📚 The Chemistry of Life: Key Biological Molecules | 生命化学：关键生物分子
Biology is the study of living organisms, but beneath every life process lies chemistry. The molecules that make up cells – carbohydrates, proteins, lipids, nucleic acids and water – determine how organisms grow, reproduce and respond to their environment. For Edexcel IGCSE Biology, you need to know these molecules in detail: their structure, function and how to test for them.
生物学是研究生物体的科学，但每个生命过程背后都隐藏着化学。构成细胞的分子——糖类、蛋白质、脂质、核酸和水——决定了生物如何生长、繁殖和应对环境。对于 Edexcel IGCSE 生物考试，你需要详细了解这些分子：它们的结构、功能以及检测方法。
1. Elements and Compounds in Living Organisms | 生命体中的元素与化合物
All living things are made from a small number of chemical elements. The most abundant are carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), which together account for about 96% of an organism’s dry mass. In addition, elements such as sulfur (S), phosphorus (P), calcium (Ca), potassium (K), sodium (Na) and chlorine (Cl) are needed in smaller but still essential amounts.
所有生命体都由少量化学元素构成。含量最多的是碳（C）、氢（H）、氧（O）和氮（N），合计约占生物体干重的96%。此外，硫（S）、磷（P）、钙（Ca）、钾（K）、钠（Na）和氯（Cl）等元素虽然需要量较小，但依然不可或缺。
These elements do not exist as separate atoms in cells; they are combined into molecules via chemical bonds. Covalent bonds hold atoms together in organic molecules, while ionic bonds are important in minerals and in many biological fluids. Understanding these bonds helps explain why biological molecules have their specific shapes and properties.
这些元素并不是以独立原子形式存在于细胞中的；它们通过化学键结合成分子。共价键将原子连接在有机分子中，而离子键在矿物质和许多生物体液中也很重要。理解这些键有助于解释生物分子为何具有特定的形状和特性。
2. Carbohydrates: Monosaccharides and Disaccharides | 碳水化合物：单糖与双糖
Carbohydrates are molecules made of carbon, hydrogen and oxygen, often with the general formula (CH₂O)ₙ. They are a major source of energy for cells and also play structural roles. The simplest carbohydrates are monosaccharides, such as glucose (C₆H₁₂O₆) and fructose. Glucose is a hexose sugar (six carbon atoms) and is the main respiratory substrate in cells.
碳水化合物是由碳、氢、氧组成的分子，通常具有通式 (CH₂O)ₙ。它们是细胞的主要能量来源，也具有结构功能。最简单的碳水化合物是单糖，例如葡萄糖（C₆H₁₂O₆）和果糖。葡萄糖是己糖（含六个碳原子），是细胞呼吸的主要底物。
Two monosaccharides can join together in a condensation reaction to form a disaccharide. In this reaction, a molecule of water is removed, and a glycosidic bond is formed. For example, maltose is formed from two glucose molecules, sucrose from glucose and fructose, and lactose from glucose and galactose.
两个单糖可以通过缩合反应连接形成双糖。该反应脱去一分子水，并形成糖苷键。例如：麦芽糖由两分子葡萄糖形成，蔗糖由葡萄糖和果糖形成，乳糖由葡萄糖和半乳糖形成。
Glucose + Glucose → Maltose + H₂O
葡萄糖 + 葡萄糖 → 麦芽糖 + 水
Disaccharides are soluble and can be broken back down into monosaccharides by hydrolysis reactions, which use water to break the glycosidic bond. This process is catalysed by enzymes during digestion.
双糖是可溶的，可以通过水解反应重新分解为单糖，水解反应利用水来断裂糖苷键。这一过程在消化过程中由酶催化。
3. Polysaccharides: Starch, Glycogen and Cellulose | 多糖：淀粉、糖原和纤维素
Polysaccharides are long chains of many monosaccharide units joined by glycosidic bonds. They are often insoluble or only partially soluble in water, making them excellent storage or structural molecules. Starch is the main energy storage molecule in plants, formed from α-glucose units. It is a mixture of amylose (unbranched chain) and amylopectin (branched chain).
多糖是由许多单糖单元通过糖苷键连接而成的长链。它们通常不溶或仅部分溶于水，因此非常适合作为储存或结构分子。淀粉是植物中主要的能量储存分子，由 α-葡萄糖单元构成，包括直链淀粉（无分支）和支链淀粉（有分支）。
Glycogen is the animal equivalent of starch. It is highly branched and stored in the liver and muscles. Because it is insoluble and compact, it can be stored without affecting the osmotic balance of cells. Cellulose, by contrast, is a structural polysaccharide made of β-glucose units. The β-glucose molecules are arranged in straight chains that hydrogen-bond together to form microfibrils, giving plant cell walls great strength.
糖原是动物储存能量的形式，高度分支，储存在肝脏和肌肉中。由于它不溶且结构紧密，储存时不会影响细胞的渗透平衡。相比之下，纤维素是由 β-葡萄糖单元构成的结构多糖。β-葡萄糖分子排列成直链，通过氢键结合形成微纤丝，为植物细胞壁提供强大的机械强度。
4. Lipids: Fats, Oils and Phospholipids | 脂质：脂肪、油和磷脂
Lipids are a diverse group of molecules that are insoluble in water but soluble in organic solvents such as ethanol. The most common lipids are triglycerides, each composed of one glycerol molecule and three fatty acid chains. The fatty acids are joined to glycerol by ester bonds formed in condensation reactions.
脂质是一类不溶于水但溶于乙醇等有机溶剂的分子。最常见的脂质是甘油三酯，每个甘油三酯由一分子甘油和三条脂肪酸链组成。脂肪酸通过缩合反应形成的酯键与甘油连接。
Glycerol + 3 Fatty Acids → Triglyceride + 3H₂O
甘油 + 3 脂肪酸 → 甘油三酯 + 3 水
Fatty acids may be saturated (no double bonds between carbon atoms) or unsaturated (containing one or more double bonds). Unsaturated fats are usually liquid at room temperature (oils) and are found in plants, whereas saturated fats are often solid (fats) and are found in animals. Lipids are important for energy storage, thermal insulation and waterproofing. Phospholipids, where one fatty acid is replaced by a phosphate group, are essential components of cell membranes.
脂肪酸可以是饱和的（碳原子之间无双键）或不饱和的（含一个或多个双键）。不饱和脂肪通常在室温下呈液态（油），存在于植物中；饱和脂肪通常为固态（脂肪），存在于动物中。脂质在能量储存、保温隔热和防水方面非常重要。磷脂中一条脂肪酸被磷酸基团取代，是细胞膜的重要组成成分。
5. Proteins: Structure and Function | 蛋白质：结构与功能
Proteins are polymers made of amino acid monomers. There are about 20 different amino acids commonly found in living organisms. Each amino acid has an amino group (-NH₂), a carboxyl group (-COOH) and a variable R group (side chain). Amino acids are joined by peptide bonds in condensation reactions, forming dipeptides and then long polypeptide chains.
蛋白质是由氨基酸单体组成的聚合物。生物体内大约有20种常见氨基酸。每个氨基酸都有一个氨基（-NH₂）、一个羧基（-COOH）和一个可变的 R 基团（侧链）。氨基酸通过缩合反应以肽键连接，形成二肽，再形成长链多肽。
Amino Acid + Amino Acid → Dipeptide + H₂O
氨基酸 + 氨基酸 → 二肽 + 水
The primary structure of a protein is its unique sequence of amino acids. This sequence determines how the chain folds into a specific three-dimensional shape, which is held together by hydrogen bonds, ionic bonds and sometimes disulfide bridges. The final shape is crucial: it allows proteins to bind to specific molecules or provide structural support.
蛋白质的一级结构是指其独特的氨基酸序列。这一序列决定了多肽链如何折叠成特定的三维结构，这种结构由氢键、离子键以及有时存在的二硫键维持。最终的形态至关重要：它使蛋白质能够结合特定分子或提供结构支持。
Proteins have many vital functions. Enzymes are catalytic proteins; antibodies are proteins that fight pathogens; haemoglobin transports oxygen in red blood cells; collagen provides strength in connective tissue; and some hormones, such as insulin, are proteins that regulate body processes.
蛋白质具有多种重要功能。酶是催化性蛋白质；抗体是抵抗病原体的蛋白质；血红蛋白在红细胞中运输氧气；胶原蛋白为结缔组织提供强度；一些激素（如胰岛素）也是蛋白质，调节身体的生理过程。
6. Enzymes: Biological Catalysts | 酶：生物催化剂
Enzymes are proteins that speed up chemical reactions without being used up themselves. They do this by lowering the activation energy of the reaction. Each enzyme has an active site – a specific region with a unique shape. The molecule on which the enzyme acts is called the substrate, and it must fit into the active site for the reaction to occur.
酶是能加速化学反应而不被消耗的蛋白质。它们通过降低反应的活化能来发挥作用。每个酶都有一个活性位点——一个具有独特形状的特定区域。被酶作用的分子称为底物，底物必须能够嵌入活性位点，反应才能发生。
This specific fit is often described using the ‘lock and key’ model: the enzyme is the lock, and the substrate is the key. Only the correct key (substrate) fits into this particular lock (enzyme). Once the substrate binds, the enzyme catalyses the conversion to the product, and then the enzyme is free to bind another substrate molecule.
这种特异性匹配常用“锁钥模型”来描述：酶是锁，底物是钥匙。只有正确的钥匙（底物）能插入这把特定的锁（酶）。一旦底物结合，酶催化其转化为产物，然后酶又可以结合下一个底物分子。
Enzymes are involved in all metabolic reactions, including respiration, photosynthesis, digestion and synthesis of new molecules. Without enzymes, these reactions would be far too slow to sustain life.
酶参与所有代谢反应，包括细胞呼吸、光合作用、消化和新分子合成。如果没有酶，这些反应将慢到无法维持生命。
7. Factors Affecting Enzyme Activity: Temperature and pH | 影响酶活性的因素：温度与 pH
Temperature and pH are the two most important factors that alter enzyme activity. As temperature increases from low values, the rate of enzyme-catalysed reaction increases, because molecules gain kinetic energy and collide more frequently. Each enzyme has an optimum temperature (often around 37°C in humans). Above this optimum, the enzyme begins to denature: the bonds holding its three-dimensional shape break, the active site changes shape, and the enzyme can no longer bind the substrate.
温度和 pH 是影响酶活性最重要的两个因素。当温度从低值升高时，酶催化反应的速率上升，因为分子获得更多动能，碰撞频率增加。每种酶都有一个最适温度（人体内的酶通常约37°C）。超过最适温度后，酶开始变性：维持其三维结构的化学键断裂，活性位点形状改变，酶无法再结合底物。
Similarly, pH affects enzyme activity. Each enzyme works best at a particular pH – for most body enzymes this is around pH 7, but pepsin in the stomach works best at pH 2, and other digestive enzymes function at alkaline pH in the small intestine. Extreme pH values can denature the enzyme permanently.
同样，pH 也会影响酶活性。每种酶都有最适 pH——大多数人体酶的最适 pH 值约为7，但胃中的胃蛋白酶在 pH 2 时活性最高，而小肠中一些消化酶在碱性 pH 下发挥作用。极端的 pH 值会使酶永久变性。
Denaturation is irreversible because the active site cannot recover its original shape.
变性是不可逆的，因为活性位点无法恢复原有形状。
8. Factors Affecting Enzyme Activity: Substrate and Enzyme Concentration | 影响酶活性的因素：底物与酶浓度
At a given temperature and pH, the rate of an enzyme-catalysed reaction also depends on the concentration of the substrate and of the enzyme. When the enzyme concentration is fixed and substrate concentration is low, increasing substrate concentration increases the rate, because more active sites are occupied at any moment.
在特定温度和 pH 下，酶促反应速率还取决于底物浓度和酶浓度。当酶浓度固定、底物浓度较低时，增加底物浓度会提高反应速率，因为单位时间内有更多活性位点被占据。
However, eventually all active sites become occupied (the enzyme is saturated), and further increases in substrate concentration have no effect on the rate. Similarly, if substrate concentration is kept constant and enzyme concentration is increased, the rate increases proportionally, because there are more active sites available.
然而，当所有活性位点最终都被占据时（酶达到饱和），继续增加底物浓度不再提高反应速率。同样，如果底物浓度恒定而酶浓度增加，反应速率也会成比例提高，因为可用的活性位点增多了。
Competitive inhibitors are molecules that resemble the substrate and compete for the active site, reducing enzyme activity. Non-competitive inhibitors bind elsewhere and change the shape of the active site. Though inhibitors are not required in detail at IGCSE, they help explain how cells regulate metabolism.
竞争性抑制剂是类似底物的分子，会竞争活性位点，从而降低酶活性。非竞争性抑制剂在别处结合，改变活性位点形状。虽然 IGCSE 不要求掌握抑制剂的细节，但它们有助于理解细胞如何调控代谢。
9. Water: The Molecule of Life | 水：生命分子
Water is the most abundant molecule in cells, typically making up 60–70% of the body mass. Its unique properties arise from the polar nature of the H₂O molecule and the hydrogen bonding between water molecules. These properties are essential for life.
水是细胞中含量最多的分子，通常占体重的60%–70%。水的独特性质源自 H₂O 分子的极性以及水分子之间的氢键。这些性质对生命至关重要。
Firstly, water is an excellent solvent. Many substances, such as glucose, amino acids and mineral ions, dissolve readily in water, allowing them to be transported in blood and sap and to react together in cells. Secondly, water has a high specific heat capacity, so it resists temperature changes and helps organisms maintain a stable internal temperature.
首先，水是很好的溶剂。许多物质（如葡萄糖、氨基酸和矿质离子）易溶于水，因此能够在血液和汁液中运输，并在细胞内参与反应。其次，水的比热容很高，因此不容易升温或降温，有助于维持生物体内温度稳定。
Water also evaporates easily, taking heat away from the body, which makes sweating an effective cooling mechanism. Additionally, water is not easily compressed, which helps maintain turgor pressure in plant cells and acts as a hydraulic skeleton in some animals. Finally, water participates directly in many reactions, notably hydrolysis and condensation reactions.
水还容易蒸发，带走体内热量，因此出汗是一种有效的降温机制。此外，水不易被压缩，这有助于维持植物细胞的膨压，并在某些动物中起到液压骨骼的作用。最后，水直接参与许多反应，特别是水解反应和缩合反应。
10. Detecting Biological Molecules: Food Tests | 检测生物分子：食物测试
You need to know simple chemical tests to identify the main biological molecules in food. These tests are quick, qualitative and rely on visible colour changes. The table below summarises the four key food tests for the Edexcel IGCSE course.
你需要掌握简单的化学测试来辨别食物中的主要生物分子。这些测试快速、定性，并依靠可见的颜色变化。下表总结了 Edexcel IGCSE 课程中四个关键食物测试。
| Molecule | 分子 | Test | 测试 | Procedure | 步骤 | Positive Result | 阳性结果 |
|---|---|---|---|
| Reducing sugars | 还原糖 | Benedict’s test | 本尼迪克特测试 | Add Benedict’s solution and heat in a water bath. | Blue → green → yellow → orange → brick red |
| Starch | 淀粉 | Iodine test | 碘液测试 | Add a few drops of iodine solution. | Yellow-brown → blue-black |
| Proteins | 蛋白质 | Biuret test | 双缩脲测试 | Add dilute sodium hydroxide, then a drop of copper(II) sulfate solution. | Blue → purple / violet |
| Lipids (fats and oils) | 脂质 | Emulsion test | 乳化测试 | Shake with ethanol, then pour into water. | White cloudy emulsion forms |
11. Metabolism: The Sum of Life’s Reactions | 代谢：生命反应的总和
All the chemical reactions taking place inside a cell or organism are collectively known as metabolism. These reactions are organised into pathways, often with one enzyme catalysing each step. Anabolic reactions build up larger molecules from smaller ones (e.g. protein synthesis from amino acids, photosynthesis). Catabolic reactions break down larger molecules into smaller ones (e.g. respiration, digestion).
细胞内或生物体内进行的所有化学反应统称为代谢。这些反应被组织成通路，通常每一步都由一种酶催化。合成代谢反应由小分子构建大分子（例如由氨基酸合成蛋白质、光合作用）。分解代谢反应将大分子分解为小分子（例如细胞呼吸、消化）。
ATP (adenosine triphosphate) is the universal energy currency of the cell. When ATP is hydrolysed to ADP and phosphate, energy is released to drive metabolic reactions such as movement, active transport and synthesis. Understanding the chemistry of biological molecules therefore provides a foundation for every topic in IGCSE Biology.
ATP（三磷酸腺苷）是细胞的通用能量货币。当 ATP 水解为 ADP 和磷酸时，会释放能量，驱动运动、主动运输和合成等代谢反应。因此，理解生物分子的化学性质为 IGCSE 生物学的所有主题奠定了基础。
12. Conclusion: A Molecular View of Life | 结论：从分子角度看生命
Carbohydrates, lipids, proteins, enzymes and water interact constantly to sustain life. Carbohydrates provide quick energy and structural support; lipids store energy and form membranes; proteins carry out catalysis, transport and immunity; enzymes control reaction rates with precision; and water creates the environment in which all of this chemistry occurs.
碳水化合物、脂质、蛋白质、酶和水不断相互作用，维持生命。碳水化合物提供快速能量和结构支持；脂质储存能量并构成细胞膜；蛋白质进行催化、运输和免疫；酶精确调控反应速率；水则创造了这一切化学活动发生的环境。
For your Edexcel IGCSE Biology exam, remember the key structures, functions, and tests described in this guide. Drawing simple diagrams and practicing exam-style questions on enzymes and food tests will help you secure full marks on this important topic.
为了应对 Edexcel IGCSE 生物考试，请牢记本指南中描述的关键结构、功能和测试。画简图以及练习有关酶和食物测试的考试题型，将帮助你在这一重要主题上获得满分。
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