IB生物核心考点：分子生物学全解析
IB生物SL和HL的核心大纲Topic 2（分子生物学）覆盖从水分子到光合作用的完整生化图景，是Paper 1选择题和Paper 2简答题的高频考点。本文用中英双语梳理十个核心知识点，帮助你在考前快速建立知识框架。
一、什么是分子生物学？| What is Molecular Biology?
分子生物学是IB生物核心大纲的Topic 2，研究生命体如何利用碳基分子完成代谢。碳原子能形成四条稳定共价键，因此可以搭建出种类繁多的大分子：从葡萄糖到蛋白质，从DNA到ATP，几乎所有重要分子都含碳。同化作用把小分子合成为大分子，需要消耗能量；异化作用把大分子分解为小分子，同时释放能量。理解这两条代谢方向，是掌握后续所有生化过程的前提。考试中常以概念辨析题出现，要求判断某个过程属于合成还是分解。
Molecular biology is Topic 2 of the IB Biology core syllabus, and it studies how living organisms use carbon-based molecules to carry out metabolism. A carbon atom can form four stable covalent bonds, so it can build an enormous variety of large molecules: from glucose to proteins, from DNA to ATP, almost every important molecule contains carbon. Anabolism builds small molecules into larger ones and requires energy, while catabolism breaks large molecules down and releases energy. Grasping these two metabolic directions is the foundation for every biochemical process that follows. Exam questions often test whether you can classify a given process as synthesis or breakdown.
二、水：生命的关键溶剂 | Water: The Key Solvent of Life
水约占细胞质量的70%，是生命活动的舞台。水分子是极性分子：氧原子带部分负电荷，氢原子带部分正电荷，相邻水分子之间形成氢键。这些氢键赋予水一系列特殊性质：比热容高，能缓冲体温变化；汽化热高，出汗可以有效降温；作为优良溶剂，能溶解离子和极性分子；内聚力强，水柱可以在植物导管中持续上升。最特别的是，冰的密度小于液态水，因此冰浮在水面，保护下方水生动物的生存环境。IB考试常要求把这些性质与生物现象一一对应。
Water makes up about 70% of cell mass and provides the stage for life. A water molecule is polar: the oxygen atom carries a partial negative charge and the hydrogen atoms carry partial positive charges, so neighbouring water molecules form hydrogen bonds. These hydrogen bonds give water a set of special properties: a high specific heat capacity that buffers body temperature, a high latent heat of vaporisation that makes sweating an effective cooling mechanism, excellent solvent power for ions and polar molecules, and strong cohesion that allows water columns to rise through plant xylem. Most strikingly, ice is less dense than liquid water, so it floats and protects aquatic life below the surface. IB exams often ask you to match each property with its biological significance.
三、碳水化合物与脂质的对比 | Carbohydrates vs Lipids
碳水化合物是主要的快速能源。单糖如葡萄糖和果糖，双糖如蔗糖和乳糖，多糖如淀粉、糖原和纤维素。单糖之间通过缩合反应形成糖苷键，脱去一分子水；水解反应则加入水分子把多糖拆回单糖。脂质更擅长长期储能：甘油三酯由一分子甘油和三个脂肪酸组成，脂肪酸链越长、饱和程度越高，室温下越接近固态。磷脂是细胞膜的基本骨架，头部亲水、尾部疏水；胆固醇能调节膜的流动性。同等质量下脂肪释放的能量约为碳水化合物的两倍，因此骆驼驼峰和北极熊皮下脂肪都是高效的储能方案。
Carbohydrates are the main fast energy source. Monosaccharides such as glucose and fructose, disaccharides such as sucrose and lactose, and polysaccharides such as starch, glycogen and cellulose all belong to this group. Monosaccharides join through condensation reactions that form glycosidic bonds and release a water molecule, while hydrolysis adds water to break polysaccharides back into monosaccharides. Lipids are better at long-term energy storage: a triglyceride consists of one glycerol molecule and three fatty acids, and the longer and more saturated the fatty acid chains, the more solid the fat at room temperature. Phospholipids form the basic framework of cell membranes with hydrophilic heads and hydrophobic tails, and cholesterol helps regulate membrane fluidity. Per unit mass, fats release about twice as much energy as carbohydrates, which is why camel humps and polar bear blubber are such efficient energy stores.
四、蛋白质：从氨基酸到四级结构 | Proteins: From Amino Acids to Quaternary Structure
蛋白质由20种氨基酸通过肽键连接而成。每个氨基酸含氨基和羧基，R基团决定了它的独特性。一级结构是氨基酸的线性排列顺序；二级结构通过氢键形成α螺旋和β折叠；三级结构是整条肽链在疏水作用、氢键、离子键和二硫键共同作用下折叠成的三维形状；四级结构由多条肽链组装而成，如血红蛋白由四条亚基组成。蛋白质功能多样：酶催化反应，抗体提供免疫，血红蛋白运输氧气，肌动蛋白和肌球蛋白驱动肌肉收缩。高温或极端pH会使蛋白质变性，氢键断裂导致空间结构被破坏，这是发烧和胃酸环境杀死病原体的原理之一。
Proteins are built from 20 kinds of amino acids joined by peptide bonds. Each amino acid has an amino group and a carboxyl group, and the R group makes every amino acid unique. Primary structure is the linear sequence of amino acids; secondary structure forms alpha helices and beta sheets through hydrogen bonds; tertiary structure is the three-dimensional shape produced when hydrophobic interactions, hydrogen bonds, ionic bonds and disulfide bonds fold the whole polypeptide chain; quaternary structure assembles several chains, as in haemoglobin with its four subunits. Proteins perform diverse functions: enzymes catalyse reactions, antibodies provide immunity, haemoglobin transports oxygen, and actin and myosin drive muscle contraction. High temperature or extreme pH causes denaturation, in which broken hydrogen bonds destroy the spatial structure; this is one reason why fever and stomach acid can disable pathogens.
五、酶：生物催化剂的工作原理 | Enzymes: How Biological Catalysts Work
酶是生物催化剂，能大幅降低反应活化能，使反应在体温条件下快速进行。酶具有专一性：活性位点的形状只与特定底物匹配。诱导契合模型认为，底物结合后活性位点会发生微小形变，从而更紧密地包裹底物。影响酶活性的因素包括：温度升高时反应速率先升后降，超过最适温度后酶变性失活；pH同样存在最适值；底物浓度增加会提高速率，直到酶被饱和。竞争性抑制剂占据活性位点，非竞争性抑制剂结合其他部位改变酶的形状。IB考试常要求画出温度或pH对酶活性影响的曲线图，并解释曲线的每一段。
Enzymes are biological catalysts that greatly lower activation energy, allowing reactions to proceed quickly at body temperature. Each enzyme is specific: the shape of its active site matches only particular substrates. According to the induced fit model, the active site changes shape slightly when the substrate binds, wrapping around it more tightly. Factors affecting enzyme activity include temperature, where the rate rises then falls as temperature increases and the enzyme denatures beyond its optimum; pH, which also has an optimum value; and substrate concentration, which raises the rate until the enzyme becomes saturated. Competitive inhibitors occupy the active site, while non-competitive inhibitors bind elsewhere and change the enzyme shape. IB exams often ask you to draw the curve of enzyme activity against temperature or pH and explain every segment of it.
六、DNA与RNA：结构差异与碱基配对 | DNA and RNA: Structure and Base Pairing
DNA是双链螺旋，由脱氧核苷酸组成，每个核苷酸含脱氧核糖、磷酸基团和四种含氮碱基之一。两条链反向平行，通过碱基互补配对结合：A与T形成两个氢键，C与G形成三个氢键，因此G-C含量高的DNA热稳定性更强。RNA是单链，用核糖代替脱氧核糖，用碱基U代替T。三种主要RNA分工明确：mRNA携带遗传信息，tRNA转运氨基酸并识别密码子，rRNA与蛋白质共同构成核糖体。DNA双螺旋模型由沃森和克里克提出，富兰克林的X射线衍射照片提供了关键证据。结构题的常见陷阱是把核糖和脱氧核糖、U和T混为一谈。
DNA is a double helix made of deoxynucleotides, each containing deoxyribose, a phosphate group and one of four nitrogenous bases. The two strands run antiparallel and are held together by complementary base pairing: A pairs with T through two hydrogen bonds, and C pairs with G through three, so DNA with a higher G-C content is more heat-stable. RNA is single-stranded, uses ribose instead of deoxyribose, and replaces thymine with uracil. Three main types of RNA divide the work: mRNA carries genetic information, tRNA transports amino acids and recognises codons, and rRNA combines with proteins to form ribosomes. The double helix model was proposed by Watson and Crick, with Rosalind Franklin's X-ray diffraction photographs providing crucial evidence. A common trap in structure questions is confusing ribose with deoxyribose, or thymine with uracil.
七、DNA半保留复制与梅塞尔森实验 | Semi-Conservative DNA Replication
复制发生在细胞周期的S期。DNA双链在解旋酶作用下解开，DNA聚合酶沿模板链合成新链，遵循碱基互补配对原则。半保留复制的含义是：每个新DNA分子保留一条母链、含一条新链。梅塞尔森和斯塔尔用含重同位素氮15的培养基培养大肠杆菌，再转入普通氮14培养基，通过密度梯度离心观察DNA条带位置，最终证明复制是半保留而非全保留或分散式。DNA聚合酶只能从5'端向3'端合成新链，因此一条链连续合成，另一条链以冈崎片段的形式不连续合成。聚合酶的校对功能使复制错误率低至十亿分之一左右。
Replication occurs during the S phase of the cell cycle. Helicase unwinds the double helix, and DNA polymerase synthesises new strands along the template strands following base pairing rules. Semi-conservative replication means that each new DNA molecule keeps one original strand and contains one newly synthesised strand. Meselson and Stahl grew E. coli in a medium containing heavy nitrogen-15, then transferred the bacteria to normal nitrogen-14 medium and observed the DNA band positions by density gradient centrifugation, proving that replication is semi-conservative rather than conservative or dispersive. DNA polymerase can only add nucleotides from the 5' end toward the 3' end, so one strand is made continuously while the other is assembled in short Okazaki fragments. Proofreading by the polymerase keeps the error rate around one in a billion bases.
八、转录与翻译：基因表达的两步 | Transcription and Translation
基因表达分为两步。转录在细胞核内进行：RNA聚合酶结合启动子，以DNA的一条链为模板合成mRNA；真核生物中转录产物还要切除内含子、拼接外显子。翻译在核糖体上进行：核糖体沿mRNA移动，每次读取三个碱基组成的一个密码子；tRNA的反密码子与密码子互补配对，把对应的氨基酸逐个加到正在延长的肽链上。遗传密码具有简并性，多个密码子可以编码同一种氨基酸，这能减轻点突变造成的伤害。起始密码子AUG同时编码甲硫氨酸，UAA、UAG、UGA是终止密码子。一条mRNA可被多个核糖体同时翻译，称为多聚核糖体，这是提高翻译效率的常用考点。
Gene expression happens in two steps. Transcription occurs in the nucleus: RNA polymerase binds to the promoter and uses one DNA strand as a template to synthesise mRNA; in eukaryotes the transcript is then processed, with introns removed and exons spliced together. Translation occurs on ribosomes: the ribosome moves along the mRNA, reading three bases at a time as one codon, and tRNA anticodons pair with the codons, adding the corresponding amino acids one by one to the growing polypeptide. The genetic code is degenerate, meaning several codons can encode the same amino acid, which softens the damage of point mutations. The start codon AUG also codes for methionine, while UAA, UAG and UGA are stop codons. A single mRNA can be translated by several ribosomes at once, forming a polyribosome, a common exam point for translation efficiency.
九、细胞呼吸：有氧与无氧产能 | Cell Respiration: Aerobic and Anaerobic
细胞呼吸把有机物中的化学能转化为ATP。糖酵解在细胞质基质中进行，一分子葡萄糖分解为两分子丙酮酸，净产生两分子ATP和两分子NADH，整个过程不需要氧气。有氧条件下，丙酮酸进入线粒体：在基质中经过连接反应和克雷布斯循环，释放二氧化碳并产生NADH和FADH2；这些电子载体把电子传递给内膜上的电子传递链，质子泵建立电化学梯度，ATP合酶利用梯度合成大量ATP，氧气作为最终电子受体生成水。无氧呼吸时，动物细胞把丙酮酸还原为乳酸，酵母菌则生成乙醇和二氧化碳，产能少得多但不需要氧气。比较有氧与无氧的产ATP数量是Paper 1的高频题。
Cell respiration converts the chemical energy in organic molecules into ATP. Glycolysis takes place in the cytoplasm: one glucose molecule is split into two pyruvate molecules, producing a net gain of two ATP and two NADH without needing oxygen. Under aerobic conditions, pyruvate enters the mitochondria: in the matrix it passes through the link reaction and the Krebs cycle, releasing carbon dioxide and producing NADH and FADH2. These electron carriers pass electrons to the electron transport chain on the inner membrane, proton pumps build an electrochemical gradient, ATP synthase uses that gradient to make large amounts of ATP, and oxygen acts as the final electron acceptor to form water. In anaerobic respiration, animal cells reduce pyruvate to lactate, while yeast produces ethanol and carbon dioxide; far less ATP is made, but no oxygen is required. Comparing ATP yields between aerobic and anaerobic respiration is a frequent Paper 1 question.
十、光合作用：光反应与卡尔文循环 | Photosynthesis: Light and Dark Reactions
光合作用发生在叶绿体中，总反应是二氧化碳和水在光能作用下生成葡萄糖和氧气。光反应在类囊体膜上进行：叶绿素吸收光能后激发电子，光系统II裂解水，释放氧气、电子和质子；电子经电子传递链传给光系统I，质子梯度驱动ATP合酶合成ATP，NADP+接受电子和质子还原为NADPH。暗反应即卡尔文循环，在基质中进行，本身不需要光，但依赖光反应提供的ATP和NADPH：二氧化碳被RuBP固定，形成三碳化合物，再被还原为三碳糖，最后再生RuBP。光反应与暗反应紧密偶联，缺一不可，这是解释光合作用曲线平台期原因的关键。
Photosynthesis takes place in chloroplasts. The overall reaction converts carbon dioxide and water into glucose and oxygen using light energy. The light-dependent reactions occur on the thylakoid membrane: chlorophyll absorbs light and excites electrons, photosystem II splits water to release oxygen, electrons and protons, electrons pass through the electron transport chain to photosystem I, the proton gradient drives ATP synthase to make ATP, and NADP+ accepts electrons and protons to become NADPH. The light-independent reactions, known as the Calvin cycle, occur in the stroma; they do not need light directly but depend on the ATP and NADPH from the light reactions: carbon dioxide is fixed by RuBP, the resulting three-carbon compound is reduced to a triose phosphate, and RuBP is finally regenerated. The two stages are tightly coupled and neither can work without the other, which is the key to explaining the plateau of photosynthesis rate curves.
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