📚 Monomers, Polymers and Biological Macromolecules | 单体、聚合物与生物大分子
In biology, life is built from a surprisingly small set of small molecules called monomers. These monomers join together in long chains to form polymers, which in turn assemble into the four major classes of biological macromolecules: carbohydrates, proteins, nucleic acids, and lipids. Understanding the rules of monomer–polymer assembly is essential for explaining how cells store energy, transmit information, and perform thousands of chemical reactions.
在生物学中，生命由一小类被称为“单体”的小分子构建而成。这些单体通过连接成长链形成“聚合物”，进而组装成四类主要的生物大分子：碳水化合物、蛋白质、核酸和脂质。理解单体—聚合物的组装规则，是解释细胞如何储存能量、传递信息以及完成成千上万种化学反应的基础。
1. What Are Monomers, Polymers and Biological Macromolecules? | 什么是单体、聚合物与生物大分子？
A monomer is a small, repeating unit that can covalently bond to other identical or similar monomers. A polymer is a large molecule made of many monomers linked together in a chain. The term “biological macromolecule” usually refers to a very large molecule present in living organisms, often built from monomers but not exclusively so.
单体是一种小分子重复单元，能够与其它相同或相似的单体形成共价键。聚合物是由许多单体连接成链的大分子。“生物大分子”通常指生物体内存在的大分子，它们大多由单体构成，但也有例外。
- Monomer / 单体的例子： glucose, amino acid, nucleotide
- Polymer / 聚合物的例子： starch, protein, DNA
- Biological macromolecule / 生物大分子： carbohydrates, proteins, nucleic acids, lipids
Note that lipids are biological macromolecules but are not true polymers, because they are not made of repeating monomer units joined by condensation.
注意：脂质是生物大分子，但并不是真正的聚合物，因为它们不是由重复单体通过缩合反应连接而成的。
2. Key Types of Monomers and Their Macromolecules | 关键单体类型及其对应的大分子
Different classes of biological macromolecules use different monomers. Each type of monomer has characteristic functional groups that determine the chemistry of the resulting polymer.
不同类别的生物大分子使用不同的单体。每种单体具有特有的官能团，这些官能团决定了所形成聚合物的化学性质。
| Macromolecule 大分子 |
Monomer(s) 单体 |
Linkage / Bond 连接键 |
|---|---|---|
| Carbohydrate 碳水化合物 |
Monosaccharide (e.g. glucose) 单糖（如葡萄糖） |
Glycosidic bond 糖苷键 |
| Protein 蛋白质 |
Amino acid 氨基酸 |
Peptide bond 肽键 |
| Nucleic acid 核酸 |
Nucleotide 核苷酸 |
Phosphodiester bond 磷酸二酯键 |
| Lipid (not a polymer) 脂质（非聚合物） |
Fatty acids + glycerol 脂肪酸 + 甘油 |
Ester bond 酯键 |
3. Condensation Reactions: Building Polymers | 缩合反应：构建聚合物
Most biological polymers are formed by condensation reactions. In a condensation reaction, two monomers become covalently linked, and a water molecule is released. This process requires energy and is catalysed by specific enzymes.
大多数生物聚合物通过缩合反应形成。在缩合反应中，两个单体共价连接，同时释放一个水分子。该过程需要能量，并由特定酶催化。
monomer + monomer → polymer + H₂O
For example, when two glucose molecules join together, a glycosidic bond forms and one water molecule is produced.
例如，当两个葡萄糖分子连接时，形成一个糖苷键并产生一个水分子。
4. Hydrolysis Reactions: Breaking Polymers | 水解反应：分解聚合物
The reverse of condensation is hydrolysis. In hydrolysis, a water molecule is used to break a covalent bond in a polymer, separating it into monomers or smaller units. This reaction releases energy when bonds are broken, but in practice the energy released from the chemical bonds is captured in metabolic pathways.
缩合的逆过程是水解。在水解中，一个水分子被用于打断聚合物中的共价键，使其分解为单体或更小的单元。该反应在断键时会释放能量，但实际上释放的化学能会在代谢途径中被捕获利用。
polymer + H₂O → monomer + monomer
Hydrolysis is essential for digestion. For example, starch in food is hydrolysed into glucose in the small intestine. Similarly, proteins are hydrolysed into amino acids.
水解对消化至关重要。例如，食物中的淀粉在小肠中被水解为葡萄糖。同样，蛋白质被水解为氨基酸。
5. Carbohydrates: Monosaccharides, Disaccharides and Polysaccharides | 碳水化合物：单糖、二糖和多糖
Carbohydrates are composed of monosaccharides, which are simple sugars with the general formula (CH₂O)ₙ, where n is typically 3–7. Common monosaccharides include glucose (C₆H₁₂O₆), fructose and ribose.
碳水化合物由单糖组成。单糖是化学通式为 (CH₂O)ₙ 的简单糖，其中 n 通常为 3–7。常见的单糖包括葡萄糖（C₆H₁₂O₆）、果糖和核糖。
Two monosaccharides combine via condensation to form a disaccharide. For example, glucose + fructose → sucrose + H₂O. The bond formed is an α-1,2-glycosidic bond in sucrose.
两个单糖通过缩合反应形成二糖。例如，葡萄糖 + 果糖 → 蔗糖 + H₂O。在蔗糖中形成的键是 α-1,2-糖苷键。
- Monosaccharide: one sugar unit (e.g. glucose, galactose)
- Disaccharide: two sugar units (e.g. maltose = glucose + glucose; sucrose = glucose + fructose)
- Polysaccharide: many sugar units (e.g. starch, glycogen, cellulose)
Polysaccharides are formed by repeated condensation reactions. Starch and glycogen are storage polysaccharides with α-glucose units, while cellulose is a structural polysaccharide with β-glucose units. The differences in glycosidic bonds (α-1,4 vs β-1,4) lead to very different shapes and properties.
多糖通过多次缩合反应形成。淀粉和糖原是储存多糖，由 α-葡萄糖单元构成；纤维素是结构多糖，由 β-葡萄糖单元构成。糖苷键的差异（α-1,4 和 β-1,4）导致它们具有截然不同的形状和性质。
6. Proteins: Amino Acids and Peptide Bonds | 蛋白质：氨基酸与肽键
Amino acids are the monomers of proteins. Each amino acid has an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R group attached to the same α-carbon. About 20 common amino acids occur in proteins.
氨基酸是蛋白质的单体。每个氨基酸含有一个氨基（-NH₂）、一个羧基（-COOH）、一个氢原子和一个可变 R 基团，它们都连接在同一个 α-碳上。蛋白质中大约有 20 种常见氨基酸。
Two amino acids join by condensation between the carboxyl group of one amino acid and the amino group of another, forming a peptide bond (‑CO‑NH‑) and releasing water.
两个氨基酸通过缩合反应连接：一个氨基酸的羧基与另一个氨基酸的氨基反应，形成肽键（‑CO‑NH‑）并释放水。
amino acid + amino acid → dipeptide + H₂O
Long chains of amino acids are called polypeptides. A protein may contain one or more polypeptide chains folded into specific three-dimensional structures. The sequence of amino acids determines the protein’s primary structure, which in turn dictates higher levels of folding.
氨基酸长链称为多肽。一个蛋白质可能含有一条或多条折叠成特定三维结构的多肽链。氨基酸序列决定了蛋白质的一级结构，而一级结构又决定了更高层次的折叠方式。
7. Nucleic Acids: Nucleotides and Phosphodiester Bonds | 核酸：核苷酸与磷酸二酯键
Nucleotides are the monomers of nucleic acids (DNA and RNA). Each nucleotide contains three components: a nitrogenous base (A, G, C, T or U), a pentose sugar (ribose in RNA, deoxyribose in DNA), and a phosphate group.
核苷酸是核酸（DNA 和 RNA）的单体。每个核苷酸包含三个部分：含氮碱基（A、G、C、T 或 U）、五碳糖（RNA 中是核糖，DNA 中是脱氧核糖）和一个磷酸基团。
When nucleotides join together, condensation occurs between the phosphate group of one nucleotide and the sugar of the next, forming a phosphodiester bond. This creates a sugar-phosphate backbone with a nitrogenous base protruding from each sugar.
当核苷酸连接时，一个核苷酸的磷酸基团与另一个核苷酸的糖之间发生缩合，形成磷酸二酯键。这产生了糖-磷酸骨架，每个糖上突出一个含氮碱基。
nucleotide + nucleotide → dinucleotide + H₂O
The sequence of bases along a nucleic acid encodes genetic information. In DNA, two antiparallel polynucleotide strands are held together by hydrogen bonds between complementary base pairs (A–T and G–C).
核酸中碱基的序列编码遗传信息。在 DNA 中，两条反向平行的多核苷酸链通过互补碱基对（A–T 和 G–C）之间的氢键结合在一起。
8. Lipids: Macromolecules but Not Polymers | 脂质：大分子但非聚合物
Lipids are a diverse group of biological macromolecules, including fats, oils, phospholipids and steroids. They are not made of repeating monomers, so they are not polymers.
脂质是一类多样的生物大分子，包括脂肪、油、磷脂和类固醇。它们不由重复单体构成，因此不属于聚合物。
Triglycerides (fats and oils) are formed by condensation between one glycerol molecule and three fatty acids. Each fatty acid joins to glycerol via an ester bond, releasing three water molecules in total.
甘油三酯（脂肪和油）由一个甘油分子和三个脂肪酸通过缩合形成。每个脂肪酸与甘油通过酯键连接，总共释放三个水分子。
glycerol + 3 fatty acids → triglyceride + 3H₂O
The hydrophobic nature of lipids arises from the long hydrocarbon tails of fatty acids. This property is crucial for forming cell membranes (phospholipid bilayers) and for energy storage.
脂质的疏水性来自脂肪酸的长烃链尾部。这一特性对于形成细胞膜（磷脂双分子层）和储存能量至关重要。
9. Structure Is Related to Function in Biological Polymers | 生物聚合物中结构与功能的关联
In A-Level Biology, you must be able to explain how the chemical properties of monomers lead to the functional properties of polymers. For example, the β-1,4-glycosidic bonds in cellulose produce straight, unbranched chains that hydrogen-bond with each other to form strong fibres, suitable for plant cell walls.
在 A-Level 生物学中，你必须能够解释单体的化学性质如何导致聚合物的功能特性。例如，纤维素中的 β-1,4-糖苷键产生直链且无分支的链，链间通过氢键结合形成坚固的纤维，适合作为植物细胞壁。
- Starch: α-glucose polymer; coiled, compact; easily hydrolysed for energy storage.
- Glycogen: highly branched; rapid release of glucose; energy storage in animals.
- Cellulose: β-glucose polymer; straight chains; tensile strength for structural support.
- Proteins: unique amino acid sequences create diverse structures, enabling enzymes, antibodies, transporters, etc.
- DNA: stable double helix; base sequence stores genetic information; hydrogen bonds allow unzipping for replication.
When revising, always link each polymer’s monomer type, bonding, and shape to its role in the organism.
复习时，始终将每种聚合物的单体类型、连接键和形状与其在生物体中的作用联系起来。
10. Common Exam Focus and Summary | 常见考点与总结
In CIE A-Level Biology, the key concepts are often tested in short-answer questions about defining condensation and hydrolysis, identifying monomers from diagrams, comparing starch and cellulose, and describing the formation of peptide and glycosidic bonds.
在 CIE A-Level 生物学中，常见考点包括：定义缩合和水解、从图中识别单体、比较淀粉和纤维素、描述肽键和糖苷键的形成等。
- Know the definitions: monomer, polymer, condensation, hydrolysis.
- Recognise the general structure of a monosaccharide, amino acid and nucleotide.
- Be able to draw and label a glycosidic bond, peptide bond and phosphodiester bond.
- Understand why lipids are not polymers.
- Relate monomer linkage to macroscopic properties (e.g. cellulose and glycogen).
A quick comparison of the three major polymeric classes can help memorisation:
快速比较三大类聚合物有助于记忆：
| Polymer 聚合物 | Monomer 单体 | Bond 连接键 | Water released per linkage 每个连接释放水分子数 |
|---|---|---|---|
| Polysaccharide 多糖 | Monosaccharide 单糖 | Glycosidic 糖苷键 | 1 H₂O |
| Polypeptide 多肽 | Amino acid 氨基酸 | Peptide 肽键 | 1 H₂O |
| Polynucleotide 多核苷酸 | Nucleotide 核苷酸 | Phosphodiester 磷酸二酯键 | 1 H₂O |
All three polymer types are formed by condensation reactions and broken down by hydrolysis.
所有三类聚合物都通过缩合反应形成，并通过水解反应分解。
Mastering this core concept allows you to answer questions about monosaccharides, amino acids, nucleotides, and their polymeric forms with confidence. Remember: one monomer unit, one functional group, one bond, and one water molecule — this repeating logic underpins all biological macromolecules.
掌握这一核心概念，你就能自信地回答关于单糖、氨基酸、核苷酸及其聚合物形式的问题。记住：一个单体单元、一个官能团、一个化学键和一个水分子——这一重复逻辑支撑着所有生物大分子。
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