AQA A-Level生物分子 碳水 脂质 蛋白质 酶
AQA 的 A-Level Biology 7402 把生物分子放在 Paper 1 的起点，Paper 1 考的是第 1 到第 4 章的内容，而这一章又铺开了后面几乎所有题目的语言：碳水、脂质、蛋白质、酶、水与无机离子。很多学生把它当成一份背诵清单，能默写葡萄糖的分子式，却在数据题里看错题目问的是哪一级结构，或者把氢键和肽键混在一起写。复习时更值得花时间的是把分子结构与它承担的功能连起来，考题经常给出一组实验结果，然后问为什么会这样。这篇按考试真正会考的顺序把线索串起来，从缩合与水解这两个贯穿全章的反应类型开始，依次走过单糖与还原糖、淀粉与纤维素的键差、脂质的酯键与磷脂双分子层、蛋白质的四级结构、酶的诱导契合模型，最后收在水的氢键与四类无机离子的具体功能上。下面每一节只讲一个知识点，同时标出最容易丢分的那一处细节，适合考前用十分钟逐条复查。
In AQA A-Level Biology (7402) biological molecules sit at the start of Paper 1, which assesses topics 1 to 4, and they supply the vocabulary for almost everything that follows: carbohydrates, lipids, proteins, enzymes, water and inorganic ions. Many students revise the chapter as a vocabulary list, quoting the formula of glucose yet misreading a data question that asks about a particular level of protein structure, or mixing hydrogen bonds up with peptide bonds. The better use of revision time is to link each structure to the job it performs, because exam questions often present a set of results and ask why they came out that way. Each section below takes one knowledge point and flags the detail that loses marks, so the whole article can be rechecked point by point in ten minutes.
一、单体与聚合物：缩合反应与水解反应
单体是能与其他相同分子连接成聚合物的较小单元，聚合物由大量单体通过缩合反应连接而成。缩合反应每形成一个新键就脱去一分子水，水解反应则消耗一分子水把同一个键断开，两个过程方向相反却共用同一套键。以两个 α-葡萄糖缩合成麦芽糖为例，一个单体脱去氢原子，另一个脱去羟基，生成一分子水并形成一个 1,4-糖苷键；要把这个键拆开就需要水分子参与，这正是淀粉在消化道中被拆解的方式。氨基酸之间形成肽键、核苷酸之间形成磷酸二酯键，走的都是同一条缩合路线，人体内的合成与消化同时进行，只是方向不同。很多学生以为缩合一定需要能量而水解一定放能，其实两者真正的区别在于水分子是被释放还是被消耗。写反应式时把水放在产物一侧的是缩合，把水写进反应物一侧的是水解，这一条比背名词更快。
Condensation joins two monomers and releases one water molecule; hydrolysis uses a water molecule to break exactly the same bond, so the two processes run in opposite directions over a shared set of linkages. In maltose formation, one alpha-glucose loses a hydrogen atom while the other loses a hydroxyl group, a water molecule leaves and a 1,4-glycosidic bond appears. Peptide bonds between amino acids and phosphodiester bonds between nucleotides form by the same route, and digestion is only hydrolysis running the opposite way to synthesis. Many students assume condensation always needs energy while hydrolysis always releases it, but the reliable distinction is whether water appears as a product or is consumed.
二、单糖与双糖：还原糖与本尼迪特检测
单糖的分子通式可以写成 (CH2O)n，n 取 3 到 7，葡萄糖、果糖和半乳糖都是己糖，分子式同为 C6H12O6。葡萄糖存在 α 与 β 两种构型，区别只在第一个碳上氢与羟基的位置，这个细节后来决定了淀粉和纤维素的性质差别。两个单糖缩合得到双糖：麦芽糖由两个 α-葡萄糖组成，蔗糖由葡萄糖与果糖组成，乳糖由葡萄糖与半乳糖组成。还原糖指能够还原本尼迪特试剂的糖，葡萄糖、果糖、半乳糖、麦芽糖和乳糖都属于这一类，蔗糖不属于，必须先与稀盐酸加热水解、再把酸中和之后才能测出还原性。检测时加过量本尼迪特试剂并在沸水浴中加热数分钟，颜色从蓝色依次经过绿、黄、橙变成砖红色沉淀。这一步最容易出错的地方是试剂加得不够或加热时间不足，颜色停在绿色时容易被误判成阴性。
Monosaccharides fit the general formula (CH2O)n with n between 3 and 7, and glucose, fructose and galactose are all hexoses with the formula C6H12O6. Glucose exists as alpha and beta forms differing only in the position of the hydrogen and hydroxyl groups on the first carbon, and that small difference explains why starch and cellulose behave so differently. Two monosaccharides condense into a disaccharide: maltose from two alpha-glucose units, sucrose from glucose and fructose, lactose from glucose and galactose. Reducing sugars such as glucose, fructose, galactose, maltose and lactose give a positive Benedict's test while sucrose does not unless it is first hydrolysed with dilute hydrochloric acid and neutralised; heating with excess reagent turns the solution from blue through green and yellow to a brick-red precipitate, and a green result usually means too little reagent or too short a heating time rather than a true negative.
三、多糖的结构差别：淀粉、糖原与纤维素的键与分支
淀粉、糖原和纤维素全部由葡萄糖组成，差别只在单体的构型与键的连接方式。淀粉是直链淀粉与支链淀粉的混合物，两者都由 α-葡萄糖通过 1,4-糖苷键连接，支链淀粉每隔一段另有 1,6-糖苷键形成分支。糖原的分支比支链淀粉密集得多，末端数量大，酶可以同时从许多末端水解，肝细胞和肌细胞因此能迅速释放葡萄糖。纤维素由 β-葡萄糖组成，相邻单体旋转 180 度后连接，链保持笔直，链与链之间形成大量氢键并聚成微纤丝，给细胞壁带来很高的抗张强度。滴加碘液检测淀粉时，直链淀粉与碘分子形成螺旋包合物而呈蓝黑色，糖原与纤维素和碘的反应要弱得多。很多学生以为分支越多越难分解，实际情况恰好相反，分支越多可作用的末端越多，水解反而越快。
Starch, glycogen and cellulose are all polymers of glucose, and the differences come down to the configuration of the monomer and the way the bonds are arranged. Starch is a mixture of unbranched amylose and branched amylopectin, both built from alpha-glucose joined by 1,4-glycosidic bonds, with occasional 1,6-glycosidic bonds producing the branches. Glycogen branches far more heavily, so enzymes can attack many free ends at once and liver and muscle cells can release glucose quickly. Cellulose uses beta-glucose with adjacent monomers rotated by 180 degrees, and hydrogen bonds between the straight chains bundle them into microfibrils with high tensile strength; iodine in potassium iodide gives a blue-black colour with amylose but only a faint response with glycogen and cellulose.
四、脂质：酯键、不饱和脂肪酸与磷脂双分子层
甘油三酯由一分子甘油与三分子脂肪酸通过酯键连接，每形成一个酯键脱去一分子水，所以合成一分子甘油三酯会释放三分子水。脂肪酸含碳碳双键时属于不饱和脂肪酸，双键让碳链弯折，分子之间无法紧密排列，熔点和凝固点都更低，植物油在室温下多为液态就是这个原因。磷脂的结构是甘油连接两条脂肪酸链和一个磷酸基团，磷酸一端亲水、脂肪酸一端疏水，这种两亲性让它在水溶液中自动排成双分子层，成为细胞膜的基本骨架。脂质在体内的用途包括储能、防水、隔热与保护，同等质量下脂质储存的能量比碳水化合物多，不带电荷、不溶于水，存进细胞不会像糖那样把水带进来改变渗透压。乳化检测的做法是先加乙醇充分振荡让脂质溶解，再加等量水，出现乳白色浑浊说明样品含脂质。直接把油样倒进水里只会看到它浮在水面，不少人把这个现象当成乳化成功，其实只有先溶于乙醇才可能形成乳浊液。
Triglycerides form when one glycerol molecule joins three fatty acids through ester bonds, and each bond that forms releases a water molecule, so synthesising one triglyceride releases three. A fatty acid containing a carbon-carbon double bond is unsaturated: the double bond bends the chain, prevents tight packing, lowers the melting point and leaves most plant oils liquid at room temperature. Phospholipids replace one fatty acid with a phosphate group, giving a hydrophilic head and hydrophobic tails that arrange into a bilayer, the framework of every cell membrane. Lipids store more energy per gram than carbohydrates and, being uncharged and insoluble in water, can be kept in cells without drawing water in and changing the water potential; in the emulsion test the sample is dissolved in ethanol first, shaken, then mixed with water, and a milky-white emulsion confirms lipid, whereas oil poured straight into water merely floats.
五、蛋白质：肽键与四级结构
氨基酸的结构是一个中心碳原子同时连接氨基、羧基、一个氢原子和一个可变基团 R，正是 R 基团的不同区分了构成蛋白质的 20 种氨基酸。两个氨基酸缩合时，一个的氨基与另一个的羧基脱去一分子水形成肽键，多个氨基酸连成多肽链，再折叠成有功能的蛋白质。一级结构是氨基酸的排列顺序，由肽键维持；二级结构是主链上的 α-螺旋与 β-折叠，靠羰基与亚氨基之间的氢键稳定；三级结构是整条链进一步折叠后的三维形状，由氢键、离子键、二硫键以及疏水基团向内、亲水基团向外的相互作用共同维持；四级结构指两条以上多肽链，或者一条多肽链加一个辅基，血红蛋白就是四条多肽链各带一个血红素辅基。结构层次和功能直接挂钩，酶与抗体都靠特定的三级构象识别底物或抗原。很多学生把二级结构里的氢键写成肽键，阅卷时这种混淆会直接丢掉整段描述分。
An amino acid has a central carbon carrying an amine group, a carboxyl group, a hydrogen atom and a variable R group, and it is the R group that distinguishes the twenty amino acids found in proteins. Condensation between the amine group of one amino acid and the carboxyl group of the next releases water and forms a peptide bond, and the resulting chain folds into a working protein. The levels of structure build on each other: the primary structure is the amino acid sequence held by peptide bonds, the secondary structure is the alpha helix or beta pleated sheet stabilised by hydrogen bonds between carbonyl and imino groups, the tertiary structure is the folded three-dimensional shape held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions, and the quaternary structure covers two or more chains or a chain plus a prosthetic group, as in haemoglobin with four chains and a haem group. Function follows shape, since enzymes and antibodies recognise their targets through a precise tertiary conformation, and in the Biuret test sodium hydroxide followed by a little dilute copper(II) sulfate turns purple where peptide bonds are present.
六、酶：诱导契合模型、温度 pH 与抑制剂
酶是球状蛋白质，靠降低反应的活化能加快反应速率，活性部位的形状与底物互补。诱导契合模型认为活性部位并不刚性，底物结合时酶会轻微改变构象把它包得更紧，这种形变会拉扯底物内部的化学键，让活化能进一步降低，因此比锁与钥匙的比喻更贴近实验证据。温度升高时分子动能增加，酶与底物碰撞频率变高，速率上升到最适温度；超过最适温度后氢键与离子键断裂，三级结构被破坏，活性部位不再与底物互补，酶发生变性，这一步不可逆。pH 改变会影响活性部位中带电基团的相互作用，每种酶都有自己的最适 pH，胃蛋白酶的最适 pH 大约在 2 附近。竞争性抑制剂与底物形状相似并占据活性部位，提高底物浓度可以削弱抑制效果；非竞争性抑制剂结合在活性部位以外的位置并改变酶的构象，提高底物浓度也不能解除抑制。常见的错误是把加热灭活写成酶被杀死，酶是蛋白质，没有生命，标准表述是变性。
Enzymes are globular proteins that speed up reactions by lowering the activation energy, and their active site is complementary in shape to the substrate. The induced-fit model describes an active site that is not rigid: as the substrate binds, the enzyme changes shape slightly and wraps around it, which strains the bonds within the substrate and lowers the activation energy further, matching the evidence better than a simple lock-and-key picture. Raising the temperature increases kinetic energy and the frequency of successful collisions up to the optimum, but beyond that hydrogen and ionic bonds break, the tertiary structure is lost and the active site is no longer complementary, so the enzyme is denatured and the change cannot be reversed. pH alters the charged groups holding the active site together, which is why pepsin works best near pH 2; a competitive inhibitor occupies the active site and can be outcompeted by more substrate, while a non-competitive inhibitor binds elsewhere, distorts the enzyme and cannot be overcome.
七、水的氢键与四类无机离子的具体功能
水分子由两个氢原子和一个氧原子组成，氧原子带部分负电、氢原子带部分正电，所以水是极性分子，分子之间靠氢键相互吸引。氢键让水的比热容很高，吸收或放出大量热量而温度变化不大，这对维持生物体内环境的温度稳定很关键；水的汽化潜热同样很大，出汗时蒸发带走大量热量；氢键还让水分子互相吸引形成内聚力，木质部中的水柱因此可以连续上升。水是最常用的极性溶剂，离子和极性分子溶解其中，物质的运输与细胞内绝大多数反应都在水溶液里进行。AQA 7402 在 3.1.8 里点名了四类无机离子的功能：氢离子决定溶液的 pH，进而影响酶和蛋白质的形状；铁离子是血红蛋白的组成成分，负责结合并运输氧气；钠离子参与小肠上皮细胞对葡萄糖和氨基酸的协同运输；磷酸根离子参与构成 DNA、RNA 和 ATP。血液与细胞质中这些离子的浓度差异很大，pH 的微小变化就足以改变酶活性，数据题经常拿这一点设计干扰项。很多学生只背离子名称不背功能，简答题要求写具体作用时只能空着。
Water is a polar molecule because the oxygen atom carries a partial negative charge and the hydrogen atoms carry partial positive charges, so hydrogen bonds form between neighbouring molecules. A high specific heat capacity means large heat gains or losses cause only small temperature changes, which keeps internal temperature stable, and a high latent heat of vaporisation makes sweating an effective cooling mechanism; hydrogen bonding also produces cohesion, allowing a continuous column of water to rise through the xylem. As a polar solvent, water dissolves ions and polar molecules, so transport and most cellular reactions happen in solution. Specification point 3.1.8 names four inorganic ions and their roles: hydrogen ions determine pH and therefore enzyme and protein shape, iron ions form part of haemoglobin and bind oxygen, sodium ions drive the co-transport of glucose and amino acids in the ileum, and phosphate ions are components of DNA, RNA and ATP.
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