📚 Answers to End-of-Chapter Questions: Biological Molecules | 章末问题答案：生物分子
This article provides detailed model answers for selected end-of-chapter questions from Cambridge International AS & A Level Biology (9700), Chapter 2: Biological Molecules. Each answer is written to match the depth and precision required by exam mark schemes, highlighting essential terminology, logical structure, and key biological concepts. The paired Chinese translations follow each English paragraph to support bilingual learners in mastering both subject knowledge and scientific expression.
本文提供了剑桥国际AS和A Level生物学（9700）第2章“生物分子”中精选章末问题的详细标准答案。每个答案均按照考试评分方案所要求的深度与精确性撰写，突出关键术语、逻辑结构和核心生物学概念。每个英文段落后均配有中文译文段落，帮助双语学习者同时掌握学科知识和科学表达。
1. Water: Structure and Properties | 水：结构与性质
Water (H₂O) is a polar molecule because oxygen has a higher electronegativity than hydrogen. This creates a dipole where the oxygen atom carries a partial negative charge (δ⁻) and each hydrogen atom carries a partial positive charge (δ⁺). As a result, water molecules can form hydrogen bonds with each other and with other polar molecules.
水（H₂O）是一种极性分子，因为氧的电负性高于氢。这形成了一个偶极，氧原子带有部分负电荷（δ⁻），每个氢原子带有部分正电荷（δ⁺）。因此，水分子能够彼此之间以及与其他极性分子形成氢键。
Hydrogen bonding gives water a high specific heat capacity, meaning it can absorb or release a large amount of heat with only a small change in temperature. This property allows aquatic environments to maintain relatively stable temperatures and helps organisms avoid rapid thermal fluctuations.
氢键赋予水较高的比热容，这意味着它能够吸收或释放大量热量而温度变化很小。这一性质使水生环境能够保持相对稳定的温度，并帮助生物体避免温度的快速波动。
Cohesion, another consequence of hydrogen bonding, enables water molecules to stick together. This is essential for the transpiration pull in xylem vessels, where columns of water are drawn up under tension without breaking. Surface tension, created by cohesion at the water–air interface, supports small organisms such as pond skaters.
内聚力是氢键的另一个结果，它使水分子相互吸引。这对木质部导管中的蒸腾拉力至关重要，水柱在张力下被向上拉动而不会断裂。水-空气界面上由内聚力产生的表面张力能够支撑水黾等小型生物。
Water is an excellent solvent for ionic and polar substances because its dipoles interact with charged solutes and surround ions, effectively separating them. This allows many vital biochemical reactions to occur in aqueous solution, for example in cytoplasm and blood plasma. The transparency of water also enables light to penetrate aquatic habitats, facilitating photosynthesis in submerged plants.
水是离子与极性物质的优良溶剂，因为它的偶极子与带电溶质相互作用并包围离子，从而有效地将它们分开。这使得许多重要的生化反应能在水溶液中发生，例如在细胞质和血浆中。水的透明性也使光线能够穿透水生栖息地，帮助沉水植物进行光合作用。
Unlike most substances, water reaches its maximum density at 4°C, making ice less dense than liquid water. Thus, ice floats and forms an insulating layer on the surface of ponds and lakes, protecting aquatic life from freezing solid during winter.
与大多数物质不同，水在4°C时密度最大，这使冰的密度小于液态水。因此，冰漂浮在水面上，并在池塘和湖泊表面形成一层绝缘层，保护水生生物在冬季不被完全冻结。
2. Carbohydrates: Monosaccharides to Polysaccharides | 碳水化合物：从单糖到多糖
Glucose is a hexose monosaccharide with the molecular formula C₆H₁₂O₆. It exists in two ring forms: α-glucose and β-glucose. In α-glucose the hydroxyl group (-OH) on carbon 1 is below the plane of the ring, while in β-glucose it is above the plane. This seemingly small difference leads to profoundly different polysaccharide structures.
葡萄糖是一种己糖单糖，分子式为C₆H₁₂O₆。它以两种环状形式存在：α-葡萄糖和β-葡萄糖。在α-葡萄糖中，碳1上的羟基（-OH）位于环平面下方，而在β-葡萄糖中则位于环平面上方。这个看似微小的差异导致了截然不同的多糖结构。
Starch, a storage polysaccharide in plants, is composed of α-glucose units. It consists of two polymers: amylose and amylopectin. Amylose is a linear chain of α-1,4 glycosidic bonds, which allows it to coil into a compact helix. Amylopectin has both α-1,4 and α-1,6 glycosidic bonds, making it branched. The branching creates many terminal glucose residues that can be rapidly hydrolysed to release energy.
淀粉是植物中的储存性多糖，由α-葡萄糖单元组成。它包含两种聚合物：直链淀粉和支链淀粉。直链淀粉通过α-1,4糖苷键形成直链，使其卷曲成紧凑的螺旋结构。支链淀粉同时含有α-1,4和α-1,6糖苷键，因此具有分支。分支结构产生了许多末端葡萄糖残基，可被快速水解以释放能量。
Glycogen is the main storage carbohydrate in animals and fungi. Its structure is very similar to amylopectin but with even more extensive branching via α-1,6 linkages. The high degree of branching means glycogen is very compact and extremely rapidly mobilised, suiting the high metabolic demands of animal tissues such as muscle and liver.
糖原是动物和真菌中的主要储存性碳水化合物。其结构与支链淀粉非常相似，但通过α-1,6键的分支更加丰富。高分支度意味着糖原非常紧凑且能被极其迅速地动用，这满足了肌肉和肝脏等动物组织较高的代谢需求。
Cellulose, a structural polysaccharide found in plant cell walls, is a polymer of β-glucose. The monomers are linked by β-1,4 glycosidic bonds. To form these bonds, every alternate glucose molecule must be inverted by 180°. This arrangement produces straight, unbranched chains that lie parallel to each other and are cross-linked by hydrogen bonds, forming microfibrils with very high tensile strength.
纤维素是存在于植物细胞壁中的结构多糖，是β-葡萄糖的聚合物。单体通过β-1,4糖苷键连接。为形成这些键，每相邻的葡萄糖分子必须旋转180°。这种排列产生笔直、无分支的链，它们平行排列并通过氢键交联，形成具有极高抗张强度的微纤维。
The key functional distinction lies in the glycosidic linkages: α-linkages form helical, compact storage molecules that are easily hydrolysed, whereas β-linkages generate rigid, straight chains ideal for structural support. This exemplifies how the configuration of just one hydroxyl group influences the properties of polymers and their biological roles.
关键的功能区别在于糖苷键的类型：α-键形成螺旋状、紧凑且易于水解的储存分子，而β-键则产生刚性的直链，非常适合结构支撑。这充分说明了仅仅一个羟基的构型如何影响聚合物的性质及其生物学角色。
3. Lipids: Triglycerides and Phospholipids | 脂质：甘油三酯和磷脂
kbone via an ester bond, and three water molecules are released. Because the fatty acid chains can be saturated (no C=C double bonds) or unsaturated (one or more C=C double bonds), the physical properties of triglycerides vary considerably.
甘油三酯由一个甘油分子和三个脂肪酸分子缩合而成。每个脂肪酸通过酯键连接到甘油骨架上，同时释放出三个水分子。由于脂肪酸链可以是饱和的（不含C=C双键）或不饱和的（含有一个或多个C=C双键），甘油三酯的物理性质变化很大。
Triglycerides serve primarily as long-term energy stores. The long hydrocarbon tails are rich in carbon–hydrogen bonds, making them a highly reduced, energy-dense source of fuel. When oxidised during respiration, they yield more than twice the energy per gram compared with carbohydrates. Moreover, because triglycerides are hydrophobic and do not affect the water potential of cells, they can be stored in large quantities without osmotic disturbance.
甘油三酯主要作为长期的能量储存。长烃链富含碳氢键，使其成为高度还原、能量密度高的燃料来源。在呼吸作用中被氧化时，它们每克释放的能量是碳水化合物的两倍以上。此外，由于甘油三酯具有疏水性，不会影响细胞的水势，因此可以大量储存而不引起渗透干扰。
Phospholipids are similar to triglycerides except that one fatty acid chain is replaced by a phosphate-containing group. This makes the molecule amphipathic: the phosphate head is hydrophilic, while the two fatty acid tails are hydrophobic. In an aqueous environment, phospholipids spontaneously arrange into a bilayer, with the hydrophilic heads facing the water and the hydrophobic tails shielded inside.
磷脂与甘油三酯相似，不同的是其中一个脂肪酸链被一个含磷酸基团取代。这使得分子具有两亲性：磷酸头是亲水的，而两个脂肪酸尾是疏水的。在水环境中，磷脂会自发地排列成双分子层，亲水头朝向水，疏水尾被屏蔽在内部。
The phospholipid bilayer is the fundamental structure of all cell membranes. It acts as a selective barrier, allowing small, non-polar molecules to pass through while restricting the movement of ions and large polar molecules. Embedded proteins then provide specific transport and signalling functions. The fluidity of the membrane is influenced by the saturation of the fatty acid tails; unsaturated tails introduce kinks that prevent tight packing and increase fluidity.
磷脂双分子层是所有细胞膜的基本结构。它充当选择性屏障，允许小的非极性分子通过，同时限制离子和大极性分子的运动。嵌入的蛋白质则提供特异性的转运和信号传导功能。膜的流动性受脂肪酸尾的饱和度影响；不饱和脂肪酸尾带来的扭结阻止紧密排列，从而增加流动性。
4. Proteins: Levels of Structure | 蛋白质：结构层次
Proteins are polymers of amino acids linked by peptide bonds. The primary structure is the linear sequence of amino acids determined by DNA. This sequence dictates all higher levels of structure because the R groups of the amino acids interact with each other and with water.
蛋白质是由肽键连接的氨基酸聚合物。一级结构是由DNA决定的氨基酸线性序列。这一序列决定了所有更高的结构层次，因为氨基酸的R基团会彼此相互作用并与水相互作用。
onyl (C=O) and amide (N-H) groups of the peptide backbone. The two main types are the α-helix and the β-pleated sheet. In an α-helix, the chain coils into a right-handed spiral, whereas in a β-pleated sheet, sections of the chain run parallel or antiparallel to one another, linked by intermolecular hydrogen bonds.
二级结构是多肽链折叠成规则局部构象时产生的，通过肽骨干上羰基（C=O）和酰胺基（N-H）之间的氢键来稳定。两种主要类型是α-螺旋和β-折叠片。在α-螺旋中，链卷曲成右手螺旋；而在β-折叠片中，链段彼此平行或反平行排列，由分子间氢键连接。
Tertiary structure is the overall three-dimensional shape of a single polypeptide. It is maintained by a variety of interactions between R groups: hydrophobic interactions, hydrogen bonds, ionic bonds, and covalent disulfide bridges. The precise folding enables proteins to form active sites, binding pockets, and other functional domains. For globular proteins such as enzymes, the tertiary structure creates a highly specific conformation that is soluble and metabolically active.
三级结构是单条多肽链的整体三维形状。它由R基团之间的多种相互作用维持：疏水相互作用、氢键、离子键和共价二硫桥。精确的折叠使蛋白质能够形成活性位点、结合口袋和其他功能域。对于酶等球状蛋白质，三级结构产生了高度特异性的构象，是可溶的且具有代谢活性。
Quaternary structure exists when a functional protein consists of two or more polypeptide chains (subunits) held together by the same types of interactions as in tertiary structure. Haemoglobin is a classic example, composed of two α-globin and two β-globin subunits, each containing a haem group. The quaternary structure enables cooperative binding of oxygen, where the binding of one O₂ molecule facilitates the binding of subsequent ones.
四级结构出现在功能性蛋白质由两条或多条多肽链（亚基）组成，并通过与三级结构中相同类型的相互作用结合在一起时。血红蛋白是一个经典例子，由两个α-珠蛋白亚基和两个β-珠蛋白亚基组成，每个亚基含有一个血红素基团。四级结构实现了氧的协同结合，即一个氧分子的结合促进了后续氧分子的结合。
If a protein is heated or placed in extreme pH, these delicate interactions are disrupted and the protein denatures. The tertiary shape is lost, leading to loss of function. The primary sequence alone remains intact, illustrating that function depends heavily on precise folding.
如果蛋白质受热或置于极端pH环境中，这些精细的相互作用会被破坏，蛋白质会变性。三级形状丧失，导致功能丧失。仅有一级序列保持完整，这说明功能高度依赖于精确的折叠。
5. Nucleic Acids: DNA and RNA | 核酸：DNA与RNA
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are both polynucleotides composed of nucleotide monomers. Each nucleotide comprises a pentose sugar, a phosphate group, and a nitrogenous base. In DNA the sugar is deoxyribose, while in RNA it is ribose. This difference is crucial: deoxyribose lacks a hydroxyl group on the 2' carbon, making DNA more chemically stable.
DNA（脱氧核糖核酸）和RNA（核糖核酸）都是由核苷酸单体组成的多核苷酸。每个核苷酸包含一个戊糖、一个磷酸基团和一个含氮碱基。在DNA中糖是脱氧核糖，而在RNA中是核糖。这一区别至关重要：脱氧核糖在2'碳上缺少一个羟基，使DNA的化学性质更稳定。
DNA stores genetic information as a sequence of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). RNA replaces thymine with uracil (U). DNA is typically double-stranded, with the two strands running antiparallel and held together by complementary base pairing: A pairs with T via two hydrogen bonds, and C pairs with G via three hydrogen bonds. This specific pairing ensures faithful replication and provides a mechanism for error correction.
DNA以四种碱基的序列储存遗传信息：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。RNA用尿嘧啶（U）代替胸腺嘧啶。DNA通常是双链结构，两条链反向平行，通过互补碱基配对结合在一起：A与T通过两个氢键配对，C与G通过三个氢键配对。这种特异性配对确保了忠实复制，并提供了错误纠正的机制。
The double helix structure of DNA, described by Watson and Crick, has a sugar–phosphate backbone on the outside and the bases stacked like rungs on the inside. The helical twist creates major and minor grooves that allow regulatory proteins to access the genetic code without unwinding the entire molecule. The high stability of DNA suits its long-term information storage role.
沃森和克里克描述的DNA双螺旋结构，外侧是糖-磷酸骨架，碱基像梯级一样堆叠在内侧。螺旋扭曲产生大沟和小沟，使调控蛋白无需解开整个分子即可接近遗传密码。DNA的高稳定性使其适合长期储存信息。
RNA is usually single-stranded and performs a variety of roles in gene expression. Messenger RNA (mRNA) carries the genetic transcript from DNA to ribosomes. Transfer RNA (tRNA) brings amino acids to the ribosome during translation, and ribosomal RNA (rRNA) forms part of the ribosomal structure and catalyses peptide bond formation. Being less stable than DNA, RNA is generally shorter-lived, allowing precise temporal control of protein synthesis.
RNA通常是单链结构，并在基因表达中执行多种功能。信使RNA（mRNA）将遗传转录本从DNA携带到核糖体。转运RNA（tRNA）在翻译过程中将氨基酸带到核糖体，核糖体RNA（rRNA）构成核糖体结构的一部分并催化肽键形成。由于稳定性低于DNA，RNA的寿命通常较短，从而能精确地控制蛋白质合成的时间。
6. Biochemical Tests | 生物化学检测
To identify reducing sugars, the Benedict's test is used. An equal volume of Benedict's reagent (alkaline copper(II) sulfate solution) is added to the sample, and the mixture is heated in a water bath at approximately 80°C for five minutes. If reducing sugars are present, the blue Cu²⁺ ions are reduced to form a brick-red precipitate of copper(I) oxide, Cu₂O. The colour change may progress through green, yellow, and orange depending on the concentration.
检测还原糖使用本尼迪克特试验。向样品中加入等体积的本尼迪克特试剂（碱性硫酸铜(II)溶液），并将混合物在水浴中加热至约80°C加热5分钟。如果存在还原糖，蓝色的Cu²⁺离子被还原，形成砖红色的氧化亚铜（Cu₂O）沉淀。根据浓度不同，颜色变化可能依次经过绿色、黄色和橙色。
For non-reducing sugars such as sucrose, a negative Benedict's test must first be confirmed. Then the sample is hydrolysed by heating with dilute hydrochloric acid, which breaks the glycosidic bond and releases reducing monosaccharides. After neutralising with sodium hydrogencarbonate, the Benedict's test is repeated. A positive result after hydrolysis confirms the presence of non-reducing sugar originally.
对于蔗糖等非还原糖，首先必须确认本尼迪克特测试结果为阴性。然后将样品与稀盐酸一起加热进行水解，这会断裂糖苷键并释放出还原性单糖。用碳酸氢钠中和后，再次进行本尼迪克特测试。水解后出现阳性结果，即可确认样品原本含有非还原糖。
Starch is detected using iodine solution (iodine dissolved in potassium iodide). Adding a few drops to the sample produces a blue-black coloration if starch is present. This colour arises from the iodine molecules fitting inside the amylose helix. The test is sensitive and does not require heating.
淀粉检测使用碘液（碘溶于碘化钾中）。向样品中滴加几滴，如果存在淀粉，则呈现蓝黑色。这种颜色是由于碘分子嵌入了直链淀粉的螺旋结构内部。该测试灵敏且无需加热。
The biuret test is specific for peptide bonds and thus indicates the presence of proteins. A small volume of dilute sodium hydroxide is added to the sample, followed by a few drops of dilute copper(II) sulfate solution. A violet or mauve colour develops if peptide bonds are present. The reaction relies on the coordination of Cu²⁺ ions with the nitrogen atoms in peptide linkages in an alkaline environment.
双缩脲试验对肽键具有特异性，因此可指示蛋白质的存在。向样品中加入少量稀氢氧化钠溶液，然后滴加几滴稀硫酸铜(II)溶液。如果存在肽键，会产生紫罗兰色或淡紫色。该反应依赖于在碱性环境中Cu²⁺离子与肽键中的氮原子配位。
For lipids, the emulsion test is used. The sample is shaken vigorously with absolute ethanol to dissolve any lipids. The resulting liquid is then poured into cold water. If lipids are present, a milky-white emulsion forms, because the lipids precipitate out of solution as the ethanol mixes with water, forming finely dispersed lipid droplets that scatter light.
脂质检测使用乳剂试验。将样品与无水乙醇剧烈振摇以溶解任何脂质。然后将得到的液体倒入冷水中。如果存在脂质，会形成乳白色乳浊液，这是因为随着乙醇与水混合，脂质从溶液中析出，形成细小的脂质微滴，散射光线。
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