📚 Monomers, Polymers and Macromolecules | 单体、聚合物与大分子
In A-Level Biology, many of the molecules that make up living organisms are large and complex, yet they are built from relatively simple repeating units. Understanding the relationship between monomers, polymers and macromolecules is fundamental to topics such as biological molecules, enzymes, genetics and cell structure.
在 A-Level 生物中，构成生物体的许多分子大而复杂，但它们是由相对简单的重复单元构建而成的。理解单体、聚合物和大分子之间的关系，是生物分子、酶、遗传学和细胞结构等主题的基础。
1. What Are Monomers and Polymers? | 什么是单体和聚合物
A monomer is a small, single molecule that can be joined together with many similar or identical units to form a larger molecule. A polymer is a long chain made up of many repeating monomer units linked by covalent bonds. Macromolecules are very large polymers, often containing hundreds or thousands of atoms, such as polysaccharides, proteins and nucleic acids.
单体是一种小分子，可以与许多相似或相同的单元连接形成更大的分子。聚合物是由许多重复的单体单元通过共价键连接而成的长链。大分子是非常大的聚合物，通常含有成百上千个原子，例如多糖、蛋白质和核酸。
- Monosaccharides are the monomers of carbohydrates. 单糖是碳水化合物的单体。
- Amino acids are the monomers of proteins. 氨基酸是蛋白质的单体。
- Nucleotides are the monomers of nucleic acids. 核苷酸是核酸的单体。
2. Condensation and Hydrolysis Reactions | 缩合与水解反应
Polymers are formed by condensation reactions, in which two monomers join together and a water molecule is removed. The bond formed is a covalent bond, such as a glycosidic bond, peptide bond or phosphodiester bond. Polymers are broken down by hydrolysis reactions, in which a water molecule is added to split the bond and release the monomers.
聚合物通过缩合反应形成，即两个单体连接在一起并脱去一个水分子。形成的键是共价键，例如糖苷键、肽键或磷酸二酯键。聚合物通过水解反应分解，即加入一个水分子使键断裂并释放单体。
monomer–OH + HO–monomer → monomer–monomer + H₂O
单体–OH + HO–单体 → 单体–单体 + H₂O（缩合反应）
The reverse reaction, hydrolysis, uses water to break the covalent bond between monomers.
逆反应即水解反应，利用水来断裂单体之间的共价键。
3. Carbohydrates: Monosaccharides | 碳水化合物：单糖
Carbohydrates contain carbon, hydrogen and oxygen, usually in the ratio Cₓ(H₂O)ᵧ. Monosaccharides are the simplest carbohydrates, with the general formula (CH₂O)ₙ where n is 3 to 7. Common examples include glucose, fructose and galactose, all with the formula C₆H₁₂O₆. Glucose exists as two isomers: α-glucose and β-glucose, which differ in the position of the -OH group on carbon 1.
碳水化合物含有碳、氢和氧，通常比例为 Cₓ(H₂O)ᵧ。单糖是最简单的碳水化合物，通式为 (CH₂O)ₙ，其中 n 为 3 到 7。常见例子包括葡萄糖、果糖和半乳糖，分子式均为 C₆H₁₂O₆。葡萄糖存在两种异构体：α-葡萄糖和 β-葡萄糖，它们碳 1 位上 -OH 基团的位置不同。
In α-glucose the hydroxyl group on carbon 1 lies below the plane of the ring, while in β-glucose it lies above the plane. This small difference has major consequences for the structure and function of the polysaccharides they form.
在 α-葡萄糖中，碳 1 上的羟基位于环平面下方，而在 β-葡萄糖中位于环平面上方。这个微小的差异对它们形成的多糖的结构和功能产生重大影响。
4. Disaccharides and Glycosidic Bonds | 双糖与糖苷键
Two monosaccharides can join by a condensation reaction to form a disaccharide, and the covalent bond formed is called a glycosidic bond. For example, maltose is formed from two α-glucose molecules linked by an α-1,4-glycosidic bond. Sucrose is formed from glucose and fructose, and lactose is formed from glucose and galactose. All three are reducing sugars except sucrose, because its monomers are linked through their anomeric carbons, preventing the ring from opening.
两个单糖可以通过缩合反应形成双糖，所形成的共价键称为糖苷键。例如，麦芽糖由两个 α-葡萄糖分子通过 α-1,4-糖苷键连接而成。蔗糖由葡萄糖和果糖形成，乳糖由葡萄糖和半乳糖形成。这三种双糖中除蔗糖外都是还原糖，因为蔗糖的单体通过异头碳连接，阻止了环的打开。
| Disaccharide 双糖 |
Monomers 单体 |
Bond / Notes 键 / 备注 |
|---|---|---|
| Maltose 麦芽糖 | α-glucose + α-glucose | α-1,4-glycosidic bond α-1,4-糖苷键 |
| Sucrose 蔗糖 | α-glucose + fructose | Non-reducing sugar 非还原糖 |
| Lactose 乳糖 | β-galactose + α-glucose | Reducing sugar 还原糖 |
5. Polysaccharides: Starch, Glycogen and Cellulose | 多糖：淀粉、糖原和纤维素
Polysaccharides are polymers of many monosaccharides joined by glycosidic bonds. Starch is the main energy storage carbohydrate in plants and consists of amylose and amylopectin. Amylose is a straight chain of α-glucose with α-1,4 bonds, coiled into a helix; amylopectin is branched with α-1,4 and some α-1,6 bonds. Glycogen is the animal storage carbohydrate, similar to amylopectin but more highly branched. Cellulose is a structural polysaccharide in plant cell walls, made of β-glucose units linked by β-1,4-glycosidic bonds, forming straight chains that hydrogen-bond to each other to form strong microfibrils.
多糖是由许多单糖通过糖苷键连接而成的聚合物。淀粉是植物中主要的储能碳水化合物，由直链淀粉和支链淀粉组成。直链淀粉是 α-葡萄糖通过 α-1,4 键连接成的直链，盘绕成螺旋；支链淀粉具有 α-1,4 键和一些 α-1,6 键的分支。糖原是动物的储能碳水化合物，与支链淀粉相似但分支更多。纤维素是植物细胞壁中的结构多糖，由 β-葡萄糖单元通过 β-1,4-糖苷键连接，形成直链，链之间通过氢键形成强韧的微纤维。
- Starch is compact and insoluble, making it ideal for storage in plant cells. 淀粉结构紧凑且不溶于水，非常适合在植物细胞中储存。
- Glycogen has many branches, allowing rapid hydrolysis to release glucose for respiration. 糖原分支多，可以快速水解释放葡萄糖用于呼吸作用。
- Cellulose chains form strong fibres that provide structural support to plant cell walls. 纤维素链形成坚韧的纤维，为植物细胞壁提供结构支撑。
6. Proteins: Amino Acids and Peptide Bonds | 蛋白质：氨基酸与肽键
Proteins are polymers made from amino acid monomers. Each amino acid has a central carbon atom bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom and a variable R group. There are 20 different amino acids commonly found in proteins, and their R groups determine the properties of each amino acid. During protein synthesis, amino acids join by condensation reactions, forming peptide bonds between the carboxyl group of one amino acid and the amino group of another. A chain of many amino acids is called a polypeptide.
蛋白质是由氨基酸单体组成的聚合物。每个氨基酸都有一个中心碳原子，连接着氨基（-NH₂）、羧基（-COOH）、一个氢原子和一个可变的 R 基团。蛋白质中常见 20 种不同的氨基酸，它们的 R 基团决定了每种氨基酸的性质。在蛋白质合成过程中，氨基酸通过缩合反应连接，在一个氨基酸的羧基和另一个氨基酸的氨基之间形成肽键。由许多氨基酸组成的链称为多肽。
H₂N–CHR–COOH
氨基酸通式：H₂N–CHR–COOH
The R group can be polar, non-polar, acidic or basic, which influences how the polypeptide folds and how the protein functions.
R 基团可以是极性的、非极性的、酸性的或碱性的，这会影响多肽的折叠方式以及蛋白质的功能。
7. Protein Structure Levels | 蛋白质结构层次
The function of a protein depends on its precise three-dimensional shape, which is described at four levels. Primary structure is the sequence of amino acids in the polypeptide chain. Secondary structure is the folding of the chain into α-helices and β-pleated sheets, held by hydrogen bonds. Tertiary structure is the overall 3D shape of a single polypeptide, maintained by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions. Quaternary structure occurs when two or more polypeptide chains associate, as in haemoglobin.
蛋白质的功能取决于其精确的三维形状，这可以用四个层次来描述。一级结构是多肽链中氨基酸的序列。二级结构是链折叠成 α-螺旋和 β-折叠片，由氢键维持。三级结构是单条多肽的整体三维形状，由氢键、离子键、二硫键和疏水相互作用维持。四级结构出现在两条或多条多肽链结合时，例如血红蛋白。
- Primary: sequence of amino acids. 一级结构：氨基酸序列。
- Secondary: α-helix and β-pleated sheet. 二级结构：α-螺旋和 β-折叠片。
- Tertiary: overall 3D folding of one chain. 三级结构：单条链的整体三维折叠。
- Quaternary: association of multiple chains. 四级结构：多条链的结合。
8. Lipids: Triglycerides and Phospholipids | 脂质：甘油三酯与磷脂
Lipids are not polymers in the strict sense because they are not made from repeating monomers, but they are still important biological macromolecules. Triglycerides are formed by condensation reactions between one glycerol molecule and three fatty acids, producing three ester bonds. They are used for energy storage, insulation and protection. Phospholipids are similar but one fatty acid is replaced by a phosphate group, giving the molecule a hydrophilic head and two hydrophobic tails. This amphipathic nature is essential for the formation of cell membranes.
脂质严格来说不是聚合物，因为它们不是由重复单体组成的，但它们仍然是重要的生物大分子。甘油三酯由一个甘油分子和三个脂肪酸通过缩合反应形成，产生三个酯键。它们用于能量储存、绝缘和保护。磷脂类似，但一个脂肪酸被磷酸基团取代，使分子具有一个亲水头部和两条疏水尾部。这种两亲性对于细胞膜的形成至关重要。
Fatty acids can be saturated or unsaturated; unsaturated fatty acids contain one or more carbon-carbon double bonds, which create bends in the hydrocarbon chain and affect membrane fluidity.
脂肪酸可以是饱和的或不饱和的；不饱和脂肪酸含有一个或多个碳碳双键，这会在烃链中产生弯曲并影响膜的流动性。
9. Nucleic Acids: Nucleotides and Phosphodiester Bonds | 核酸：核苷酸与磷酸二酯键
Nucleic acids such as DNA and RNA are polymers made from nucleotide monomers. Each nucleotide consists of a pentose sugar, a nitrogenous base and one or more phosphate groups. In DNA the sugar is deoxyribose; in RNA it is ribose. Nucleotides join by condensation reactions between the phosphate group of one nucleotide and the hydroxyl group on the sugar of another, forming a phosphodiester bond. This creates a sugar-phosphate backbone with bases projecting from it.
DNA 和 RNA 等核酸是由核苷酸单体组成的聚合物。每个核苷酸由一个五碳糖、一个含氮碱基和一个或多个磷酸基团组成。在 DNA 中糖是脱氧核糖；在 RNA 中是核糖。核苷酸通过缩合反应连接，一个核苷酸的磷酸基团与另一个核苷酸糖上的羟基之间形成磷酸二酯键。这形成了糖-磷酸骨架，碱基从骨架上伸出。
The chain has a 5′ end and a 3′ end, and nucleotides are always added to the 3′ end during synthesis, so nucleic acids grow in the 5′ to 3′ direction.
该链有一个 5′ 端和一个 3′ 端，核苷酸在合成过程中总是添加到 3′ 端，因此核酸沿 5′ 到 3′ 方向延伸。
10. DNA and RNA Polymers | DNA 和 RNA 聚合物
DNA is a double-stranded polymer in which two polynucleotide chains run antiparallel to each other and are held together by hydrogen bonds between complementary base pairs: adenine with thymine, and cytosine with guanine. The sequence of bases carries genetic information. RNA is usually single-stranded and contains uracil instead of thymine. Messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA) play roles in protein synthesis.
DNA 是一种双链聚合物，两条多核苷酸链反向平行排列，通过互补碱基对之间的氢键结合在一起：腺嘌呤与胸腺嘧啶配对，胞嘧啶与鸟嘌呤配对。碱基序列携带遗传信息。RNA 通常是单链的，含有尿嘧啶而不是胸腺嘧啶。信使 RNA（mRNA）、转运 RNA（tRNA）和核糖体 RNA（rRNA）在蛋白质合成中发挥作用。
In DNA, the two strands are not identical but complementary, which allows accurate replication and transcription of the genetic code.
在 DNA 中，两条链不是相同的而是互补的，这使得遗传密码能够准确复制和转录。
11. Comparison of Major Macromolecules | 主要大分子的比较
It is useful to compare the four major groups of biological macromolecules in terms of their monomers, bonds, examples and functions. The table below summarises key points for revision.
比较四大类生物大分子的单体、键、例子和功能非常有用。下表总结了复习要点。
| Class 类别 |
Monomer 单体 |
Bond 键 |
Examples 例子 |
Main Functions 主要功能 |
|---|---|---|---|---|
| Carbohydrates 碳水化合物 |
Monosaccharides 单糖 |
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