📚 Biological Molecules | 生物分子
Biological molecules are the building blocks of life, forming the structural and functional basis of all living organisms. In Cambridge A-Level Biology, understanding these molecules—water, carbohydrates, lipids, proteins, and nucleic acids—is essential for grasping how cells work, how energy is stored and transferred, and how genetic information is passed on. This article provides an in-depth yet accessible coverage of the key biological molecules, their structures, properties, and roles, along with the common biochemical tests used to identify them.
生物分子是生命的基石，构成了所有生物体的结构和功能基础。在剑桥A-Level生物学中，理解这些分子——水、碳水化合物、脂质、蛋白质和核酸——对于掌握细胞如何工作、能量如何储存和转移以及遗传信息如何传递至关重要。本文深入而通达地介绍了关键生物分子、它们的结构、性质和功能，以及用于鉴定它们的常见生化检测。
1. Introduction to Biological Molecules | 生物分子概述
Biological molecules are organic compounds that contain carbon and are synthesised by living organisms. They range from small, simple molecules such as water and monosaccharides to large, complex polymers like proteins and nucleic acids. Most macromolecules are polymers formed by the condensation of smaller monomer units, and they can be broken down by hydrolysis. The main groups are carbohydrates, lipids, proteins, and nucleic acids, each with distinct chemical properties and biological roles.
生物分子是由生物体合成的含碳有机化合物。它们涵盖从小分子（如水和单糖）到大而复杂的聚合物（如蛋白质和核酸）的范围。大多数大分子是由较小的单体通过缩合反应形成的聚合物，并可通过水解分解。主要类别包括碳水化合物、脂质、蛋白质和核酸，每一类都有独特的化学性质和生物学作用。
2. Water: The Solvent of Life | 水：生命之溶剂
Water is a dipolar molecule because oxygen is more electronegative than hydrogen, creating an unequal sharing of electrons. The oxygen atom carries a partial negative charge (δ⁻), while each hydrogen atom has a partial positive charge (δ⁺). This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules, giving water its unique properties. As a result, water is an excellent solvent for ionic and polar substances, a medium for metabolic reactions, a transport agent, and a temperature buffer. Its high specific heat capacity helps organisms maintain stable internal temperatures, while its high latent heat of vaporisation enables effective cooling through sweating or transpiration. Cohesion and surface tension support water transport in plants and provide habitats for small organisms.
水是一种极性分子，因为氧的电负性比氢强，导致电子对分布不均。氧原子带有部分负电荷（δ⁻），而每个氢原子带有部分正电荷（δ⁺）。这种极性使水分子之间以及与其他极性分子之间形成氢键，赋予水独特的性质。因此，水是离子和极性物质的优良溶剂，是代谢反应的介质、运输载体和温度缓冲液。其高比热容有助于生物体维持稳定的内部温度，而高汽化潜热使出汗或蒸腾作用能有效散热。内聚力和表面张力支持植物体内的水分运输，并为小型生物提供栖息环境。
3. Carbohydrates: Monosaccharides | 碳水化合物：单糖
Monosaccharides are the simplest carbohydrates, with the general formula (CH₂O)n, where n ranges from 3 to 7. Glucose, fructose, and galactose are all hexose sugars (C₆H₁₂O₆) but differ in structural arrangement. Glucose exists in two isomeric forms: α-glucose and β-glucose, which differ in the position of the hydroxyl (–OH) group on carbon-1. In α-glucose, the –OH on C1 points downward; in β-glucose, it points upward. This seemingly minor difference has major biological consequences, affecting the properties of the polysaccharides they form. Monosaccharides are reducing sugars because they have a free aldehyde or ketone group that can donate electrons to Benedict’s reagent, causing a colour change upon heating. Their primary role is as a source of energy in respiration, but they also serve as building blocks for larger carbohydrates.
单糖是最简单的碳水化合物，通式为 (CH₂O)n，其中n为3至7。葡萄糖、果糖和半乳糖都是己糖 (C₆H₁₂O₆)，但结构排列不同。葡萄糖存在两种异构形式：α-葡萄糖和β-葡萄糖，它们仅在碳1上羟基 (–OH) 的方向不同。在α-葡萄糖中，C1上的–OH指向下方；而在β-葡萄糖中则指向上方。这种微小的差异具有重大的生物学影响，会影响它们形成的多糖的性质。单糖都是还原糖，因为它们具有游离的醛基或酮基，可以向本尼迪克特试剂提供电子，加热时产生颜色变化。单糖的主要功能是作为呼吸作用的能量来源，但它们也是构建更大碳水化合物的基本单位。
4. Disaccharides and the Glycosidic Bond | 二糖与糖苷键
Disaccharides are formed by a condensation reaction between two monosaccharides, resulting in the formation of a glycosidic bond and the elimination of a water molecule. For example, maltose consists of two α-glucose molecules linked by an α-1,4-glycosidic bond, and it is a reducing sugar because one of the glucose units still has a free aldehyde group. Sucrose (table sugar) is formed from α-glucose and fructose, bonded by an α-1,2-glycosidic bond; since both anomeric carbons are involved in the bond, sucrose is a non-reducing sugar. Lactose, the sugar in milk, is made of β-galactose and glucose linked by a β-1,4-glycosidic bond and is also reducing. The condensation and hydrolysis of disaccharides are catalysed by specific enzymes such as maltase, sucrase, and lactase.
二糖由两个单糖通过缩合反应生成，其间形成糖苷键并脱去一分子水。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，它属于还原糖，因为其中一个葡萄糖单元仍保留游离的醛基。蔗糖（食用糖）由一个α-葡萄糖和一个果糖经α-1,2-糖苷键连接而成；由于两个异头碳均参与了键的形成，蔗糖是非还原糖。乳糖是牛奶中的糖，由β-半乳糖和葡萄糖通过β-1,4-糖苷键连接，也具有还原性。二糖的缩合与水解反应分别由特定的酶（如麦芽糖酶、蔗糖酶和乳糖酶）催化。
5. Polysaccharides: Starch, Glycogen, and Cellulose | 多糖：淀粉、糖原和纤维素
Polysaccharides are long-chain polymers of monosaccharides joined by glycosidic bonds. Starch is the main energy storage carbohydrate in plants and exists in two forms: amylose and amylopectin. Amylose is an unbranched chain of α-glucose units linked by α-1,4 bonds, forming a helical structure compact enough for storage. Amylopectin is branched, with α-1,4 chains and α-1,6 branch points, allowing rapid enzymatic release of glucose. Glycogen is the animal equivalent of starch, found mainly in liver and muscle cells; its structure resembles amylopectin but is more extensively branched, enabling even quicker mobilisation of glucose during intense activity. Cellulose, in contrast, is a structural polysaccharide composed of β-glucose units joined by β-1,4-glycosidic bonds. The alternating orientation of glucose molecules allows long, straight chains to form microfibrils that are held together by hydrogen bonds, imparting high tensile strength to plant cell walls. The difference in the glycosidic bond explains why starch and glycogen are digestible by humans but cellulose is not; we lack the enzyme cellulase required to hydrolyse β-1,4 bonds.
多糖是由糖苷键连接的单糖长链聚合物。淀粉是植物中主要的储能碳水化合物，以两种形式存在：直链淀粉和支链淀粉。直链淀粉是由α-葡萄糖单元通过α-1,4键连接而成的无分支链，形成紧凑的螺旋结构，适合储存。支链淀粉具有分支，通过α-1,4键和α-1,6分支点连接，这使得葡萄糖能被酶迅速释放。糖原相当于动物体内的淀粉，主要存在于肝脏和肌肉细胞中；其结构与支链淀粉相似，但分支更为密集，可在剧烈运动时更快动员葡萄糖。相比之下，纤维素是一种结构多糖，由β-葡萄糖单元通过β-1,4-糖苷键连接而成。葡萄糖分子交替翻转，使长的直链形成微纤维，并由氢键维系，赋予植物细胞壁很高的抗拉强度。糖苷键的不同解释了为什么淀粉和糖原可被人体消化而纤维素不能；我们缺乏水解β-1,4键所需的纤维素酶。
6. Lipids: Triglycerides | 脂质：甘油三酯
Triglycerides are non-polar, hydrophobic molecules composed of one glycerol molecule bonded to three fatty acid chains via ester bonds. The condensation reaction between a carboxyl group of a fatty acid and a hydroxyl group of glycerol releases three water molecules and forms a triester. Fatty acids can be saturated (no double bonds between carbon atoms, solid at room temperature, e.g. animal fats) or unsaturated (one or more C=C double bonds, liquid at room temperature, e.g. plant oils). The presence of double bonds introduces kinks in the hydrocarbon chain, preventing tight packing and lowering the melting point. Triglycerides serve as long-term energy storage in adipose tissue, providing about twice the energy per gram compared to carbohydrates. They also act as thermal insulators and protect delicate organs.Being insoluble in water, they do not disturb the osmotic balance of cells.
甘油三酯是非极性、疏水的分子，由一个甘油分子通过酯键与三条脂肪酸链结合而成。脂肪酸的羧基与甘油的羟基发生缩合反应，释放三分子水，形成三酯。脂肪酸可以是饱和的（碳原子间无双键，室温下为固体，如动物脂肪）或不饱和的（含一个或多个C=C双键，室温下为液体，如植物油）。双键的存在使烃链产生扭结，阻止分子紧密排列，从而降低熔点。甘油三酯作为脂肪组织中的长期能量储存形式，其每克供给的能量约为碳水化合物的两倍。它们还可充当隔热层并保护脆弱器官。由于不溶于水，甘油三酯不会影响细胞的渗透平衡。
7. Phospholipids and Steroids | 磷脂与固醇
A phospholipid is similar to a triglyceride but has one of the three fatty acid chains replaced by a phosphate-containing group. This gives phospholipids a hydrophilic ‘head’ (phosphate group) and two hydrophobic ‘tails’ (fatty acid chains), making them amphipathic. In an aqueous environment, phospholipids spontaneously form a bilayer, with the hydrophilic heads facing outwards and the hydrophobic tails tucked inside. This bilayer is the fundamental structure of all cell membranes, providing a selectively permeable barrier. Steroids are another class of lipids characterised by four fused carbon rings. Cholesterol,an important steroid, sits within the phospholipid bilayer of animal cell membranes, where it modulates membrane fluidity and stability. It is also the precursor for the synthesis of steroid hormones such as oestrogen and testosterone.
磷脂与甘油三酯相似，但其中的一条脂肪酸链被含磷酸基团取代。这使得磷脂具有一个亲水的“头部”（磷酸基团）和两条疏水的“尾部”（脂肪酸链），从而具备两性。在水性环境中，磷脂自发形成双分子层，亲水头部朝外，疏水尾部朝内。这种双分子层是所有细胞膜的基本结构，提供了选择性渗透屏障。固醇是另一类脂质，其特征是含有四个稠合的碳环。胆固醇是一种重要的固醇，存在于动物细胞膜的磷脂双分子层中，调节膜的流动性和稳定性。它也是合成类固醇激素（如雌激素和睾酮）的前体。
8. Proteins: Amino Acids and the Peptide Bond | 蛋白质：氨基酸与肽键
Amino acids are the monomers of proteins. Each amino acid has a central carbon atom (the α-carbon) attached to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group that determines its identity. There are 20 different R groups used in protein synthesis, conferring a wide range of chemical properties. Amino acids join together via condensation reactions, where the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule and forming a covalent peptide bond (–CONH–). A chain of amino acids is called a polypeptide, and it has directionality, with an N-terminus (free amino group) and a C-terminus (free carboxyl group). The sequence of amino acids is genetically determined and defines the protein’s primary structure.
氨基酸是蛋白质的单体。每个氨基酸都有一个中心碳原子（α-碳），分别连接一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子以及一个决定其特性的可变R基团。在蛋白质合成中共使用20种不同的R基团，赋予了蛋白质多种多样的化学性质。氨基酸通过缩合反应连接在一起：一个氨基酸的羧基与另一个氨基酸的氨基反应，脱去一分子水，形成共价肽键（–CONH–）。一条氨基酸链称为多肽，具有一定的方向性，其N端含有游离氨基，C端含有游离羧基。氨基酸序列由遗传信息决定，并决定了蛋白质的一级结构。
9. Levels of Protein Structure | 蛋白质的结构层次
Protein structure is organised into four levels. Primary structure is the linear sequence of amino acids. Secondary structure refers to local folding patterns stabilised by hydrogen bonds between the backbone –NH and –C=O groups, producing α-helices and β-pleated sheets. Tertiary structure is the overall three-dimensional folding of a single polypeptide chain, maintained by interactions between R groups: hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges (covalent S–S bonds between cysteine residues). Quaternary structure exists in proteins made of more than one polypeptide chain, such as haemoglobin, which consists of four subunits (two alpha and two beta chains) held together by the same types of bonds as tertiary structure, along with a haem prosthetic group. A protein’s function depends critically on its precise three-dimensional shape, and even minor changes in pH or temperature can cause denaturation, where the protein loses its structure and function without breaking peptide bonds.
蛋白质结构可分为四个层次。一级结构是氨基酸的线性序列。二级结构是指多肽主链上的–NH 和–C=O 基团之间由氢键稳定的局部折叠模式，形成α-螺旋和β-折叠片。三级结构是单一多肽链的整体三维折叠，由R基团之间的相互作用维持：氢键、离子键、疏水相互作用以及二硫桥（半胱氨酸残基之间形成的共价S–S键）。四级结构存在于由多于一条多肽链组成的蛋白质中，例如血红蛋白，它由四个亚基（两条α链和两条β链）组成，这些亚基通过类似三级结构中的键以及一个血红素辅基结合在一起。蛋白质的功能严格依赖于其精确的三维形状，即使pH或温度的微小改变也可能导致变性，使蛋白质在不破坏肽键的情况下丧失结构和功能。
10. Protein Functions and Key Examples | 蛋白质功能与关键实例
Proteins perform an astonishing variety of functions. Enzymes are biological catalysts that speed up metabolic reactions; for instance, amylase breaks down starch into maltose. Structural proteins like collagen provide mechanical support in connective tissues; collagen is a fibrous protein with a triple helix, rich in glycine and proline, offering high tensile strength. Haemoglobin is a globular transport protein that carries oxygen from the lungs to respiring tissues; its quaternary structure allows cooperative binding of oxygen, described by a sigmoidal dissociation curve. Antibodies (immunoglobulins) are defensive proteins that recognise and neutralise foreign antigens. Hormones such as insulin (a small protein) regulate blood glucose concentration. The diversity of protein function arises from the endless combinations of 20 amino acids and the subsequent folding patterns.
蛋白质具有极其多样的功能。酶是加速代谢反应的生物催化剂；例如，淀粉酶将淀粉分解为麦芽糖。像胶原蛋白这样的结构蛋白为结缔组织提供机械支撑；胶原蛋白是一种具有三股螺旋的纤维蛋白，富含甘氨酸和脯氨酸，能提供很高的抗拉强度。血红蛋白是一种球状运输蛋白，将氧气从肺部运送到需氧组织；其四级结构允许氧的协同结合，氧合曲线呈S形。抗体（免疫球蛋白）是防御蛋白，能识别并中和外来抗原。激素如胰岛素（一种小分子蛋白质）能够调节血糖浓度。蛋白质功能的多样性源于20种氨基酸的无限组合以及随后的折叠方式。
11. Nucleic Acids: DNA and RNA | 核酸：DNA 与 RNA
Nucleic acids are vital biological molecules that store and transmit genetic information. The monomer unit is a nucleotide, which consists of a pentose sugar, a phosphate group, and a nitrogenous base. In DNA, the sugar is deoxyribose; in RNA, it is ribose. DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G); RNA contains uracil (U) instead of thymine. Nucleotides join via phosphodiester bonds between the phosphate of one nucleotide and the hydroxyl group of the sugar of another, forming a sugar–phosphate backbone with bases projecting. DNA is a double helix held by hydrogen bonds between complementary base pairs (A=T and C≡G), while RNA is usually single-stranded. DNA carries the genetic code, whereas various types of RNA (mRNA, tRNA, rRNA) participate in protein synthesis. Understanding nucleic acids is central to genetics and molecular biology, but a detailed discussion extends beyond this article’s main scope.
核酸是储存和传递遗传信息的重要生物分子。其单体是核苷酸，由戊糖、磷酸基团和含氮碱基组成。DNA中的糖是脱氧核糖，RNA中则是核糖。DNA的碱基包括腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)；RNA中含有尿嘧啶 (U) 而非胸腺嘧啶。核苷酸通过一个核苷酸的磷酸基团与另一个核苷酸的糖羟基之间形成的磷酸二酯键连接，形成糖-磷酸骨架，碱基向外伸出。DNA是由互补碱基对之间的氢键（A=T 和 C≡G）维系的右手双螺旋结构，而RNA通常为单链。DNA携带遗传密码，而多种RNA（mRNA、tRNA、rRNA）参与蛋白质合成。理解核酸对于遗传学和分子生物学至关重要，但详细讨论已超出本文的主要范围。
12. Biochemical Tests for Biological Molecules | 生物分子的生化检测
Identifying the presence of key biological molecules is a practical skill in A-Level Biology. The test for reducing sugars uses Benedict’s reagent: a sample in the reagent is heated in a water bath; a brick-red precipitate indicates a positive result. For non-reducing sugars like sucrose, the sample is first hydrolysed with dilute acid, neutralised, and then tested with Benedict’s reagent. Starch is detected using iodine solution, which changes from yellow-brown to blue-black. The biuret test for proteins involves adding a few drops of dilute copper sulfate solution to a sample that has been made alkaline with sodium hydroxide; a colour change from blue to lilac confirms the presence of peptide bonds. The emulsion test for lipids requires mixing the sample with ethanol, then pouring the mixture into water; a cloudy white emulsion indicates the presence of lipids. These tests rely on specific chemical reactions and provide a reliable way to distinguish between different biomolecules in the lab.
鉴定关键生物分子的存在是A-Level生物学的一项实践技能。还原糖检测使用本尼迪克特试剂：将样品与试剂混合并在水浴中加热，出现砖红色沉淀即为阳性。对于蔗糖等非还原糖，需先用稀酸水解，中和后再用本尼迪克特试剂检测。淀粉的检测使用碘液，颜色由黄褐色变为蓝黑色。蛋白质的双缩脲检测需向已被氢氧化钠碱化的样品中加入几滴稀硫酸铜溶液，颜色由蓝变紫即表示存在肽键。脂质的乳剂检测需将样品与乙醇混合，然后将混合物倒入水中，出现云状白色乳剂表示含有脂质。这些检测依赖于特定的化学反应，是实验室区分不同生物分子的可靠方法。
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