, these millions of compounds fall intofour major groups: carbohydrates, proteins, lipids, and nucleic acids.Though all of these groups are organized around carbon, each group hasits own special structure and function.
[b] Carbohydrates[/b]
Carbohydrates are compounds that havecarbon, hydrogen, and oxygen atoms in a ratio of about 1:2:1. If you’restuck on an SAT II Biology question about whether a compound is acarbohydrate, just count up the atoms and see if they fit this ratio.Carbohydrates are often sugars, which provide energy for cellularprocesses.
Like all of the biologically importantclasses of compounds, carbohydrates can be monomers, dimers, orpolymers. The names of most carbohydrates end in “-ose”: [b]glucose[/b], fructose, sucrose, and maltose are some common examples.
[b] Monosaccharides[/b]
Carbohydrate monomers are known as monosaccharides. This group includes glucose, C6H12O6,which is a key substance in biochemistry. Sugars that an animal eatsare converted into glucose, which is then converted into energy to fuelthe animal’s activities by respiration (see Cell Processes).
Glucose has a cousin called fructose with the same chemical formula. But these two compounds have different structures:
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/cace66c0214dc99a0bb1d14eaf08287b.gif[/img][/align]
Glucose and fructose differ in one importantway: glucose has a double-bonded oxygen on the top carbon, whilefructose has its double-bonded carbon on the second carbon. Thisdifference is most apparent when the two monosaccharides are in theirring forms. Glucose generally forms a hexagonal ring (six sided), whilefructose forms a pentagonal ring (five sided). Whereas fructose is thesugar most often found in fruits, glucose is most often used as themajor source of energy for cellular activities.
[b] Disaccharides[/b]
Disaccharidesare carbohydratedimers. These dimers are formed from two monomers by dehydrationsynthesis. Any two monosaccharides can form a disaccharide. Forexample, maltose is formed by the dehydration synthesis of two glucosemolecules. Sucrose, common table sugar, comes from the linkage of onemolecule of glucose and one of fructose.
[b] Polysaccharides[/b]
Polysaccharides can consist of as few asthree and as many as several thousand monosaccharides. Depending ontheir structure and the monosaccharides they contain, polysaccharidescan function as a means of storing excess energy or provide structuralsupport.
When cells ingest more carbohydrates thanthey need for fuel, they link the sugars together to formpolysaccharides. The structure of these polysaccharides is different inplants and animals: in plants, polysaccharides take the form of [b]starch[/b], whereas in animals, they are linked in a structure called [b]glycogen[/b].
Polysaccharides can also have structuralroles in plants and animals. Cellulose, which forms the cell walls ofplant cells, is a structural polysaccharide. In animals, thepolysaccharide chitin forms the hard outer armor of insects, crabs,spiders, and other arthropods. Many fungi also use chitin as astructural carbohydrate.
[b] Proteins[/b]
More than half of the organic compounds incells are proteins, which play an important function in almost everycellular process. Proteins, for example, provide structural support tothe cell in the cytoskeleton and make up many of the hormones that sendmessages around the body. [b]Enzymes[/b], which regulate chemical reactions in the cell, are also proteins.
[b] Amino Acids[/b]
Proteins are made up of monomers calledamino acids. The names of many, but not all, amino acids end in -ine:methionine, lysine, serine, etc. Each amino acid consists of a centralcarbon atom attached to a set of three designated groups: an atom ofhydrogen (–H), an amino group (–NH2), and a carboxyl group (–COOH). The final group, designated (–R) in the diagram below, varies between different amino acids.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/1cc27174f7b604969291584f3a0de7e0.gif[/img][/align]
It is possible to make an infinite number of amino acids by attaching different compounds to the R position of the central carbon. However, only 20 types of R groups exist in nature, so there are only 20 naturally occurring amino acids.
[b] Polypeptides[/b]
All proteins are made of chains of some orall of these 20 amino acids. The bond formed between two amino acids bydehydration synthesis is known as a [b]peptide bond[/b].
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/767e3c7c7cf01fbc8ab44c919c487f26.gif[/img][/align]
A particular protein has a specific sequence of amino acids, which is known as its [b]primary structure[/b].Every protein also winds, coils, and folds in three-dimensional spacein specific and predetermined ways, taking on a unique secondary(initial winding and coiling) and tertiary structure (overall folding).In harsh conditions, such as high temperature or extreme pH, proteinscan lose their normal tertiary shape and cease to function properly.When a protein unfolds in harsh conditions, it has been “denatured.”
[b] Lipids[/b]
Lipids are carbon compounds that do notdissolve in water. They are distinguished from other macromolecules bycharacteristic [b]hydrocarbon[/b] chains—long strings of carbonmolecules with hydrogens attached. Such chains do not dissolve well inwater because they are nonpolar.
[b] Triglycerides[/b]
Triglycerides consist of three long hydrocarbon chains known as [b]fatty acids[/b] attached to each other by a molecule called glycerol.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/86a293a655556dd683364742a7ab4dde.gif[/img][/align]
Because they include three fatty acids, fatsand oils are also known as triglycerides. As you might expect by thispoint, glycerol and each fatty acid chain are joined to each other bydehydration synthesis.
Some fats are saturated, while others areunsaturated. These terms refer to the presence or absence of doublebonds in the fatty acids of fats. Saturated fats have no double bonds,whereas unsaturated fats contain one or more such bonds. In general,plant fats are unsaturated and animal fats are saturated. Saturatedfats are generally solid at room temperature, while unsaturated fatsare typically liquid.
[b] Phospholipids[/b]
Phospholipids, which are importantcomponents of cell membranes, consist of a glycerol molecule attachedto two fatty acid chains and one phosphate group (PO4–2):
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/dc50794e262ffcf5341a4717785ff3a7.gif[/img][/align]
Like all fats, the hydrocarbon tails ofphospholipids do not dissolve in water. However, phosphate groups dodissolve in water because they are polar. The different solubilities ofthe two ends of phospholipid molecules allow them to form the bilayersthat make up the cell membrane.
[b] Steroids[/b]
Steroids are the primary structure inhormones, substances that play important signaling roles in the body.Structurally, steroids are made up of four fused carbon rings attachedto a hydrocarbon chain.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/17fe06ef2bc54d3810489fbb399195d6.gif[/img][/align]
The linked rings indicate that each carbonatom is attached to other carbon atoms that form multiple loops.Cholesterol, the steroid in the image above, is the central steroidfrom which other steroids, such as the sex hormones, are synthesized.Cholesterol is only found in animal cells.
[b] Nucleic Acids[/b]
Cells use a class of compounds callednucleic acids to store and use hereditary information. Individualnucleic acid monomers, known as [b]nucleotides[/b], consist of three main units: a [b]nitrogenous base[/b] (a compound made with nitrogen), a phosphate group, and a sugar:
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/ccd24d3bffdf157b5803e326857966e6.gif[/img][/align]
There are two main types of nucleotides, differentiated by their sugars: [b]deoxyribonucleic acid (DNA)[/b] and [b]ribonucleic acid (RNA)[/b].DNA nucleotides have one less oxygen than RNA nucleotides. The “deoxy”in deoxyribonucleic acid refers to the missing oxygen molecule. Interms of function, DNA molecules store genetic information for thecell, while RNA molecules carry genetic messages from the DNA in thenucleus to the cytoplasm for use in protein synthesis and otherprocesses.
Within both DNA and RNA, there are furthersubdivisions of nucleotides by nitrogenous bases. For DNA, there arefour kinds of nitrogenous bases:
[list=1][*]adenine (A)[*]guanine (G)[*]cytosine (C)[*]thymine (T)[/list]
The nitrogenous base of a nucleotideprovides it with its chemical identity, so the nucleotides are calledby the name of their nitrogenous base. RNA also has four nitrogenousbases. Three—adenine, guanine, and cytosine—are identical to thosefound in DNA. The fourth, uracil, replaces thymine.
[b] DNA and RNA[/b]
In 1953, James Watson and Francis Crickpublished the discovery of the three-dimensional structure of DNA.Watson and Crick hypothesized that DNA nucleotides are organized into apolymer that looks like a ladder twisted into a coil. They called thisstructure the [b]double helix[/b].
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/e81e651c153cb163809472b8d0c03279.gif[/img][/align]
Two separate DNA polymers make up each sideof the ladder. The sugar and phosphate molecules of the DNA form thevertical supports, while the nitrogenous bases stick out to form therungs. The rungs attach to each other by hydrogen bonding.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/e0aa86a95999c4a2194db61e762b4161.gif[/img][/align]
The nitrogen bases attach to each otheraccording to two simple rules: adenine (A) pairs with thymine (T), andguanine (G) pairs with cytosine (C). The exclusivity of the attachmentsbetween nitrogen bases is known as [b]base pairing[/b].
The rules of base pairing are frequently tested on the SAT II Biology. A test question might ask, “What is the [b]complementary[/b]DNA strand to ‘CAT’?” Following the rules of DNA base pairing, you candeduce that the answer is “CAT.” (“DOG” is the wrong answer, smartguy.)
[b] RNA Structure[/b]
Unlike the double-stranded DNA, RNA issingle stranded. It looks like a ladder cut down the middle. As youwill see when we discuss protein synthesis in the chapter on CellProcesses, this structure of RNA is very important to its functions asa messenger from the DNA in the nucleus to the cytoplasm.
[table=350][tr][td]
[/td][td]DNA[/td][td]RNA[/td][/tr][tr][td]Bases[/td][td]Adenine, guanine, cytosine, thymine[/td][td]Adenine, guanine, cytosine, uracil[/td][/tr][tr][td]Structure[/td][td]Double helix[/td][td]Single helix[/td][/tr][tr][td]Function[/td][td]Stores genetic material and passes it from generation to generation[/td][td]Carries messages from the nucleus to the cytoplasm[/td][/tr][/table]
[b] Summary of the Molecules of Life[/b]
[table=350][tr][td]
[/td][td]Proteins[/td][td]Lipids[/td][td]Nucleic Acids[/td][td]Carbohydrates[/td][/tr][tr][td]Function[/td][td]Structure, signaling, catalysis[/td][td]Energy storage, signaling, membrane constituents[/td][td]Store genetic material[/td][td]Energy source, energy storage, structural[/td][/tr][tr][td]Monomer[/td][td]Amino acid[/td][td]
[/td][td]Nucleotide[/td][td]Monosaccharide[/td][/tr][tr][td]Polymer[/td][td]Polypeptide, protein[/td][td]
[/td][td]RNA, DNA[/td][td]Polysaccharide[/td][/tr][tr][td]Example[/td][td]Insulin, transcriptase (an enzyme)[/td][td]Corn oil[/td][td]A chromosome[/td][td]Glucose[/td][/tr][/table]
端木·宇 2008-7-24 08:21
分子的生活
生命过程所涉及的要素可以，形成数百万不通化合物。多亏，这些以百万计的化合物，这些化合物有：碳水化合物，蛋白质，脂类，核酸.所有这些团体周围都是由碳的结构组成，每组hasits？拥有特殊的结构和功能。
碳水化合物
碳水化合物是含碳 ，氢，氧原子的比率约为1：2：1的化合物。如果你对SAT二生物学中是否对一种复合acarbohydrate?有问题，那么就数一下原子和看看他们是否适合这个比。碳水化合物往往是提供能源给细胞的过程的糖类 。
像所有的生物重要化合物的种类如：碳水化合物，可单体，二聚体， 或者 聚合体 。大部分碳水化合物名字的后面有“-OSE” ：葡萄糖，果糖，蔗糖，麦芽糖是一些常见的例子。
单糖
碳水化合物单体被称为单糖。这个小组包括葡萄糖， 酒精 ，这是生物化学中一个关键的实质。一个动物通过消化转化为葡萄糖的糖，--------
[color=Red][b]以上部分感谢[/b][/color] [url=space-uid-56708.html]bixian[/url]
那些动物通过进食获得的糖类转化成葡萄糖，这些将最终转化成能量提供给动物的呼吸作用。（参见细胞分裂）
[[i] 本帖最后由 端木·宇 于 2008-7-27 00:02 编辑 [/i]]
bixian 2008-7-26 18:53
分子的生活
生命过程所涉及的要素可以，形成数百万不通化合物。多亏，这些以百万计的化合物，这些化合物有：碳水化合物，蛋白质，脂类，核酸.所有这些团体周围都是由碳的结构组成，每组hasits？拥有特殊的结构和功能。
碳水化合物
碳水化合物是含碳 ，氢，氧原子的比率约为1：2：1的化合物。如果你对SAT二生物学中是否对一种复合acarbohydrate?有问题，那么就数一下原子和看看他们是否适合这个比。碳水化合物往往是提供能源给细胞的过程的糖类 。
像所有的生物重要化合物的种类如：碳水化合物，可单体，二聚体， 或者 聚合体 。大部分碳水化合物名字的后面有“-OSE” ：葡萄糖，果糖，蔗糖，麦芽糖是一些常见的例子。
单糖
碳水化合物单体被称为单糖。这个小组包括葡萄糖， 酒精 ，这是生物化学中一个关键的实质。一个动物通过消化转化为葡萄糖的糖，--------望高人往下翻译
duoduo1991 2008-7-29 00:38
[b][size=5]凭借我高二的生物知识 一个晚上全翻译完了 有些地方可能和原文由出入 但我觉得我这么翻译更好全是生物的专业术语 有些地方与原文有出入 我是把他的深层意思写上了 看看吧 有点累了 语法不难 词好难啊 、 表格也修改好了 [/size][/b] 最后我全文又修改了一下 更加流畅了
分子的生活
生命过程所涉及的元素可以形成数百万种不同化合物。这些化合物有：碳水化合物，蛋白质，脂类，核酸.所有这些周围都是由碳的结构组成，每组hasits？拥有特殊的结构和功能。
碳水化合物
碳水化合物是含碳 ，氢，氧原子构成，比例约为1：2：1。如果在SAT2生物学考试中，关于是否是一种复水碳水化合物的问题，那么就数一下原子和看看他们是否适合这个比。碳水化合物往往是提供能源给细胞的过程的糖类 。
像所有的生物重要化合物的种类如：碳水化合物，单体，二聚体， 或者聚合体 。大部分碳水化合物名字的后面有“-OSE” ：葡萄糖，果糖，蔗糖，麦芽糖是一些常见的例子。[size=5] [/size]
单糖
碳水化合物单体被称为单糖。单糖包括葡萄糖， 酒精 ，这是生物化学中一个关键的实质。一个动物通过消化转化为葡萄糖的糖，然后把它们转成能量，供动物消耗，主要是呼吸作用消耗的能量。
葡萄糖与果糖具有相同的化学分子式（C6H1206)但是这两个化合物具有不同的结构(我没记错的话是一个有醛基，一个没有）
在一个重要的方面葡萄糖和果糖不同：葡萄糖有一个双键对顶端的碳，而果糖的双键碳在第二个碳上。当两个单糖是在其环的形式这种差异是最明显的。葡萄糖一般形成了一个六角环（ 6片面） ，而果糖形成了一个五角环（ 5片面） 。在水果中的经常发下的糖是果糖，葡萄糖是作为主要的能源来源，为细胞活动提供能量。
二糖
二糖化合物，这些二聚体是由两个单体脱水缩合形成。任何两个单糖可以形成一个糖。举例来说，麦芽糖是由脱水合成二葡萄糖分子。蔗糖，淀粉，是由葡萄糖和果糖组成的。
多糖
多糖可以构成多达数千种。根据其结构和它们含有单糖，多糖的所具有的功能，所以多糖可以储存过剩的能源或提供结构性支持。
当细胞摄取碳水化合物比他们需要消耗的多时，他们链接起来在一起，形成多糖结构，植物和动物的多糖不相同：在植物中，多糖存在形式是淀粉，而在动物，他们链接一起后存在形式是糖元。
多糖也可以有结构性的角色，在植物和动物。在植物细胞中纤维素构成细胞壁，纤维素就是是一个结构性的多糖。在动物方面，多糖甲壳素形式铠甲如昆虫，螃蟹，蜘蛛，和其他节肢动物的壳。许多真菌也利用甲壳素作为一个结构性碳水化合物。
蛋白质
蛋白质在半数以上的有机化合物细胞中发挥了重要的作用。例如，支持细胞膜结构的骨架和输送激素到身体的各个部位。在细胞中酶调节化学反应，也是靠蛋白质。（因为酶就是一种蛋白质）
氨基酸
氨基酸是蛋白质的单体。他的名称很多，都以INE结尾：蛋氨酸，赖氨酸，丝氨酸等，每个氨基酸组成的中央碳原子上都连有3个基团这是不会变的： 1原子氢（ - H ）的，一氨基酸组（-NH2） ，和一个羧基（ -COOH） 。最后一组，都是R基（ - R ）在如下图，R基决定氨基酸的不同。
这也就是说可以有无数种的氨基酸，只要中央碳的支链上有不同的R基。但是，在性质上只有20个类型的R群体存在所以只有20种氨基酸。
多肽
这20个氨基酸的部分或全部制成所有的蛋白质的链。两个氨基酸间-NH2与-OH之间称作一个肽键2个集团组合到一起的过程就叫脱水缩合
氨基酸为其主要结构， 某一特定蛋白质有特定的氨基酸序列，这是众所周知。每一个蛋白质在三维立体空间中经过一系列的过程如卷等这些都是都预定好的。具有一个独特的2级（初步绕组和卷取）和三级结构（整体折叠） 。在苛刻的条件，如高温或极端的pH值，蛋白质可能会失去其正常的形状和停止运作。一个蛋白生活在苛刻的条件，它会发生“变性” 。
血脂
血脂是碳水化合物但不溶于水。他们是有别于其他生物大分子的特点在长的链上碳与氢相连。这种链不溶于水，因为他们非极性。
甘油三酯
甘油三酸酯的构成为三条长链烃末端连有不饱和脂肪酸，叫做甘油。
因为他们包括3个脂肪酸，脂肪油砂也被称为甘油三酯。由于这点 ，甘油和脂肪酸，互相进入对方脱水的合成。
一些饱和脂肪，而另一些不饱和。这些的分别指的是存在或没有碳碳双键，在不饱和脂肪酸的油脂。饱和脂肪没有碳碳双键，不饱和脂肪酸，而包含一个或多个这样碳碳双键。在一般情况下，植物是不饱和脂肪和动物脂肪是饱和。饱和脂肪一般在室温下为固体，而不饱和脂肪通常是液体。
磷脂
细胞膜重要组成部分是磷脂，构成一个甘油分子上连接二个不饱和脂肪酸链和一个磷酸基团（ po4 - 2 ） ：
像所有脂肪，磷脂的烃的后部不溶于水。不过，磷酸盐集团溶解在水中，因为他们是极性分子。不同的溶解度两端的磷脂分子为磷脂双分子层称为细胞膜。
类固醇
激素 ，类固醇是荷尔蒙的主要结构，在人体中这种物质起着重要的信号作用。在结构上，类固醇组成的4融合碳环连接到一个烃链。
联系表明，每一个碳原子连接到其他的碳原子形成多个环.胆固醇，类固醇在上图中，中央的是类固醇，其他是类固醇，如性激素是合成的。胆固醇是只在动物细胞中发现。
核酸
细胞使用的一类化合物称为核酸功能是储存和使用的遗传信息。个别核酸单体称为核苷酸，组成三个主要单位：碱基 核糖或脱氧核糖以及磷酸：
主要有两种类型的核苷酸，区别他们的核糖或脱氧核糖：脱氧核糖核酸（ DNA ）和核糖核酸（ RNA ）的。 DNA的核苷酸比RNA的核苷酸少了一个氧。 “脱氧”在脱氧核糖核酸指失去的氧分子。 DNA分子储存遗传信息的细胞，而RNA分子携带的遗传信息从DNA的细胞核的核孔中出来，让在蛋白质合成和其他进程细胞质使用
这两个DNA和RNA ，有进一步区分核苷酸由碱基。 DNA的，有四种碱基
腺嘌呤（A）
鸟嘌呤（G）
胞嘧啶（C）
胸腺嘧啶（T）
该碱基的一个核苷酸决定了它由于它的化学品特性，因此，核苷酸的名称有他们的碱基来决定。 RNA的也有4 碱基 。 前3个腺嘌呤，鸟嘌呤，胞嘧啶在DNA-也存在。而特殊的是RNA中是尿嘧啶，DNA中是胸腺嘧啶。
DNA与RNA
1953年，沃森和克里克发表了发现了三维结构的dna.沃森和克里克推测DNA的核苷酸是由聚合物构成，看起来像梯子扭曲成线圈组成的。他们称这个结构叫双螺旋结构。
两个独立的DNA是由两条链组成的。DNA中的脱氧核糖和磷酸交替连接，排列在外侧构成基本骨架，碱基排列在内侧。DNA两条连上的碱基通过氢键连接成碱基对。
碱基遵循简单的原则叫碱基互补配对原则：腺嘌呤（A）对与胸腺嘧啶（ T ） ，鸟嘌呤（G）对胞嘧啶（C） 。
规则中相应的配对经常测试，SAT2生物学。测试的问题可能会问， “什么是互补DNA链'CAT” ？根据DNA碱基配对，您可以推断答案是“CAT” （ “DOG”是错误的答案。 ） (这块好像是作者在开玩笑吧） RNA结构
不像双练的DNA ， RNA是单链。正如您看到的，当我们讨论蛋白质合成的一章，对细胞的过程，这种结构的RNA是非常重要的，其职能是在细胞核中先把DNA的遗传信息传递给RNA。
[size=5] [/size][table=464][tr][td] [/td][td] DNA[/td][td] RNA[/td][/tr][tr][td] 基础[/td][td] 腺嘌呤，鸟嘌呤，胞嘧啶，胸腺嘧[/td][td] 腺嘌呤，鸟嘌呤，胞嘧啶，尿嘧啶[/td][/tr][tr][td] 结构[/td][td] 双螺旋 [/td][td]单螺旋[/td][/tr][tr][td] 功能[/td][td] 贮存基因物质并传递[/td][td] 从核仁往细胞质中运送物质[/td][/tr][/table]
综述分子的生活
[size=5] [/size][table=50%][tr][td] [/td][td] 蛋白质[/td][td] 油脂[/td][td] 核酸[/td][td] 碳水化合物[/td][/tr][tr][td]作用 [/td][td] 结构支持，催化作用，传塑[/td][td] 膜，乳酸成分，能量储存和传输[/td][td] 储存基因物质[/td][td] 能量来源，能量储存[/td][/tr][tr][td] 单体[/td][td] 氨基酸[/td][td] [/td][td] 核苷酸 [/td][td] 单糖[/td][/tr][tr][td] 聚合体[/td][td] 蛋白质，多肽[/td][td] [/td][td] RNA DNA[/td][td] 多糖[/td][/tr][tr][td] 实例[/td][td] 胰岛素，转录酶[/td][td] 玉米油[/td][td] 一个染色体[/td][td] 葡萄糖[/td][/tr][/table]
[[i] 本帖最后由 duoduo1991 于 2008-7-29 08:15 编辑 [/i]]
Horse 2008-7-29 15:23
LS很牛逼……(Horse19 (Horse19 (Horse19
