📚 Monomers, Polymers and Macromolecules | 单体、聚合物与大分子
Biological macromolecules are large, complex assemblies that form the structural and functional basis of all living organisms. Many of these macromolecules are built by linking together smaller repeating units called monomers through polymerisation reactions. Understanding the relationship between monomers, polymers and macromolecules is essential for grasping how carbohydrates, proteins, nucleic acids and lipids carry out their diverse roles in cells.
生物大分子是构成所有生命体结构和功能基础的大型复杂组装体。其中许多大分子是通过聚合反应将称为单体的小重复单元连接在一起而形成的。理解单体、聚合物和大分子之间的关系，对于掌握碳水化合物、蛋白质、核酸和脂质如何在细胞中发挥其多样作用至关重要。
1. Introduction to Biological Macromolecules | 生物大分子简介
Biological macromolecules are giant molecules with high relative molecular mass. In living systems they mainly belong to four classes: carbohydrates, proteins, nucleic acids and lipids. Three of these classes – carbohydrates, proteins and nucleic acids – are true polymers, meaning they consist of long chains of covalently bonded monomers. Lipids are also considered macromolecules, yet they are not built from repeating monomeric units in the same way.
生物大分子是具有高相对分子质量的巨型分子。在生命体系中它们主要分为四类：碳水化合物、蛋白质、核酸和脂质。其中三类——碳水化合物、蛋白质和核酸——是真正的聚合物，即它们由以共价键连接的长链单体组成。脂质也被视为大分子，但它们并非以相同方式由重复单体单元构建而成。
2. Monomers and Polymers: The Building Blocks | 单体与聚合物：构建基石
A monomer is a relatively small, simple molecule that can join with other similar or identical monomers to form a polymer. The process is analogous to linking identical beads on a string. In biological systems the most important monomers are monosaccharides (e.g. glucose), amino acids and nucleotides. Each type of monomer possesses specific functional groups that enable the formation of characteristic covalent bonds.
单体是一种相对较小、简单的分子，它可以与其他相似或相同的单体连接形成聚合物。这一过程类似于将相同的珠子串在绳上。在生物体系中最主要的单体是单糖（如葡萄糖）、氨基酸和核苷酸。每种单体都带有特定的官能团，能够形成特征性的共价键。
A polymer is a long chain molecule made up of many repeating monomer units linked by covalent bonds. Polysaccharides, polypeptides and polynucleotides are natural polymers. The sequence of monomers within a polymer determines its three‑dimensional shape and biological function. Even a small change in monomer order can drastically alter the properties of the resulting macromolecule.
聚合物是由许多重复单体单元以共价键连接而成的长链分子。多糖、多肽和多核苷酸都是天然聚合物。聚合物内单体的序列决定了它的三维形状和生物学功能。即使单体顺序发生微小改变，也可能显著改变最终大分子的性质。
3. Condensation Reactions: Joining Monomers | 缩合反应：连接单体
Monomers are joined together by condensation reactions, sometimes called dehydration synthesis. In a condensation reaction, the hydroxyl (–OH) group of one monomer combines with a hydrogen atom (–H) from another monomer, releasing a water molecule (H₂O) and forming a new covalent bond. This type of reaction is catalysed by specific enzymes and requires an input of energy.
单体通过缩合反应（有时称为脱水合成）连接在一起。在缩合反应中，一个单体的羟基（–OH）与另一个单体的氢原子（–H）结合，释放出一分子水（H₂O），同时形成一个新的共价键。这类反应由特定的酶催化，并且需要能量输入。
Condensation reactions are responsible for building all the biological polymers: glycosidic bonds in carbohydrates, peptide bonds in proteins, and phosphodiester bonds in nucleic acids. Each of these bonds is formed with the loss of a water molecule. The reverse process, hydrolysis, uses water to break these bonds.
缩合反应负责构建所有生物聚合物：碳水化合物中的糖苷键、蛋白质中的肽键以及核酸中的磷酸二酯键。这些键在形成时都伴随着一分子水的失去。其逆过程——水解反应，则利用水来断裂这些键。
4. Hydrolysis Reactions: Breaking Polymers | 水解反应：分解聚合物
Hydrolysis is the chemical breakdown of a polymer into its constituent monomers by the addition of water. A water molecule is split so that its –H and –OH components are attached to the two resulting monomers. Enzymes such as amylase, proteases and nucleases catalyse hydrolysis reactions during digestion and within cells, making the stored chemical energy or building blocks available for reuse.
水解是通过加水分子的方式将聚合物化学分解为其组成单体的反应。一分子水被拆分，其–H和–OH部分分别连接到所产生的两个单体上。在消化过程以及细胞内部，淀粉酶、蛋白酶和核酸酶等酶催化水解反应，从而使储存的化学能或构建单元得以重新利用。
The condensation–hydrolysis cycle is fundamental to metabolism. It allows organisms to assemble complex structures when needed and disassemble them when monomers are required for energy release or synthesis of other biomolecules. This reversible nature ensures a dynamic balance between storage and utilisation.
缩合-水解循环是代谢的基础。它使生物体能够在需要时组装复杂结构，在需要单体释放能量或合成其他生物分子时又将其分解。这种可逆特性确保了储存与利用之间的动态平衡。
5. Carbohydrates: Monosaccharides as Monomers | 碳水化合物：单糖作为单体
Monosaccharides are the simplest carbohydrates and serve as the monomeric units for larger carbohydrates. Common examples include glucose, fructose and galactose, all with the molecular formula C₆H₁₂O₆. These hexose sugars exist in both straight‑chain and ring forms; the ring structure is predominant in aqueous solution and is crucial for polymer formation.
单糖是最简单的碳水化合物，也是更大糖类分子的单体单元。常见的例子包括葡萄糖、果糖和半乳糖，它们的分子式均为C₆H₁₂O₆。这些已糖既存在直链形式也存在环状形式；环状结构在水溶液中占主导地位，并且对聚合物的形成至关重要。
Monosaccharides are classified according to the number of carbon atoms (trioses, pentoses, hexoses) and the functional group present (aldoses with an aldehyde group, ketoses with a ketone group). Glucose is an aldose, while fructose is a ketose. The α and β isomers of glucose differ in the orientation of the –OH group on carbon‑1, a small difference that has profound consequences for polymer structure, as seen in starch and cellulose.
单糖根据碳原子数目（丙糖、戊糖、己糖）和所含官能团（醛糖具有醛基，酮糖具有酮基）进行分类。葡萄糖是一种醛糖，而果糖是一种酮糖。葡萄糖的α和β异构体在碳‑1上–OH基团的取向不同，这微小的差异对聚合物结构产生深远影响，正如淀粉和纤维素中所见。
6. Disaccharides and Polysaccharides: Carbohydrate Polymers | 二糖与多糖：碳水化合物聚合物
Two monosaccharides can undergo a condensation reaction to form a disaccharide, linked by a glycosidic bond. For example, α‑glucose and α‑glucose form maltose; α‑glucose and β‑fructose form sucrose; β‑galactose and α‑glucose form lactose. The glycosidic bond can be α‑1,4, β‑1,4 or other types, determining the digestibility and properties of the resulting sugar.
两分子单糖可通过缩合反应形成二糖，它们之间以糖苷键连接。例如，α‑葡萄糖与α‑葡萄糖形成麦芽糖；α‑葡萄糖与β‑果糖形成蔗糖；β‑半乳糖与α‑葡萄糖形成乳糖。糖苷键可以是α‑1,4、β‑1,4或其他类型，这决定了生成糖的消化性和性质。
Polysaccharides are polymers containing hundreds to thousands of monosaccharide units. Starch (a mixture of amylose and amylopectin) and glycogen are energy storage polysaccharides composed of α‑glucose. Cellulose, a structural polysaccharide found in plant cell walls, consists of β‑glucose units linked by β‑1,4 glycosidic bonds. The β‑linkages cause cellulose chains to pack straight and form strong microfibrils that many organisms cannot digest without specialised enzymes.
多糖是含有数百至数千个单糖单元的聚合物。淀粉（直链淀粉和支链淀粉的混合物）和糖原是由α‑葡萄糖组成的储能多糖。纤维素是一种存在于植物细胞壁中的结构多糖，由以β‑1,4糖苷键连接的β‑葡萄糖单元组成。β‑键使得纤维素链笔直排列，形成坚固的微纤维，许多生物体若无专门的酶则无法消化。
7. Proteins: Amino Acids and Peptide Bonds | 蛋白质：氨基酸与肽键
Proteins are polymers in which the monomers are amino acids. There are 20 standard amino acids, each with a central carbon atom bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable R group (side chain). The R group determines the chemical nature of the amino acid – whether it is non‑polar, polar, acidic or basic.
蛋白质是由氨基酸单体组成的聚合物。有20种标准氨基酸，每种都有一个中心碳原子，连接着一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子以及一个可变的R基团（侧链）。R基团决定了氨基酸的化学性质——它是非极性、极性、酸性还是碱性。
Amino acids are linked by peptide bonds formed through condensation reactions between the carboxyl group of one amino acid and the amino group of another. A chain of amino acids is called a polypeptide. The specific linear sequence of amino acids (primary structure) dictates how the polypeptide will fold into functional conformations, ultimately giving rise to enzymes, antibodies, transport proteins and structural components.
氨基酸通过肽键连接，肽键是在一个氨基酸的羧基与另一个氨基酸的氨基之间发生缩合反应形成的。一条氨基酸链称为多肽。氨基酸特定的线性序列（一级结构）决定了多肽链将如何折叠成具有功能性的构象，最终形成酶、抗体、转运蛋白和结构组分。
8. Levels of Protein Structure | 蛋白质结构的层次
The primary structure is the linear sequence of amino acids in a polypeptide chain. Secondary structure refers to local folding patterns such as α‑helices and β‑pleated sheets, stabilised by hydrogen bonds between the backbone –NH and –CO groups. Tertiary structure is the overall three‑dimensional folding of a single polypeptide, maintained by interactions between R groups including hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges.
一级结构是多肽链中氨基酸的线性序列。二级结构指局部折叠模式，如α‑螺旋和β‑折叠片，它们由主链–NH和–CO基团之间的氢键稳定。三级结构是单条多肽链的整体三维折叠，由R基团之间的相互作用维持，包括氢键、离子键、疏水作用和二硫桥。
Quaternary structure arises when two or more polypeptide chains (subunits) assemble to form a functional protein. Haemoglobin, for instance, is a tetramer of two α‑globin and two β‑globin chains. The precise assembly of subunits is vital for cooperative oxygen binding. The hierarchy of protein structure illustrates how a simple linear polymer can give rise to immensely complex and specific molecular machines.
当两条或多条多肽链（亚基）组装形成功能性蛋白质时，便产生了四级结构。例如，血红蛋白是由两条α‑珠蛋白链和两条β‑珠蛋白链组成的四聚体。亚基的精确组装对于氧气的协同结合至关重要。蛋白质结构的分层体系展示了如何由简单的线性聚合物产生极其复杂和特异的分子机器。
9. Nucleic Acids: Nucleotides as Monomers | 核酸：核苷酸作为单体
Nucleic acids (DNA and RNA) are polymers of nucleotides. Each nucleotide consists of three components: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. The bases are divided into purines (adenine and guanine) and pyrimidines (cytosine, thymine in DNA, uracil in RNA).
核酸（DNA和RNA）是由核苷酸组成的聚合物。每个核苷酸由三部分组成：一个戊糖（DNA中为脱氧核糖，RNA中为核糖）、一个磷酸基团和一个含氮碱基。碱基分为嘌呤（腺嘌呤和鸟嘌呤）以及嘧啶（胞嘧啶，DNA中为胸腺嘧啶，RNA中为尿嘧啶）。
Nucleotides are joined by condensation reactions that create phosphodiester bonds between the phosphate group of one nucleotide and the hydroxyl group on the 3' carbon of the sugar of another. This forms a sugar–phosphate backbone with the bases projecting to the side. The directional chain runs from 5' to 3', and this polarity is crucial for replication and transcription.
核苷酸通过缩合反应连接在一起，反应在一个核苷酸的磷酸基团与另一个核苷酸糖的3'碳上的羟基之间形成磷酸二酯键。这样就形成了糖-磷酸骨架，碱基伸出到一侧。链具有方向性，从5'到3'，这种极性对于复制和转录至关重要。
10. DNA and RNA: Information Polymers | DNA与RNA：信息聚合物
DNA typically consists of two antiparallel polynucleotide strands wound into a double helix. The strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine (A–T) via two hydrogen bonds, and cytosine pairs with guanine (C–G) via three hydrogen bonds. This base‑pairing specificity allows faithful replication of the genetic code.
DNA通常由两条反向平行的多核苷酸链缠绕成双螺旋结构。这两条链通过互补碱基对之间的氢键维系在一起：腺嘌呤与胸腺嘧啶（A–T）通过两个氢键配对，胞嘧啶与鸟嘌呤（C–G）通过三个氢键配对。这种碱基配对的特异性确保了遗传密码的精确复制。
RNA is usually single‑stranded and contains ribose and uracil instead of thymine. Messenger RNA (mRNA) carries the genetic blueprint from DNA to ribosomes; transfer RNA (tRNA) brings specific amino acids during translation; and ribosomal RNA (rRNA) forms the catalytic core of ribosomes. All these polymers are synthesised by condensation of nucleotide triphosphates.
RNA通常是单链的，含有核糖以及尿嘧啶而非胸腺嘧啶。信使RNA（mRNA）将遗传蓝图从DNA携带到核糖体；转运RNA（tRNA）在翻译过程中携带特定的氨基酸；核糖体RNA（rRNA）构成核糖体的催化核心。所有这些聚合物都是由核苷三磷酸的缩合反应合成的。
11. Lipids: Macromolecules but Not Polymers | 脂质：是大分子但非聚合物
Lipids are a diverse group of hydrophobic macromolecules that are not built from repeating monomer units. Triglycerides, the primary energy storage lipids, are formed by esterification of one glycerol molecule with three fatty acids. This condensation reaction produces three ester bonds and releases three water molecules, but the product is not considered a polymer because it lacks a repeating monomer structure.
脂质是一类多样的疏水性大分子，它们并非由重复的单体单元构成。甘油三酯是最主要的储能脂质，通过一个甘油分子与三个脂肪酸的酯化反应形成。这个缩合反应产生三个酯键并释放三分子水，但该产物不被视为聚合物，因为它缺乏重复的单体结构。
Phospholipids, composed of glycerol, two fatty acids, a phosphate group and a polar head group, are major components of cell membranes. Steroids such as cholesterol have a four‑ring structure completely unlike the glycerol‑based lipids. Although all these lipids are macromolecules, their synthesis does not follow the polymerisation model seen in carbohydrates, proteins and nucleic acids.
磷脂由甘油、两分子脂肪酸、一个磷酸基团和一个极性头部组成，是细胞膜的主要成分。类固醇如胆固醇具有四环结构，完全不同于基于甘油的脂质。尽管所有这些脂质都是大分子，它们的合成并不遵循在碳水化合物、蛋白质和核酸中见到的聚合模式。
12. Comparing Biological Macromolecules | 生物大分子的比较
A clear comparison of the four major classes of biological macromolecules helps consolidate their key features. While carbohydrates, proteins and nucleic acids are true polymers with distinct monomers, lipids stand apart as non‑polymeric macromolecules. The table below summarises their properties.
对四大类生物大分子进行清晰比较有助于巩固其关键特征。碳水化合物、蛋白质和核酸是具有明确单体的真正聚合物，而脂质则是非聚合型大分子。下表演总结了它们的性质。
| Macromolecule 大分子 |
Monomer 单体 |
Bond Type 键型 |
Polymer? 是否为聚合物？ |
Major Function 主要功能 |
|---|---|---|---|---|
| Carbohydrate 碳水化合物 |
Monosaccharide (e.g. glucose) 单糖（如葡萄糖） |
Glycosidic bond 糖苷键 |
Yes 是 |
Energy storage, structure 能量储存，结构 |
| Protein 蛋白质 |
Amino acid 氨基酸 |
Peptide bond 肽键 |
Yes 是 |
Catalysis, transport, structure, signalling 催化，运输，结构，信号传导 |
| Nucleic acid 核酸 |
Nucleotide 核苷酸 |
Phosphodiester bond 磷酸二酯键 |
Yes 是 |
Genetic information storage and transfer 遗传信息储存与传递 |
| Lipid 脂质 |
No monomer (glycerol + fatty acids) 无单体（甘油+脂肪酸） |
Ester bond 酯键 |
No 否 |
Energy storage, membrane components, insulation 能量储存，膜组分，保温 |
The unifying theme across all these macromolecules is that their structure is intimately linked to their function. Alterations in monomer sequence, bond type or three‑dimensional conformation can have dramatic biological consequences. This structure‑function relationship is a central principle of molecular biology.
贯穿所有这些大分子的统一主题是，它们的结构与其功能紧密相关。单体序列、键型或三维构象的改变都可能产生巨大的生物学影响。这种结构-功能关系是分子生物学的核心原则。
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