📚 Combined Science 157: Biological Molecules | 综合科学157：生物分子
Welcome to this in-depth revision guide for Edexcel Combined Science Topic 157: Biological Molecules. This chapter covers the fundamental building blocks of life, from the unique properties of water to the intricate structures of carbohydrates, lipids, proteins, and the action of enzymes. Understanding these molecules is essential not only for your A-Level exams but also for grasping how living organisms function at the molecular level. We will explore key concepts, common exam questions, and practical techniques to help you master this core area of the specification.
欢迎阅读这篇针对爱德思综合科学主题157：生物分子的深入复习指南。本章涵盖生命的基本组成单元，从水的独特性质到碳水化合物、脂质、蛋白质的复杂结构以及酶的作用。理解这些分子不仅对你的A-Level考试至关重要，对于掌握生物体在分子水平上的功能也同样关键。我们将探索关键概念、常见考试题型和实验技术，帮助你精通考纲的这一核心部分。
1. Introduction to Biological Molecules | 生物分子简介
Biological molecules, also known as biomolecules, are the chemical compounds found in living organisms. They are predominantly organic molecules based on a carbon skeleton and can be divided into four main groups: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are often polymers built from smaller repeating units called monomers. The process of joining monomers together via condensation reactions releases a water molecule, while breaking them down through hydrolysis consumes a water molecule. In Edexcel Combined Science Unit 1, you need to recognise the structures and functions of these molecules and understand how their arrangements determine their roles.
生物分子，也称生物大分子，是存在于生命体内的化合物。它们主要是基于碳骨架的有机分子，可分为四大类：碳水化合物、脂质、蛋白质和核酸。这些大分子通常是由称为单体的小重复单元构成的聚合物。通过缩合反应将单体连接在一起会释放一分子水，而通过水解反应将其分解则消耗一分子水。在爱德思综合科学第一单元中，你需要识别这些分子的结构和功能，并理解它们的排列如何决定其作用。
2. The Importance of Water | 水的重要性
Water (H₂O) is the most abundant molecule in living organisms, making up about 70–90% of the mass of a cell. Its unique properties arise from its polar nature and the ability to form hydrogen bonds between molecules. The high specific heat capacity of water means it can absorb a lot of energy with only a small rise in temperature, providing thermal stability for aquatic environments and intracellular reactions. Its high latent heat of vaporisation is crucial for cooling mechanisms such as sweating. Water's cohesion and adhesion allow it to move through xylem vessels in plants via transpiration pull, while its solvent properties make it an excellent medium for metabolic reactions and transport of ions and polar molecules.
水（H₂O）是生物体内最丰富的分子，约占细胞质量的70–90%。其独特性质源自其极性和分子间形成氢键的能力。水的高比热容意味着它可吸收大量能量而温度仅小幅上升，为水生环境和细胞内反应提供了热稳定性。其高汽化潜热对出汗等冷却机制至关重要。水的内聚力和附着力使其能通过蒸腾作用在植物木质部导管中移动，而作为溶剂的性质则使其成为代谢反应以及离子和极性分子运输的优良介质。
3. Carbohydrates: Monosaccharides & Disaccharides | 碳水化合物：单糖与双糖
Carbohydrates are composed of carbon, hydrogen, and oxygen, with the general formula (CH₂O)n. Monosaccharides are the simplest sugars, such as glucose, fructose, and galactose, all with the molecular formula C₆H₁₂O₆. Glucose exists as two isomers, α-glucose and β-glucose, differing in the orientation of the hydroxyl group on carbon 1. Disaccharides form when two monosaccharides undergo a condensation reaction, creating a glycosidic bond. For example, maltose is formed from two α-glucose molecules linked by an α-1,4 glycosidic bond; sucrose is made from glucose and fructose; and lactose from glucose and galactose. All monosaccharides and some disaccharides are reducing sugars, which can donate electrons to Benedict's reagent, turning it brick‑red upon heating.
碳水化合物由碳、氢、氧组成，通式为(CH₂O)n。单糖是最简单的糖类，如葡萄糖、果糖和半乳糖，分子式均为C₆H₁₂O₆。葡萄糖以两种异构体存在，即α-葡萄糖和β-葡萄糖，区别在于1号碳上羟基的朝向。当两个单糖发生缩合反应时形成双糖，生成糖苷键。例如麦芽糖由两分子α-葡萄糖通过α-1,4糖苷键连接而成；蔗糖由葡萄糖和果糖组成；乳糖由葡萄糖和半乳糖组成。所有单糖和部分双糖都是还原糖，能在加热时向本尼迪克特试剂提供电子，使其变为砖红色。
4. Polysaccharides: Starch, Glycogen, Cellulose | 多糖：淀粉、糖原、纤维素
Polysaccharides are long chains of monosaccharides joined by glycosidic bonds. Starch is the main energy store in plants, consisting of two polymers: amylose, which is a linear chain of α-glucose with α-1,4 bonds forming a helical structure, and amylopectin, which is branched with both α-1,4 and α-1,6 bonds. This structure makes starch compact and insoluble, ideal for storage in granules. Glycogen is the animal equivalent, highly branched with α-1,4 and more frequent α-1,6 bonds, allowing rapid release of glucose. Cellulose, a structural polysaccharide in plant cell walls, is made of β-glucose monomers linked by β-1,4 glycosidic bonds. The alternating orientation of β-glucose allows long straight chains to form, which are held together by hydrogen bonds, producing microfibrils of great tensile strength.
多糖是由糖苷键连接而成的长链单糖。淀粉是植物中的主要能量储存形式，由两种聚合物组成：直链淀粉是α-葡萄糖通过α-1,4键形成的线性螺旋链；支链淀粉则含有α-1,4和α-1,6键的分支结构。这样的结构使淀粉致密且不溶，非常适合以颗粒形式储存。糖原是动物体内的对应物，高度分支且含有更多α-1,6键，能快速释放葡萄糖。纤维素是植物细胞壁中的结构多糖，由β-葡萄糖单体通过β-1,4糖苷键连接而成。β-葡萄糖的交替朝向使长链笔直伸展，并由氢键维系，形成具有高度抗拉强度的微纤丝。
5. Lipids: Triglycerides & Phospholipids | 脂质：甘油三酯与磷脂
Lipids are a diverse group of hydrophobic molecules. Triglycerides are formed by the condensation of one glycerol molecule and three fatty acid molecules. Each fatty acid consists of a long hydrocarbon chain with a carboxyl group; if the chain contains no double bonds it is saturated, while the presence of one or more double bonds makes it mono‑ or poly‑unsaturated. The resulting ester bonds link glycerol to the fatty acids. Triglycerides are excellent energy stores because the long hydrocarbon tails yield a high number of hydrogen atoms for oxidative phosphorylation. Phospholipids have one of the fatty acid tails replaced by a phosphate‑containing group, making the molecule amphipathic – with a hydrophilic head and hydrophobic tails. This property allows them to form the bilayer of cell membranes.
脂质是一类疏水分子。甘油三酯由一分子甘油与三分子脂肪酸通过缩合反应生成。每条脂肪酸含有一个长烃链和一个羧基；若链中无双键则为饱和，若含一个或多个双键则为单不饱和或多不饱和。生成的酯键将甘油与脂肪酸连接起来。甘油三酯是极佳的能量储存物质，因为长烃尾含有大量氢原子，可用于氧化磷酸化。磷脂的一个脂肪酸尾被含磷酸基团取代，使分子具有两亲性——亲水头部和疏水尾部。这一特性使其能够形成细胞膜的双层结构。
6. Proteins: Amino Acids & Peptide Bonds | 蛋白质：氨基酸与肽键
Proteins are polymers of amino acids. 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. The R group determines the chemical nature of the amino acid – whether it is polar, non‑polar, charged, or contains sulfur. During protein synthesis, a condensation reaction between the carboxyl group of one amino acid and the amino group of another forms a peptide bond. A dipeptide contains two amino acids, while a polypeptide contains many. The sequence and number of amino acids are dictated by the genetic code and are crucial for the protein's final shape and function.
蛋白质是氨基酸的聚合物。每个氨基酸都有一个中心碳原子，分别连接一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子和一个可变的R基团。R基团决定了氨基酸的化学性质——是极性、非极性、带电荷还是含硫。在蛋白质合成过程中，一个氨基酸的羧基与另一个氨基酸的氨基发生缩合反应，形成肽键。二肽包含两个氨基酸，而多肽包含多个。氨基酸的顺序和数目由遗传密码决定，对蛋白质的最终形状和功能至关重要。
7. Levels of Protein Structure | 蛋白质结构层次
sequence of amino acids. The secondary structure arises from hydrogen bonding between the –NH and –C=O groups of the polypeptide backbone, forming patterns such as the α‑helix and β‑pleated sheet. The tertiary structure is the overall three‑ ons, and disulfide bridges between cysteine residues. Quaternary structure occurs when two or more polypeptide chains associate, as in haemoglobin, which consists of four subunits, each containing a haem group. The specific shape of a protein determines its function; denaturation, caused by high temperature or extreme pH, disrupts these bonds and leads to loss of function.
蛋白质具有四个不同的结构层次。一级结构是氨基酸的线性序列。二级结构由多肽骨架中–NH和–C=O之间的氢键形成，如α-螺旋和β-折叠片。三级结构是多肽链整体的三维折叠，由R基团之间的相互作用稳定：氢键、离子键、疏水相互作用以及半胱氨酸残基间的二硫键。四级结构发生在两条或多条多肽链组合时，例如血红蛋白由四个亚基组成，每个亚基含有一个血红素基团。蛋白质的特定形状决定其功能；由高温或极端pH引起的变性会破坏这些键，导致功能丧失。
8. Enzyme Action & Factors Affecting | 酶作用及影响因素
Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy. The substrate molecule fits into the enzyme's active site, forming an enzyme‑substrate complex. The induced‑fit model suggests that the active site changes shape slightly to accommodate the substrate, placing strain on bonds and facilitating the reaction. The rate of enzyme‑controlled reactions is affected by temperature, pH, enzyme concentration, and substrate concentration. At low temperatures, molecules have low kinetic energy; as temperature rises, the rate increases until an optimum where bonds maintaining the tertiary structure begin to break, denaturing the enzyme. Each enzyme has an optimum pH; changes alter the ionisation of R groups, disrupting bonding and the active site's shape. Competitive inhibitors have a similar shape to the substrate and block the active site; non‑competitive inhibitors bind elsewhere, altering the active site's conformation.
酶是生物催化剂，通过降低活化能来加速代谢反应。底物分子嵌入酶的活性位点，形成酶-底物复合物。诱导契合模型认为活性位点会略微变形以容纳底物，施加键的张力并促进反应。酶促反应速率受温度、pH、酶浓度和底物浓度影响。低温时分子动能低；随着温度升高，反应速率增加，直至最适温度时维持三级结构的键开始断裂，酶变性。每种酶都有最适pH；pH变化会改变R基团的电离状态，破坏键和活性位点的形状。竞争性抑制剂与底物形状相似，阻塞活性位点；非竞争性抑制剂结合在别处，改变活性位点构象。
9. Practical: Testing for Biological Molecules | 实验：生物分子的检测
The Edexcel specification expects you to be familiar with qualitative tests for key biomolecules. For reducing sugars, add Benedict's reagent and heat in a water bath; a colour change from blue through green, yellow, orange to brick‑red indicates a positive result. The test for starch uses iodine solution; a blue‑black colour confirms its presence. To detect proteins, add biuret reagent (sodium hydroxide followed by dilute copper sulfate solution); a violet/mauve colour forms if peptide bonds are present. For lipids, the emulsion test involves shaking the sample with ethanol and then pouring into water; a cloudy white emulsion indicates lipids. Remember to state appropriate safety precautions and the importance of control experiments.
爱德思考纲要求你熟悉关键生物分子的定性检测。检测还原糖时，加入本尼迪克特试剂并水浴加热；颜色从蓝变绿、黄、橙，最后呈砖红色表示阳性结果。淀粉测试使用碘液；出现蓝黑色确认其存在。检测蛋白质时，加入双缩脲试剂（氢氧化钠后再加入稀硫酸铜溶液）；若存在肽键则形成紫/淡紫色。脂质检测采用乳浊液测试，将样品与乙醇振荡，然后倒入水中；出现白色混浊乳浊液表明含有脂质。务必陈述适当的安全预防措施和对照实验的重要性。
10. Summary & Exam Tips | 总结与考试技巧
Biological molecules form a cohesive topic where structure directly relates to function. Be able to draw and label the ring structures of α‑ and β‑glucose, the formation of a glycosidic bond, a generalised amino acid, and the formation of a peptide bond. In exams, use accurate terminology – condensation, hydrolysis, monomer, polymer, saturated, unsaturated, denaturation – and link properties to biological roles, such as the insolubility of starch for storage or the amphipathic nature of phospholipids for membrane formation. Common exam questions ask you to interpret the results of food tests or to explain how molecular structure relates to function. Practice drawing clear diagrams and always answer in the context of the question.
生物分子构成了一个结构直接关联功能的连贯主题。你需要能够绘制并标注α-和β-葡萄糖的环状结构、糖苷键的形成、一个通式氨基酸的结构以及肽键的形成。考试中要使用准确的术语——缩合、水解、单体、聚合物、饱和、不饱和、变性——并将性质与生物学角色联系起来，例如淀粉的不溶性利于储存，或磷脂的两亲性利于膜的形成。常见的考题要求你解释食物测试的结果，或说明分子结构如何与功能相关联。练习绘制清晰的图示，并始终根据题目背景作答。
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