📚 Biochemical Polymers | 生物化学聚合物
Biochemical polymers are giant molecules synthesised by living organisms through condensation polymerisation. The three main classes – polysaccharides, proteins, and nucleic acids – perform vital functions ranging from energy storage and structural support to catalysis and information transfer. Understanding their monomer units, the bonds that link them, and how their three‑dimensional structures relate to their roles is a core topic in A‑Level Chemistry.
生物化学聚合物是由生物体通过缩聚反应合成的巨大分子。三大类——多糖、蛋白质和核酸——承担着从能量储存、结构支撑到催化和信息传递等关键功能。理解它们的单体单元、连接键以及三维结构如何与其功能相关联，是A-Level化学中的一个核心主题。
1. Monomers and Condensation Polymerisation | 单体与缩聚反应
Biochemical polymers are formed when monomers join with the elimination of a small molecule, usually water. Each monomer must carry at least two functional groups capable of forming covalent bonds with neighbouring units. This type of step‑growth polymerisation creates long chains where the repeating unit has a slightly different composition from the monomer.
生化聚合物是由单体在脱去一个小分子（通常是水）的条件下连接而成。每个单体必须带有至少两个能够与相邻单元形成共价键的官能团。这种逐步增长聚合反应产生长链，其中重复单元的组成与单体略有不同。
The general equation for a condensation polymerisation involving bifunctional monomers can be written as:
涉及双官能团单体的缩聚反应通式可以写成：
n A–X–B → [A–X–B]ₙ + (n−1) H₂O
In polysaccharides the monomers are monosaccharides, in proteins they are amino acids, and in nucleic acids they are nucleotides. The removal of a water molecule each time a new bond is formed accounts for the slight mass difference between the monomer and the polymer repeat unit.
在多糖中单体是单糖，在蛋白质中是氨基酸，在核酸中是核苷酸。每形成一个新键时脱去一个水分子，这解释了单体与聚合物重复单元之间轻微的质量差异。
2. Polysaccharides: Built from Sugars | 多糖：由糖构建
Polysaccharides are polymers of monosaccharides, primarily glucose. Glucose exists as a six‑membered ring in its pyranose form, and the position of the –OH group on carbon‑1 determines whether it is α‑glucose (OH below the ring) or β‑glucose (OH above the ring). This small stereochemical difference has profound structural consequences.
多糖是单糖（主要是葡萄糖）的聚合物。葡萄糖以六元环的吡喃糖形式存在，碳‑1上–OH基团的位置决定了它是α‑葡萄糖（OH在环平面下方）还是β‑葡萄糖（OH在环平面上方）。这一微小的立体化学差异带来了深远的结构影响。
Glycosidic bonds link the sugar units. An α‑1,4‑glycosidic bond forms between carbon‑1 of one α‑glucose and carbon‑4 of the next, while a β‑1,4‑glycosidic bond does the same for β‑glucose units. Branches are introduced through α‑1,6‑glycosidic bonds.
糖苷键连接糖单元。α‑1,4‑糖苷键在一个α‑葡萄糖的碳‑1与下一个的碳‑4之间形成，而β‑1,4‑糖苷键则对β‑葡萄糖单元起同样作用。支链则通过α‑1,6‑糖苷键引入。
The overall polymerisation reaction for starch or cellulose can be summarised as:
淀粉或纤维素的总聚合反应可以概括为：
n C₆H₁₂O₆ → (C₆H₁₀O₅)ₙ + (n−1) H₂O
Because every linkage generates a water molecule, polysaccharides are classic condensation polymers.
因为每个连接产生一个水分子，多糖是典型的缩聚聚合物。
3. Starch: Energy Storage in Plants | 淀粉：植物中的能量储存
Starch is a mixture of two α‑glucose polymers: amylose and amylopectin. Amylose is a linear chain of α‑1,4‑linked glucose units that coils into a helix, while amylopectin is highly branched, having both α‑1,4‑ and α‑1,6‑glycosidic bonds. The compact, helical structure allows starch to be stored in semi‑crystalline granules inside plant cells.
淀粉是两种α‑葡萄糖聚合物的混合物：直链淀粉和支链淀粉。直链淀粉是由α‑1,4‑连接的葡萄糖单元构成的线性链，并卷曲成螺旋；而支链淀粉高度分支，同时含有α‑1,4‑和α‑1,6‑糖苷键。紧凑的螺旋结构使淀粉能以半结晶颗粒的形式储存在植物细胞内。
When iodine solution is added to starch, iodine molecules fit inside the amylose helix, producing a deep blue‑black colour. This test is used to detect starch in the laboratory. Animals digest starch using the enzyme amylase, which hydrolyses α‑1,4‑linkages, releasing maltose and eventually glucose for respiration.
当碘溶液加到淀粉上时，碘分子嵌入直链淀粉的螺旋内部，产生深蓝黑色。该试验用于在实验室中检测淀粉。动物利用淀粉酶消化淀粉，该酶水解α‑1,4‑键，释放出麦芽糖并最终生成用于呼吸作用的葡萄糖。
4. Glycogen: The Animal Equivalent of Starch | 糖原：动物体内的"淀粉"
Glycogen serves as the main energy‑storage polysaccharide in animals and fungi. It resembles amylopectin but is even more extensively branched, with α‑1,6‑glycosidic bonds occurring roughly every 8–12 glucose residues. This high degree of branching makes glycogen extremely compact and allows rapid mobilisation of glucose when energy is needed.
糖原是动物和真菌中主要的能量储存多糖。它与支链淀粉相似，但分支程度更大，大约每隔8–12个葡萄糖残基就有一个α‑1,6‑糖苷键。这种高度分支使糖原极为紧凑，并能在需要能量时快速动员葡萄糖。
Glycogen is stored mainly in the liver and muscle cells. The enzymes glycogen phosphorylase and debranching enzyme work together to release glucose‑1‑phosphate, keeping blood sugar levels within a narrow range. The polymerisation reaction is the same condensation process found in starch, but the branching pattern is optimised for metabolic turnover rather than long‑term compact storage.
糖原主要储存在肝脏和肌肉细胞中。糖原磷酸化酶和脱支酶共同作用，释放葡萄糖‑1‑磷酸，使血糖水平保持在狭窄范围内。其聚合反应与淀粉中的缩合过程相同，但分支模式针对代谢周转而非长期紧凑储存进行了优化。
5. Cellulose: Structural Polymer of Plant Cell Walls | 纤维素：植物细胞壁的结构聚合物
Cellulose is a linear polymer of β‑glucose units joined by β‑1,4‑glycosidic bonds. Every other glucose residue is rotated 180° relative to its neighbours, allowing the formation of long, straight chains. These chains align side‑by‑side, held together by extensive intermolecular hydrogen bonds between the –OH groups on adjacent chains.
纤维素是由β‑1,4‑糖苷键连接的β‑葡萄糖单元的线性聚合物。每隔一个葡萄糖残基相对于其相邻单元旋转180°，从而形成长而直的链。这些链并排排列，通过相邻链上–OH基团之间广泛的分子间氢键被固定在一起。
Bundles of about 60–70 cellulose chains form microfibrils, which are embedded in a matrix of hemicellulose and pectin to give plant cell walls tremendous tensile strength. Most organisms lack the enzyme cellulase needed to hydrolyse β‑1,4‑glycosidic bonds, so cellulose is indigestible by humans. However, it provides essential dietary fibre.
大约60–70条纤维素链组成的束形成微纤维，它们镶嵌在半纤维素和果胶的基质中，赋予植物细胞壁巨大的抗张强度。大多数生物缺乏水解β‑1,4‑糖苷键所需的纤维素酶，因此纤维素不能被人类消化。然而，它提供了必需的膳食纤维。
6. Comparing the Major Polysaccharides | 主要多糖的比较
The table below summarises the key features of starch, glycogen, and cellulose.
下表总结了淀粉、糖原和纤维素的关键特征。
| Property / 性质 | Starch / 淀粉 | Glycogen / 糖原 | Cellulose / 纤维素 |
|---|---|---|---|
| Monomer unit / 单体 | α‑Glucose | α‑Glucose | β‑Glucose |
| Glycosidic bonds / 糖苷键 | α‑1,4 and some α‑1,6 | α‑1,4 and extensive α‑1,6 | β‑1,4 only |
| Chain shape / 链形状 | Helical (amylose); branched (amylopectin) | Highly branched, globular | Straight, extended |
| Function / 功能 | Plant energy store | Animal energy store | Structural support in cell walls |
| Digestibility by humans / 人类可消化性 | Yes / 是 | Yes / 是 | No / 否 |
The difference between α‑ and β‑glycosidic linkages is responsible for the dramatic functional divergence: helical, accessible chains for storage versus straight, rigid chains for structure.
α‑和β‑糖苷键的差异导致了功能的巨大差异：螺旋状、易于接近的链用于储存，而笔直、刚性的链用于结构。
7. Proteins: Amino Acid Monomers and Peptide Bonds | 蛋白质：氨基酸单体与肽键
Proteins are condensation polymers of amino acids. Each amino acid has the general structure H₂N–CHR–COOH, where R is one of about 20 different side‑chain groups. The amino group of one amino acid reacts with the carboxyl group of another, releasing a water molecule and forming a peptide link (–CO–NH–).
蛋白质是氨基酸的缩聚聚合物。每个氨基酸的通式为H₂N–CHR–COOH，其中R是大约20种不同侧链基团之一。一个氨基酸的氨基与另一个的羧基反应，释放一个水分子并形成肽键（–CO–NH–）。
When two amino acids join, a dipeptide is produced; chains of many amino acids are called polypeptides. The backbone of the polypeptide consists of the repeating sequence –N–C–C–N–C–C–, with the R groups projecting outwards.
两个氨基酸连接时产生二肽；许多氨基酸连接成的链称为多肽。多肽的主链由重复的–N–C–C–N–C–C–序列组成，R基团向外伸出。
The condensation reaction can be represented as:
缩合反应可表示如下：
H₂N–CHR₁–COOH + H₂N–CHR₂–COOH → H₂N–CHR₁–CO–NH–CHR₂–COOH + H₂O
The peptide bond has partial double‑bond character due to resonance, restricting rotation and influencing the folding of the chain.
由于共振，肽键具有部分双键性质，限制了旋转并影响链的折叠。
8. Protein Structure: Primary to Quaternary | 蛋白质结构：从一级到四级
The primary structure of a protein is the linear sequence of amino acids joined by peptide bonds. This sequence is determined by the genetic code. Any change in the order of amino acids can alter the protein’s overall shape and function, as seen in sickle‑cell anaemia where a single glutamic acid is replaced by valine in haemoglobin.
蛋白质的一级结构是通过肽键连接的氨基酸的线性序列。该序列由遗传密码决定。氨基酸顺序的任何变化都可能改变蛋白质的整体形状和功能，如在镰状细胞贫血中，血红蛋白中的一个谷氨酸被缬氨酸取代。
The secondary structure arises from hydrogen bonds between the –NH and –C=O groups of the peptide backbone. Common patterns are the α‑helix, stabilised by hydrogen bonds parallel to the helix axis, and the β‑ en bonds between adjacent extended strands. Tertiary structure describes the overall three‑ olding of a single polypeptide chain, held together by hydrogen bonds, disulfide bridges (–S–S–), ionic interactions, and hydrophobic effects between R groups.
二级结构源于肽链主链上–NH和–C=O基团之间的氢键。常见的模式是α‑螺旋（由平行于螺旋轴的氢键稳定）和β‑折叠片（由相邻展开链之间的氢键稳定）。三级结构描述单条多肽链的整体三维折叠，由R基团之间的氢键、二硫键（–S–S–）、离子相互作用和疏水效应共同维持。
Quaternary structure exists in proteins composed of more than one polypeptide subunit. Haemoglobin, for example, contains four subunits working cooperatively to bind oxygen. All structural levels are ultimately governed by the order of amino acids in the primary sequence.
四级结构存在于由多个多肽亚基组成的蛋白质中。例如，血红蛋白含有四个亚基，协同工作以结合氧气。所有结构层次最终都受一级序列中氨基酸顺序的控制。
9. Nucleic Acids: DNA and RNA | 核酸：DNA与RNA
Nucleic acids are the information‑storage polymers of the cell. Deoxyribonucleic acid (DNA) holds the genetic blueprint, while ribonucleic acid (RNA) translates that information into proteins and performs catalytic functions. Both are condensation polymers of nucleotide monomers.
核酸是细胞中储存信息的聚合物。脱氧核糖核酸（DNA）承载遗传蓝图，而核糖核酸（RNA）将该信息翻译为蛋白质并执行催化功能。两者都是核苷酸单体的缩聚聚合物。
A nucleotide consists of three components: a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base. The sugar‑phosphate backbone is formed by phosphodiester bonds between the 3′‑hydroxyl group of one sugar and the 5′‑phosphate of the next. The repeated formation of these bonds through condensation reactions generates the long, directional strands with a free 5′ end and a free 3′ end.
核苷酸由三个组分构成：一个磷酸基团、一个五碳糖（DNA中为脱氧核糖，RNA中为核糖）和一个含氮碱基。糖‑磷酸骨架是通过一个糖的3′‑羟基与下一个糖的5′‑磷酸之间的磷酸二酯键形成的。通过这些缩合反应反复形成键，产生了具有游离5′端和游离3′端的长定向链。
DNA is double‑stranded, with the two antiparallel strands held together by complementary base pairing: adenine (A) pairs with thymine (T) via two hydrogen bonds, and cytosine (C) pairs with guanine (G) via three hydrogen bonds. RNA is usually single‑stranded and contains uracil (U) in place of thymine.
DNA是双链的，两条反向平行的链通过互补碱基配对结合在一起：腺嘌呤（A）与胸腺嘧啶（T）通过两个氢键配对，胞嘧啶（C）与鸟嘌呤（G）通过三个氢键配对。RNA通常为单链，并含有尿嘧啶（U）代替胸腺嘧啶。
10. Nucleotides and the Phosphodiester Linkage | 核苷酸与磷酸二酯键
The formation of a phosphodiester bond involves the condensation between the phosphate group attached to the 5′ carbon of one nucleotide and the hydroxyl group on the 3′ carbon of another. Two water molecules are released for every two nucleotides linked in the growing chain, one from the 3′‑OH and another from the phosphate’s –OH group.
磷酸二酯键的形成涉及一个核苷酸5′碳上的磷酸基团与另一个核苷酸3′碳上的羟基之间的缩合。在生长链中每连接两个核苷酸释放两个水分子，一个来自3′‑OH，另一个来自磷酸的–OH基团。
The resulting linkage can be depicted as R–O–P(O)(O⁻)–O–R′, where R and R′ represent the 3′ and 5′ sugar residues. The negative charge on the phosphate group contributes to the overall hydrophilicity of the DNA backbone, which is oriented outward in the double helix while the hydrophobic bases stack inside.
所形成的键可以表示为R–O–P(O)(O⁻)–O–R′，其中R和R′代表3′和5′糖残基。磷酸基团上的负电荷有助于DNA骨架的整体亲水性，该骨架在双螺旋中朝外，而疏水性碱基则堆积在内侧。
In RNA, the presence of the 2′‑OH group on ribose makes the polymer more susceptible to hydrolysis, explaining why RNA is less chemically stable than DNA and why it often functions as a short‑lived messenger.
在RNA中，核糖上2′‑OH基团的存在使该聚合物更易水解，这解释了为什么RNA的化学稳定性不如DNA，以及它为什么通常作为短寿命的信使起作用。
11. Comparison of DNA and RNA | DNA与RNA的比较
| Feature / 特征 | DNA | RNA |
|---|---|---|
| Pentose sugar / 戊糖 | Deoxyribose (no 2′‑OH) | Ribose (has 2′‑OH) |
| Bases / 碱基 | A, T, C, G | A, U, C, G |
| Strands / 链数 | Double helix / 双螺旋 | Usually single‑stranded / 通常单链 |
| Stability / 稳定性 | High / 高 | Lower, prone to hydrolysis / 较低，易水解 |
| Function / 功能 | Long‑term genetic storage / 长期遗传储存 | Protein synthesis, catalysis / 蛋白质合成，催化 |
Both nucleic acids use the same phosphodiester chemistry, but the substitution of uracil for thymine and the presence of the 2′‑OH group in ribose lead to functional specialisation in the cell.
两种核酸使用相同的磷酸二酯化学，但用尿嘧啶替代胸腺嘧啶以及核糖中2′‑OH基团的存在导致了细胞内的功能特化。
12. Biological Roles and Summary | 生物学作用与总结
Biochemical polymers illustrate how controlled condensation reactions assemble small monomers into macromolecules with vastly different properties. Polysaccharides serve as compact fuel reserves (starch, glycogen) or robust structural fibres (cellulose). Proteins fold into precise three‑dimensional shapes that enable catalysis, transport, and signalling. Nucleic acids store and transmit genetic information with remarkable fidelity.
生物化学聚合物展示了受控缩合反应如何将小单体组装成具有截然不同性质的大分子。多糖可作为紧凑的燃料储备（淀粉、糖原）或坚固的结构纤维（纤维素）。蛋白质折叠成精确的三维形状，实现催化、运输和信号传导。核酸以极高的保真度储存和传递遗传信息。
Key revision points for A‑Level Chemistry include recognising the monomers, the type of linkage formed (glycosidic, peptide, phosphodiester), the role of stereochemistry (α vs β glucose, peptide bond planarity), and the relationship between structure and function. Being able to draw simplified representations of these polymers, interpret charts of hydrolysis products, and explain why certain polymers are biodegradable while others persist is essential for success in examinations.
A-Level化学的关键复习要点包括识别单体、形成的键的类型（糖苷键、肽键、磷酸二酯键）、立体化学的作用（α与β葡萄糖，肽键平面性）以及结构与功能的关系。能够绘制这些聚合物的简化表示图、解释水解产物图表，并说明为什么某些聚合物可生物降解而另一些则持久存在，对于考试成功至关重要。
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